Name: Una panoramica dell'analisi genetica
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Description: An organism&#8217;s physical traits, or phenotype, are a product of its genotype, which is the combination of alleles (gene variants) inherited from its ...

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Research

2D NMR: Heteronuclear Single-Quantum Correlation Spectroscopy (HSQC)

Heteronuclear single-quantum correlation spectroscopy (HSQC) is a 2D NMR technique that reveals one-bond correlations between hydrogen and a heteronucleus. The HSQC experiment is similar to the heteronuclear correlation experiment (HETCOR) but is more sensitive. In the HSQC spectrum, the proton chemical shift is plotted on the horizontal F2 axis, while the 13C chemical shift is plotted on the vertical F1 axis. The corresponding proton and 13C spectra are also shown. The HSQC contour plot does not have any diagonal peaks. All the cross-peaks indicate the protons directly attached to each carbon in a molecule. Similar to the COSY spectrum, imaginary lines are drawn in horizontal and vertical directions to identify the correlated peaks.
For example, the HSQC spectrum of 2-chlorobutane has a carbon signal at 11 ppm that can be assigned to the shielded methyl group and correlated to the proton multiplet at 1.01 ppm. The carbon signal at 24.8 ppm arises from the deshielded methyl group, which correlates to the proton multiplet at 1.52 ppm. Similarly, the methylene carbon signal at 33.3 ppm and methine carbon signal at 60.4 ppm correlate to the multiplets at 1.72 and 3.96 ppm, respectively.&#160; HSQC is a very useful technique for structurally elucidating complex molecules and is routinely used in protein NMR studies.

Heteronuclear single-quantum correlation spectroscopy (HSQC) is a 2D NMR technique that reveals one-bond correlations between hydrogen and a ...

2D NMR: Overview of Heteronuclear Correlation Techniques

Heteronuclear correlation spectroscopy is an analytical technique that investigates the coupling between different types of nuclei, often a proton and an X-nucleus, such as carbon-13 or nitrogen-15. This method is commonly used in nuclear magnetic resonance (NMR) spectroscopy to gain insights into complex chemical compounds&#39; structural and compositional aspects. A typical heteronuclear correlation spectrum displays X-nucleus chemical shifts on one axis and a proton spectrum on the other axis. Cross-peaks reveal the connection between specific protons and X-nuclei.
The main heteronuclear correlation experiments include HETCOR, HSQC, HMQC, and HMBC. HSQC, HMQC, and HMBC record the proton spectrum, whereas the X-nucleus spectrum is recorded by standard heteronuclear correlation spectroscopy or HETCOR. The HETCOR-HMQC-HSQC group of experiments cannot detect X-nuclei without an attached proton. HETCOR is used when ultrahigh peak resolution is required along the X-nucleus axis.
The heteronuclear single quantum correlation technique, or HSQC, investigates proton-to-X-nucleus single bond correlations. In an HSQC experiment, polarization is transferred from a proton nucleus to a neighboring X-nucleus and then back to the proton nucleus. The signal from the proton nuclei is then recorded. HSQC generates a spectrum like that of heteronuclear multiple-quantum correlation spectroscopy or HMQC but uses a different suppression method, providing better X-nucleus resolution along the axis.
Heteronuclear multiple-bond correlation, or HMBC, is a proton-detected experiment, like HMQC but with a longer initial delay time. This experiment is used to observe long-range proton to X-nucleus connectivity separated by 2-3 bonds, with some experiments going out to 4 or 5 bonds. In an HMBC experiment, direct one-bond correlations are suppressed as part of the sequence. The spectra of HMBC give rise to cross peaks that correlate which protons are coupled to what other specific X-nucleus is more than one bond away. The gradient HMBC (gHMBC) pulse sequence offers better suppression of interfering signals, making it particularly useful for analyzing complex molecules with overlapping signals.

Heteronuclear correlation spectroscopy is an analytical technique that investigates the coupling between different types of nuclei, often a proton and an ...

2D NMR: Homonuclear Correlation Spectroscopy (COSY)

Homonuclear correlation spectroscopy, or COSY, is a 2-dimensional NMR technique that provides information about coupled protons. Typically, the geminal and vicinal coupling are observed. For example, consider the COSY spectrum of ethyl acetate, where its 1D proton NMR spectrum is plotted along the vertical and horizontal axes with their corresponding chemical shift scale. Three spots on the diagonal corresponding to the three peaks in the 1D proton spectrum are called diagonal peaks. The COSY spectrum is symmetric about the diagonal, with signals above and below the diagonal called cross-peaks or off-diagonal peaks. An imaginary line is drawn from diagonal peaks parallel to the horizontal axis, leading to a cross peak. A vertical line is then drawn from the same cross peak towards the diagonal peak to correlate the peaks. The off-diagonal peaks correlating the multiplets at 1.25 and 4.2 ppm indicate that the methyl protons are coupled to the methylene protons. The signal at 2 ppm is not associated with cross-peaks because the protons are not involved in homonuclear coupling. A COSY spectrum is useful when overlapping multiplets complicate the corresponding 1D spectrum.

Homonuclear correlation spectroscopy, or COSY, is a 2-dimensional NMR technique that provides information about coupled protons. Typically, the geminal ...

Insensitive Nuclei Enhanced by Polarization Transfer (INEPT)

Insensitive Nuclei Enhanced by Polarization Transfer (INEPT) is an advanced Nuclear Magnetic Resonance (NMR) technique specifically designed to detect and enhance the signals of low-abundance nuclei, such as carbon-13 and nitrogen-15, in small molecules. The fundamental principle behind INEPT is the transfer of polarization from a more abundant and highly polarizable nucleus, typically hydrogen-1, to the low-abundance nucleus of interest. This process effectively boosts the NMR signal of the low-abundance nuclei, making them more easily detectable.
The INEPT technique involves a series of carefully timed radiofrequency (RF) pulses applied to the sample. These RF pulses excite the more abundant nuclei, causing them to align their spins in a specific manner. Through the process of spin coupling, the polarization of the more abundant nuclei is then transferred to the low-abundance nuclei. As a result, the NMR signal of these low-abundance nuclei is significantly enhanced, allowing for a more detailed analysis of the molecular structure.
Various pulse sequences have been developed to improve the observability of these low-abundance nuclei when coupled to a high-receptivity nucleus. The INEPT pulse sequence combines a spin echo and selective population inversion, which includes 90&#176; and 180&#176; pulses on the high-sensitivity (the I spins) and low-sensitivity nuclei (the S spins), respectively. Coupling-constant-based delays between pulses facilitate magnetization transfer through space or bond interactions. Additional RF pulses and delays invert the low-gamma magnetization, producing an observable NMR signal. The net result is an enhanced NMR signal from low-gamma nuclei that would otherwise be challenging to detect.
Various pulse sequences have been developed to improve the detection of low-abundance nuclei when coupled with high-receptivity nuclei such as protons. The INEPT sequence yields enhanced signals for insensitive nuclei, with half of the CH group producing negative peaks and the other half positive peaks. This pattern allows for discrimination between different types of carbon-hydrogen bonds within the molecule.
Decoupling methods can be employed to enhance sensitivity and resolution in distinguishing various carbon-hydrogen bonds, making INEPT an invaluable tool in the chemical and pharmaceutical industries.

Insensitive Nuclei Enhanced by Polarization Transfer (INEPT) is an advanced Nuclear Magnetic Resonance (NMR) technique specifically designed to detect and ...

Two-Dimensional (2D) NMR: Overview

The 1D NMR spectrum of large and complex molecules like natural products has complicated splitting patterns and overlapping signals, which can be easily interpreted using 2-dimensional (2D) NMR. Unlike 1D NMR, 2D NMR has two frequency axes that provide the coupling information between the nucleus A and nucleus B in a molecule. The process from which 2D spectra are obtained has four steps.
The first step is the preparation period, during which nucleus A is excited with a radiofrequency pulse. The second step is the evolution or wait period t1, which is incremented systematically, and no data is observed during this period. The next step is the mixing period, in which a second radiofrequency pulse is introduced, which transfers the magnetization to the spins of nucleus B. In the last step, the magnetization of nuclear spins from B is detected during the acquisition time t2 for each t1. Time domains t1 and t2 are Fourier transformed into frequency domains f1 and f2, and these correspond to two dimensions of 2D spectra, while the third dimension is the intensity. In 2D experiments, coupling between the same kind of nucleus is called homonuclear 2D experiments, whereas coupling between two types of nucleus is called heteronuclear 2D experiments. 2D spectra can be displayed as stacked or contour plots. When presented as stacked plots, 2D spectra are difficult to interpret. The contour plot is the horizontal cross-section of a stacked plot where the size of the circles shows the signal intensity. Its interpretation is straightforward and it is preferred for data analysis.

The 1D NMR spectrum of large and complex molecules like natural products has complicated splitting patterns and overlapping signals, which can be easily ...

2D NMR: Overview of Homonuclear Correlation Techniques

Homonuclear correlation spectroscopy (COSY) is a powerful technique used in Nuclear Magnetic Resonance (NMR) spectroscopy to study the correlations between nuclei of the same type within a molecule. It provides information about scalar couplings between adjacent nuclei, which helps determine connectivity and structural information. There are several COSY variants, each with its unique strengths and experimental parameters.
COSY90 is the standard two-dimensional (2D) COSY experiment that utilizes a second 90-degree pulse to achieve high sensitivity and resolution. It excels in analyzing small to medium-sized molecules and provides information on directly coupled protons. COSY45, similar to COSY90, uses 45-degree pulses instead of 90-degree pulses. This results in reduced sensitivity but improved resolution.
Long-range COSY (LR-COSY) extends the COSY experiment to detect scalar couplings between nuclei further apart in the molecule. It achieves this by combining multiple pulse sequences and magnetization transfers.
Relayed COSY is a variant of COSY that adds extra pulses to the standard pulse sequence to produce cross-peaks between cross-peaks, showing specific molecular connections hidden by overlapping signals.
TOCSY (Total Correlation Spectroscopy), also known as HOHAHA (Homonuclear Hartmann-Hahn Spectroscopy), reveals all through-bond connectivity between all coupled nuclei. TOCSY is especially useful for analyzing molecules with overlapping signals and complex coupling patterns.
NOESY (Nuclear Overhauser Effect Spectroscopy) measures the nuclear overhauser effect, which arises from dipole-dipole interactions between nuclei. It provides information about spatial proximity and can be used to determine the three-dimensional structure of a molecule.
ROESY (Rotating-frame Overhauser Enhancement Spectroscopy) is similar to NOESY but employs a different mechanism to obtain through-space correlations. It provides information about spatial proximity and is particularly useful for studying molecules with weak NOE signals.
In conclusion, choosing a COSY variant depends on the molecule or reaction under investigation and the desired information. The right COSY variant can be determined based on molecular size, coupling complexity, and the need for spatial or long-range connectivity data.

Homonuclear correlation spectroscopy (COSY) is a powerful technique used in Nuclear Magnetic Resonance (NMR) spectroscopy to study the correlations ...

Double Resonance Techniques: Overview

Double resonance techniques in Nuclear Magnetic Resonance (NMR) spectroscopy involve the simultaneous application of two different frequencies or radiofrequency pulses to manipulate and observe two distinct nuclear spins. One important application of double resonance is spin decoupling, which selectively suppresses coupling with one type of nucleus while observing the NMR signal from another nucleus, simplifying the spectrum and enhancing resolution.
Spin decoupling is usually achieved by irradiating the sample atoms with a suitable radio frequency (rf) pulse sequence, effectively eliminating all the coupling with one nuclide. This simplifies the observed spectrum, making it easier to analyze and understand the relationships between different nuclei.
Double resonance experiments can be classified as heteronuclear or homonuclear, depending on whether the two sets of nuclei are of different isotopes or the same isotope, respectively. They can also be selective or nonselective, depending on whether the irradiating frequency covers only a portion or all of the resonance frequencies.
In nonselective heteronuclear decoupling, samples are exposed to an appropriate radiofrequency range to remove all coupling with one nuclide. However, increased external field strengths necessitate more powerful irradiation across broader frequency ranges. In certain instruments, continuous irradiation creates enough heat to cause damage to thermally sensitive samples.
To overcome these problems, modern methods using a series of pulses and exact timing delays, like J-modulated spin echo pulse sequences, can be employed to eliminate or adjust coupling effects within the spectrum.
For example, the attached proton transfer (APT) experiment applies a J-modulated spin echo pulse sequence to focus on the phase of detected carbon signals. Carbon atoms attached to an even number of protons exhibit positive signals in the spectrum, while those bonded to an odd number of protons appear as negative signals.
It utilizes a combination of a 180&#176; proton pulse and broadband decoupling to simplify spectral interpretation and assign multiplicities. The 180&#176; pulse is applied to the protons during the APT experiment. Its role is to refocus the spin coupling evolution that naturally occurs due to interactions between protons and carbons.
Specifically, after the 90&#176; pulse creates transverse magnetization, the 180&#176; pulse ensures that the effects of coupling are frozen during the initial period where the decoupler is turned off. This means the chemical shifts evolve only during the time 1/J&#8203; (the coupling constant period) after the first 180&#176; pulse.
By carefully timing this pulse sequence, the experiment isolates carbon multiplicities by utilizing the J-coupling between carbons and their attached protons. After the coupling evolution period (1/J&#8203;), broadband decoupling is turned on. This removes the J-coupling signals, collapsing multiplets into single peaks for each carbon resonance.
The decoupling ensures that carbon signals are free of splitting, allowing for a clear differentiation of signals based on phase (positive or negative) rather than splitting patterns.
The APT technique selectively enhances carbon signals, providing valuable information about carbon-proton connectivity, the number of attached hydrogen atoms, and the overall molecular structure.

Double resonance techniques in Nuclear Magnetic Resonance (NMR) spectroscopy involve the simultaneous application of two different frequencies or ...

&sup1;H NMR of Labile Protons:  Deuterium (&sup2;H) Substitution

This lesson illustrates the role of deuterium substitution in simplifying the NMR spectrum of compounds comprising labile protons. One method employed is the use of deuterium. Amongst the three isotopes of hydrogen, deuterium (2H) has a nucleus composed of one proton and one neutron. When the D2O solvent is added to a pure dry ethanol solution, its labile proton is substituted with deuterium.
<img alt="Figure1" src="/files/ftp_upload/12280/12280-PT_Figure_1.svg" style="text-align: center; height: 700px;" />
Figure 1. The comparison of the proton NMR spectra of pure dry ethanol and the same in D2O solvent.
The signals from deuterium are not observed in a proton NMR spectrum, as deuterium&#8211;proton coupling is negligible, and no spin-spin splitting occurs between them. As shown in Figure 1, the NMR spectra of pure ethanol and ethanol in deuterium, the disappearance of the triplet in the latter, confirms that it corresponds to the labile &#8211;OH proton. Furthermore, the methylene protons that initially showed up as a multiplet transform to a quartet, proving that the multiplet is due to the coupling with the &#8211;OH proton and the methyl protons.

This lesson illustrates the role of deuterium substitution in simplifying the NMR spectrum of compounds comprising labile protons. One method employed is ...

Nuclear Overhauser Enhancement (NOE)

Irradiation of a spin-active nucleus causes an increase or decrease in the signal intensity of neighboring nuclei that are not necessarily chemically bonded or involved in J-coupling. &#160;This phenomenon, called the Nuclear Overhauser Enhancement (NOE), results from through-space interactions between the nuclear spins. The NOE effect decreases with increasing internuclear distance and is generally not observed beyond 4 angstroms. In NOE, dipole-dipole interactions between neighboring spin-active nuclei result in the transfer of nuclear spin polarization. These interactions can cause an increase or decrease in the signal intensity, which results in a positive or negative NOE, respectively.
NOE can be applied to improve the signal intensity of less sensitive nuclei such as carbon-13. Proton-decoupled carbon-13 spectra show a positive NOE effect, where the carbon signal intensities are substantially increased compared to those in proton-coupled spectra.
Targeted irradiation of specific nuclei can also reveal through-space proximity, which helps confirm stereochemistry and verify spectral peak assignments. The NOE effect is also applied in 2D NMR experiments, such as Nuclear Overhauser Effect Spectroscopy (NOESY), to determine the 3D structure of proteins.

Irradiation of a spin-active nucleus causes an increase or decrease in the signal intensity of neighboring nuclei that are not necessarily chemically ...

&sup1;H NMR of Labile Protons: Temporal Resolution

Protons bonded to heteroatoms such as nitrogen and oxygen exhibit a range of chemical shift values. This is due to the varying degree of hydrogen bonding between the proton and the heteroatom in other molecules. The extent of hydrogen bonding affects the electron density around the proton, thereby giving different chemical shift values for the protons in the proton NMR spectrum.
The &#8211;OH proton in alcohols typically appears in the range of &#948; 2 to 5 ppm but can vary depending on the specific alcohol and conditions. This &#8211;OH proton undergoes rapid proton exchange with other molecules and is referred to as a labile proton. So, the exact &#948; value of the &#8211;OH proton and the spin-spin coupling involving the &#8211;OH proton and the other protons in the alcohol depends on the rate of the &#8211;OH proton exchange. The proton exchange rate, in turn, depends on multiple factors, including the purity of the alcohol sample, temperature, and solvent.
<img alt="Figure1" src="/files/ftp_upload/12279/12279-PT_Figure_1.svg" style="text-align: center; height: 350px;" />
Figure 1. The triplet of a &#8211;OH proton in the proton NMR spectrum of neat ethanol.
For example, the &#8211;OH proton exchange in a pure dry ethanol sample is very slow on the NMR time scale, giving enough time for the spin-spin coupling. Consequently, as depicted in Figure 1, the proton NMR spectrum of pure dry ethanol shows a triplet for the &#8211;OH proton due to the spin-spin coupling with adjacent methylene protons. The methylene protons are seen as a multiplet due to the coupling with the &#8211;OH proton and the methyl protons.
The presence of trace amounts of an acid or base increases the proton exchange rate. The proton exchange becomes rapid, and the NMR spectrometer only records an average of all the possible environments for the &#8211;OH proton.
<img alt="Figure2" src="/files/ftp_upload/12279/12279-PT_Figure_2.svg" style="text-align: center; height: 325px;" />
Figure 2. The broad singlet of a &#8211;OH proton in the proton NMR spectrum of neat ethanol.
As a result, as recorded in Figure 2, the &#8211;OH proton exhibits a broad singlet. The protons from the adjacent methylene group present a quartet where the splitting is caused only by the adjoining methyl group.

Protons bonded to heteroatoms such as nitrogen and oxygen exhibit a range of chemical shift values. This is due to the varying degree of hydrogen bonding ...

&sup1;H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR

The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.
<img alt="Figure1" src="/files/ftp_upload/13016/13016-PT_Figure_1.svg" style="text-align: center; height: 480px;" />
Figure 1. The temperature-dependent proton NMR spectra of cyclohexane.
Figure 1 depicts the proton NMR spectrum of the deuterium substituted sample, recorded at various temperatures. At room temperature, the rapid ring flipping of cyclohexane results in a single sharp peak. As the temperature is lowered to &#8722;60&#176;C, the rate of chair-chair interconversion reduces, resulting in the broadening of the peak.
Further lowering of the temperature broadens the peak, forming a saddle that splits into two peaks. On reaching a temperature lower than &#8722;89&#176;C, two sharp, well-resolved peaks corresponding to the equatorial and axial protons are formed. The peaks corresponding to the equatorial and axial protons occur at &#948; 1.62 and 1.14, respectively. At such low temperatures, the rate of chair-chair interconversion is reduced significantly, enabling the detection of the two distinct sets of protons on the NMR time scale.

The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one ...

&sup1;H NMR of Conformationally Flexible Molecules: Temporal Resolution

At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the axial and equatorial protons, resulting in a single peak at &#948; 1.4.
Similarly, NMR averages all the conformations of any molecule with rapid conformational equilibria. For example, the CH3 protons in bromoethane have a single resonance and coupling constant for splitting by the CH2 protons due to the rapid internal rotation of the carbon-carbon bond. Lastly, besides conformational equilibria, the time-averaging effect of NMR spectroscopy is also observed in some chemical reactions.

At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately ...

Mass Spectrometry: Carboxylic Acid, Ester, and Amide Fragmentation

The fragmentation patterns observed for compounds such as carboxylic acids, esters, and amides in the mass spectra include &#9082;-cleavage and McLafferty rearrangement. Fragmentation by &#9082;-cleavage preferentially occurs at the carbon-carbon bond at the &#9082;-position next to the carboxylic group to generate a neutral radical and a cation. Long chain compounds with hydrogen at their &#947;-carbon undergo McLafferty rearrangement to give a radical cation and a neutral alkene.
For example, the fragmentation of butyramide is demonstrated in Figure 1. The molecular ion peak of butyramide, which contains one nitrogen atom, shows an odd mass-to-charge ratio of 87. Fragmentation of butyramide molecular ion by &#9082;-cleavage produces a cation at a mass-to-charge ratio 44. In addition to &#9082;-cleavage, butyramide molecular ion also undergoes fragmentation via McLafferty rearrangement. The base peak at the mass-to-charge ratio 59 arises due to McLafferty rearrangement.
<img alt="Figure1" src="/files/ftp_upload/13051/13051_PT_Figure_1.svg" style="text-align: center; height: 250px;" />

The fragmentation patterns observed for compounds such as carboxylic acids, esters, and amides in the mass spectra include &#9082;-cleavage and McLafferty ...

Chemical Ionization (CI) Mass Spectrometry

The molecular ion peak of a molecule in the mass spectrum provides vital information for molecular identification. However, conventional electron impact ionization can lead to the rapid dissociation of some molecular ions before they reach the detector. A milder ionization method is required to increase the lifetime of such ionized analyte molecules. Chemical ionization (CI) is a gas-phase protonation reaction useful for mass-analyzing analyte molecules that are easily protonated to yield the corresponding conjugate acid. In this process, the sample is mixed with an excess reagent gas, which ensures the electron impact occurs primarily on the reagent gas. The charged species formed from the reagent protonates the analyte molecule, producing a relatively stable protonated analyte (conjugate acid) compared to the molecular ion. This results in an M+1 peak in the mass spectrum. The conjugate acid may then undergo fragmentation that generates additional signals.
For example, the CI of di-sec-butyl ether in methane gas illustrates this process. When the ether is mixed with excess methane gas as a reagent, electron impact occurs on methane rather than the ether. The resulting methane radical cation can react with another methane molecule to generate a methane radical and a methanium ion. The methanium ion is a source of gas-phase protons, which can protonate the ether to form its conjugate acid. The sequential reactions during the chemical ionization of di-sec-butyl ether are depicted in Figure 1.
<img alt="Figure1" src="/files/ftp_upload/12300/12300_PT_Figure_1.svg" style="text-align: center; height: 175px;" />
Figure 1: Chemical ionization of di-sec-butyl ether methane mixture.
This conjugate acid of the ether (m/z = 131) is relatively more stable than the molecular ion (m/z = 130) of ether formed via the conventional electron impact ionization. In the traditional route, the molecular ion undergoes fragmentation via &#945; cleavage, giving a signal at m/z = 101. Figure 2 illustrates the reactions that occur during electron impact ionization directly on di-sec-butyl ether.
<img alt="Figure2" src="/files/ftp_upload/12300/12300_PT_Figure_2.svg" style="text-align: center; height: 175px;" />
Figure 2: Electron impact ionization of di-sec-butyl ether and fragmentation of the molecular ion.
So, the mass spectra of di-sec-butyl ether ionized via CI feature an M+1 peak at m/z = 131. On the other hand, mass spectra of di-sec-butyl ether ionized via electron impact ionization do not show any peak at the m/z value of its molecular weight. Figures 3a and 3b show the mass spectra of di-sec-butyl ether ionized via the electron impact and chemical ionization methods, respectively.
<img alt="Figure3" src="/files/ftp_upload/12300/12300_PT_Figure_3.svg" style="text-align: center; height: 225px;" />
Figure 3:&#160;a) No m/z = 130 peak is visible in the mass spectrum obtained via electron impact ionization of di-sec-butyl ether. b) The m/z = 131 peak is clearly visible in the mass spectrum obtained via chemical ionization of di-sec-butyl ether.

The molecular ion peak of a molecule in the mass spectrum provides vital information for molecular identification. However, conventional electron impact ...

Mass Spectrometry: Alkyl Halide Fragmentation

Chlorine isotopes exist as 35Cl and 37Cl in a 3:1 ratio, while bromine isotopes exist as 79Br and 81Br in a 1:1 ratio. The mass spectrum of alkyl halides typically produces two distinct molecular ion peaks, the molecular ion peak, [M], and the molecular ion plus two, [M + 2] peak. The relative heights of these two peaks are proportional to the isotopic abundance ratios of the halide. For example, 2&#8208;chloropropane and 1&#8208;bromopropane display two peaks with relative peak heights in a 3:1 and 1:1 ratio, respectively.
Alkyl halides typically lose a non-bonding electron from the halogen to form molecular ions. The base peak results from the heterolytic cleavage of the carbon&#8211;halogen bond, where the bonding electrons go to the most electronegative atom, forming a cation and halogen atom. Interestingly, the &#9082;-cleavage is observed only in alkyl chlorides, not alkyl bromides. In alkyl chlorides, the&#160; C&#8211;Cl and C&#8211;C bonds exhibit similar strengths, while in alkyl bromides, the C&#8211;Br bond is weaker than the C&#8211;C bond. Consequently, the alkyl chlorides form a stable cation, where the positive charge is shared between the two atoms.
While less commonly discussed, fluoroalkanes and iodoalkanes exhibit unique fragmentation behavior. Fluoroalkanes, due to the strength of the C&#8211;F bond and the presence of only one isotope (19F), show limited fragmentation and lack isotopic peaks. Iodoalkanes, with their weak C&#8211;I bond and a single isotope (127I), predominantly fragment by losing iodine, resulting in simpler spectra with less isotopic splitting.

Chlorine isotopes exist as 35Cl and 37Cl in a 3:1 ratio, while bromine isotopes exist as 79Br and 81Br in a 1:1 ratio. The mass spectrum of alkyl halides ...

Electrospray Ionization (ESI) Mass Spectrometry

Higher molecular weight biomolecules are nonvolatile compounds that may decompose before ionizing or vaporizing during mass analysis with conventional electron impact ionization methods. Accordingly, electrospray ionization (ESI) is the favored method for vaporizing and ionizing biomolecules as it circumvents rapid fragmentation and enables the recording of mass signals for the entire biomolecule.
ESI utilizes electrical energy to transfer ions from the liquid phase of the sample into the gaseous phase. The analyte biomolecule is premixed with an ionic liquid&#8212;a nonmolecular compound serving as the ionization source. The solution is sprayed through a high-voltage capillary to generate a fine aerosol. The voltage difference applied between the capillary and the entrance of the analysis chamber causes the movement of charged droplets toward the analysis chamber. The ionic liquid from the aerosol evaporates during this movement, leaving behind a variably protonated or ionized analyte molecule. These charged species are detected in the mass spectrum at an m/z value equivalent to the sum of the analyte molecule&#39;s molecular mass and the ions&#39; molecular weight from the ionic liquid associated with the analyte molecule.
The position of mass signals varies depending on the type of cations present in the ionic liquid. For example, the presence of sodium ions in the ionic liquid leads to an (M+23) peak in the mass spectra, while the presence of a proton results in an (M+1) peak.

Higher molecular weight biomolecules are nonvolatile compounds that may decompose before ionizing or vaporizing during mass analysis with conventional ...

Matrix-Assisted Laser Desorption Ionization (MALDI)

Matrix-assisted laser desorption ionization (MALDI) is a powerful analytical technique used in mass spectrometry. It enables the identification and characterization of various biomolecules, including proteins, peptides, nucleic acids, and carbohydrates. MALDI spectrometry is widely employed in biological and medical research, as well as in fields like pharmacology and biochemistry.
The analyte of interest, a biomolecule or a mixture of biomolecules, is mixed with a suitable matrix material. The matrix is usually an organic compound, such as a small organic acid or a crystalline substance, that facilitates ionization. This analyte-matrix mixture is then deposited onto a target surface, inserted into the mass spectrometer, and dried. During drying, the matrix forms small crystals that incorporate the analyte molecules within their structure.
Once the sample is prepared, a pulsed laser is directed onto the target surface. The matrix molecules absorb the laser beam energy, causing them to heat up and vaporize rapidly. This process is known as laser desorption. As the matrix molecules vaporize, they carry the embedded analyte molecules. The vaporized analyte molecules become ionized through a process called proton transfer. The proton transfer occurs between the matrix and the analyte molecule. The matrix molecule donates a proton to the analyte molecule, resulting in the formation of protonated ions. The analyte ions are then accelerated by an electric field and further separated based on their mass-to-charge ratio in a time-of-flight mass analyzer.
The mass spectrum obtained from MALDI spectrometry provides valuable information about the mass and relative abundance of the analyzed biomolecules.

Matrix-assisted laser desorption ionization (MALDI) is a powerful analytical technique used in mass spectrometry. It enables the identification and ...

Mass Spectrometry: Aldehyde and Ketone Fragmentation

In mass spectrometry, the fragmentation of aliphatic aldehydes and ketones generally occurs through three key mechanisms: &#945;-cleavage, inductive cleavage, and the McLafferty rearrangement.
<ol>
	<li>&#945;-Cleavage 
	&#945;-Cleavage is a common fragmentation in carbonyl compounds where the bond adjacent to the carbonyl group is broken. This process produces an alkyl radical and an acylium cation, which is often detected in the mass spectrum due to its stability.</li>
	<li>Inductive Cleavage 
	In inductive cleavage, fragmentation yields an acyl radical and an alkyl cation. This occurs when electrons are drawn toward the carbonyl, weakening adjacent bonds and resulting in a distinctive fragmentation pattern. For example, in the mass spectrum of 5-methyl-2-hexanone, inductive cleavage produces an alkyl cation at a mass-to-charge (m/z) ratio of 71.</li>
	<li>McLafferty Rearrangement 
	The McLafferty rearrangement is a specific fragmentation pattern for carbonyl compounds with &#947;-hydrogen (hydrogen on the third carbon from the carbonyl group). The molecule undergoes a six-membered cyclic transition in this rearrangement, yielding a radical cation and a neutral alkene. For 5-methyl-2-hexanone, this rearrangement results in a peak at m/z 58.</li>
</ol>
Example: Fragmentation of 5-methyl-2-hexanone 
In the mass spectrum of 5-methyl-2-hexanone, the molecular ion undergoes these fragmentation patterns:
<ol style="list-style-type: circle;">
	<li>Inductive cleavage produces an alkyl cation at m/z 71.</li>
	<li>&#945;-Cleavage results in the base peak at m/z 43.</li>
	<li>McLafferty rearrangement creates a distinct peak at m/z 58.</li>
</ol>
An additional diagnostic feature in aldehydes is the M&#8722;1 peak, which arises due to the &#945;-cleavage of the aldehyde proton. In mass spectrometric analysis, this peak can help identify aldehydes, specifically among other carbonyl compounds.
Understanding these fragmentation pathways is crucial for interpreting mass spectra, particularly in identifying functional groups and distinguishing between similar compounds.

In mass spectrometry, the fragmentation of aliphatic aldehydes and ketones generally occurs through three key mechanisms: &#945;-cleavage, inductive ...

Mass Spectrometry: Alcohol Fragmentation

Alcohols (R-OH) ionize to lose one non-bonded electron from the oxygen atom, forming molecular ions. Due to their tendency to fragment rapidly, the intensity of the molecular ion peak in the mass spectrum is weak or sometimes absent. The fragmentation patterns for alcohols occur in two ways, i.e. &#9082;-cleavage and dehydration. During &#9082;-cleavage, the bond at the &#9082;-position adjacent to the hydroxyl group cleaves to give a resonance-stabilized cation and a radical. However, intramolecular dehydration involves losing a water molecule from an alcohol, forming an alkene cation, which shows a peak at M&#8722;18. Consider the fragmentation of butanol as shown below. &#9082;-cleavage produces a base peak at a mass-to-charge ratio of 31. In the case of dehydration, the fragmentation forms an alkene radical cation at a mass-to-charge ratio of 56.
<img alt="Figure1" src="/files/ftp_upload/13046/13046-PT_Figure_1.svg" style="text-align: center; height: 175px;" />

Alcohols (R-OH) ionize to lose one non-bonded electron from the oxygen atom, forming molecular ions. Due to their tendency to fragment rapidly, the ...

Mass Spectrometry: Alkyne Fragmentation

The fragmentation of alkynes preferentially occurs at the carbon&#8211;carbon bond between the &#945; and &#946; carbon of the alkyne bond to generate a 3-propynyl cation (or propargyl cation). In terminal alkynes, there is the only type of fragmentation that yields the 3-propynyl cation. The unsubstituted 3-propynyl cation exhibits a peak at a mass-to-charge ratio of 39. In internal alkynes, the 3-propynyl cation is substituted. For example, 2-pentyne fragments into methyl-substituted 3-propynyl cation, which subsequently features as a mass signal at the mass-to-charge ratio of 56. The 3-propynyl cation is resonance-stabilized, and the corresponding mass signal is intense in the mass spectrum of alkynes. Figure 1 shows the fragmentation of 1-pentyne into an ethyl radical and a resonance stabilized 3-propynyl cation.
<img alt="Figure1" src="/files/ftp_upload/13045/13045-PT_Figure_1.svg" style="text-align: center; height: 115px;" />
Figure 1: Fragmentation of 1-pentyne molecular ion to yield the 3-propynyl cation.
Another fragmentation pathway observed in terminal alkynes is the loss of hydrogen from the terminal sp carbon. This results in a mass signal with a mass-to-charge ratio of one unit less than the molecular ion. This M&#8211;1 peak is very intense and becomes the base peak in the mass spectra of many alkynes, so it can be used to identify alkynes.
<img alt="Figure2" src="/files/ftp_upload/13045/13045-PT_Figure_2.svg" style="text-align: center; height: 275px;" />
Figure 2: The mass spectrum of 1-pentyne.

The fragmentation of alkynes preferentially occurs at the carbon&#8211;carbon bond between the &#945; and &#946; carbon of the alkyne bond to generate a ...

Mass Spectrometry: Aromatic Compound Fragmentation

Upon ionization, aromatic compounds generate a molecular ion that is observed as a prominent peak in their mass spectra. For example, the molecular ion peak for benzene appears at a mass-to-charge ratio of 78, while toluene is observed at a mass-to-charge ratio of 92. The molecular ion benzene is highly stable and does not readily undergo further fragmentation due to the significant amount of energy required to disrupt the aromatic stability of the benzene ring. In contrast, the molecular ion of alkyl&#8208;substituted benzenes, such as toluene, can undergo fragmentation by losing a hydrogen atom, forming a benzylic carbocation. This carbocation can rearrange to form the resonance-stabilized tropylium cation, which appears in the mass spectra as a strong peak. In the case of toluene, this strong peak is observed at a mass-to-charge ratio of 91.

Upon ionization, aromatic compounds generate a molecular ion that is observed as a prominent peak in their mass spectra. For example, the molecular ion ...

Mass Spectrometry: Cycloalkene Fragmentation

The molecular ions of cycloalkenes undergo fragmentation via a retro-Diels&#8211;Alder reaction.

The reaction proceeds via the cleavage of two carbon-carbon bonds in the cycloalkene to yield ethene. The remaining part of the cycloalkene structure is a dienyl radical cation with a molecular weight of 28 u lower than the molecular ion. This fragmentation pathway is similar to the fragmentation of cycloalkanes releasing ethene, differing only in the resultant radical cation. An alkyl species forms from the cycloalkane, whereas a dienyl species originates from the cycloalkene.

Retro-Diels&#8211;Alder reactions of cycloalkenes are also similar to the McLafferty-rearrangement-type fragmentation observed in acyclic alkenes with hydrogen on the &#947; carbon. In both cases, cleavage occurs at two positions, yielding an ethene molecule and an alkenyl species. In acyclic alkenes, the cleavage occurs at one carbon-carbon bond and a carbon-hydrogen bond, along with rearranging hydrogen from one carbon to another. In cyclic alkenes, cleavage occurs at two carbon&#8211;carbon bonds without any atomic rearrangement. Figure 1 depicts the fragmentation of cyclohexene via a retro-Diels&#8211;Alder reaction.
<img alt="Figure1" src="/files/ftp_upload/13044/13044-PT_Figure_1.svg" style="text-align: center; height: 90px;" />
Figure 1: Fragmentation of cyclohexene via a retro-Diels&#8211;Alder reaction.
In addition to the retro-Diels&#8211;Alder reaction, branched cycloalkenes exhibit fragmentation via cleavage of the side chain to form a cycloalkene cation. For example, the fragmentation of 1-methyl-1-cyclohexene via a retro-Diels&#8211;Alder reaction and side-chain cleavage are shown in Figure 2.
<img alt="Figure2" src="/files/ftp_upload/13044/13044-PT_Figure_2.svg" style="text-align: center; height: 225px;" />
Figure 2: Fragmentation of 1-methyl-1-cyclohexene via (top) a retro-Diels&#8211;Alder reaction; (bottom) side-chain cleavage.
The corresponding signals of the isoprenyl radical cation and the cyclohexenyl cation are evident in the mass spectrum of 1-methyl-1-cyclohexene shown in Figure 3.
<img alt="Figure3" src="/files/ftp_upload/13044/13044-PT_Figure_3.svg" style="text-align: center; height: 250px;" />
Figure 3: Mass spectrum of 1-methyl-1-cyclohexene.

The molecular ions of cycloalkenes undergo fragmentation via a retro-Diels&#8211;Alder reaction.

The reaction proceeds via the cleavage of two ...

Mass Spectrometry: Amine Fragmentation

Amines can be identified using mass spectroscopy based on their characteristic fragmentation patterns. The molecular ions of amines undergo fragmentation via &#9082;-cleavage. The &#9082;-cleavage of the carbon-carbon bonds in amines generates an alkyl radical and resonance-stabilized nitrogen-containing cation.
In amines, the number of nitrogen atoms affects the mass of the molecular ion, which is described by the nitrogen rule of mass spectrometry. This rule states that a compound containing a single or an odd number of nitrogen atoms produces a molecular ion with an odd molecular weight, while a compound with no or an even number of nitrogen atoms generates a molecular ion with an even molecular weight.
For example, the mass spectrum of 3-methyl-1-butanamine, which contains one nitrogen atom, displays a molecular ion having an odd mass-to-charge ratio of 87. Fragmentation of this molecular ion via &#9082;-cleavage produces a base peak, which features a mass signal at a mass-to-charge ratio of 30, corresponding with the loss of the 2-methyl propyl radical.
Similarly, the fragmentation of triethylamine via &#9082;-cleavage generates a cation at a mass-to-charge ratio of 86. The molecular ion peak shows an odd mass-to-charge ratio of 101, as shown below.
<img alt="Figure1" src="/files/ftp_upload/13048/13048_PT_Figure_1.svg" style="text-align: center; height: 105px;" />

Amines can be identified using mass spectroscopy based on their characteristic fragmentation patterns. The molecular ions of amines undergo fragmentation ...

Mass Spectrometry: Long-Chain Alkane Fragmentation

The molecular ions of linear alkanes prefer to fragment at the carbon-carbon bond away from the end of the chain since the cleavage of an inner bond creates a stable carbocation and a stable radical. Consequently, the mass signals of linear alkanes feature intense peaks in the middle of the mass-to-charge ratio plot with weaker peaks on either end. The fragmentation of each carbon-carbon bond with the release of a methyl group in each splitting leads to prominent peaks in the mass spectra separated by 14 u. Smaller mass signals near each prominent peak result arise from secondary fragmentation at the carbon-hydrogen bond.
For example, consider the fragmentation pattern of n-hexane shown in Figure 1 and the corresponding mass spectra in Figure 2.
<img alt="Figure1" src="/files/ftp_upload/12299/12299-PT_Figure_1.svg" style="text-align: center; height: 500px;" />
Figure 1: Fragmentation of n-hexane molecular ion.
<img alt="Figure2" src="/files/ftp_upload/12299/12299-PT_Figure_2.svg" style="text-align: center; height: 275px;" />
Figure 2: Mass spectrum of n-hexane.
The signals at a mass-to-charge ratio of 71, 57, 43, 29, and 15 indicate that the cleavage can occur at all carbon-carbon bonds in the molecule. However, the relative abundance of these peaks varies. The difference between molecular ion signal and signal at 71 indicates the separation of methyl groups (M.W. = 15 u) group from molecular ions. Further release of a CH2 group from that fragment (M.W. = 14 u) results in a signal at 57. The signals at 43, 29, and 15 are due to the subsequent releases of CH2 groups. The abundance of signals at 15 and 71 is minimal due to the least probability for the formation of unstable methyl carbocation or a methyl radical, respectively. The base peak at 43 indicates the most preferred fragmentation, which yields a pair of stable carbocation and radical.

The molecular ions of linear alkanes prefer to fragment at the carbon-carbon bond away from the end of the chain since the cleavage of an inner bond ...

An Overview of Genetic Analysis

An organism&#8217;s physical traits, or phenotype, are a product of its genotype, which is the combination of alleles (gene variants) inherited from its parents. To varying degrees, genes interact with each other and environmental factors to generate traits. The distribution of alleles and traits within a population is influenced by a number of factors, including natural selection, migration, and random genetic drift.In this video, JoVE introduces some of the foundational discoveries in genetics, from Gregor Mendel&#8217;s elucidation of the genetic basis of inheritance, to how natural processes affect allele distributions within populations, to the modern synthesis of biology that brought together Mendelian genetics and Darwinian evolution. We then review the questions asked by geneticists today regarding how genes influence traits, and some of the main tools used to answer these questions. Finally, several applications of techniques such as genetic crosses, screens and evolution experiments will be presented.

Genetic Analyses, Genetic Crosses and Screens

Una panoramica dell'analisi genetica

An organism&#8217;s physical traits, or phenotype, are a product of its genotype, which is the combination of alleles (gene variants) inherited from its ...

Genetic Crosses

To dissect genetic processes or create organisms with novel suites of traits, scientists can perform genetic crosses, or the purposeful mating of two organisms. The recombination of parental genetic material in the offspring allows researchers to deduce the functions, interactions, and locations of genes.
This video will examine how genetic crosses were influential in developing Mendel's three laws of inheritance, which form the basis of our understanding of genetics. One genetic crossing technique that was first developed for single-celled organisms such as yeast, known as tetrad analysis, will then be presented in detail, followed by some examples of how this classical tool is used in genetic studies today.

Mendel&#039;s Laws, Genetic Crosses and Tetrad Analysis

Incroci genetici

To dissect genetic processes or create organisms with novel suites of traits, scientists can perform genetic crosses, or the purposeful mating of two ...

Genetic Screens

Genetic screens are critical tools for defining gene function and understanding gene interactions. Screens typically involve mutating genes and then assessing the affected organisms for phenotypes of interest. The process can be &#8220;forward&#8221;, where mutations are generated randomly to identify unknown genes responsible for the phenotypes, or it can be &#8220;reverse&#8221;, where specific genes are targeted for mutation to observe what phenotypes are produced.Here, JoVE reviews various types of genetic screens, including those that depend on either loss-of-function or gain-of-function mutations, which respectively decrease or increase the activity of genes. We then explore general protocols for forward and reverse screens in a popular model organism, the nematode worm. Finally, we highlight how screens are applied in research today, for example to better understand gene interactions that may contribute to neurodegenerative diseases.

Forward and Reverse Genetic Screens

Screening genetici

Genetic screens are critical tools for defining gene function and understanding gene interactions. Screens typically involve mutating genes and then ...

An Overview of Genetics and Disease

Many human diseases are associated with mutations or variations in genetic sequences. Some of these genetic variants are heritable, passed down from generation to generation, while others arise sporadically during an organism&#8217;s life and cause diseases such as cancer. Researchers are trying to identify and characterize these genetic alterations in the hopes of improving diagnosis and therapeutic options for patients.In this video, we will examine the history of genetic disease research, and explore key questions asked by medical geneticists. Various tools used to identify the genetic basis of diseases are then discussed, including genotyping techniques and genome-wide association studies (GWAS). Finally, several current examples of medical genetics research are presented.

Medical Genetics Research: History and Modern Techniques

Una panoramica di genetica e malattia

Many human diseases are associated with mutations or variations in genetic sequences. Some of these genetic variants are heritable, passed down from ...

SNP Genotyping

Single nucleotide polymorphisms, or SNPs, are the most common form of genetic variation in humans. These differences at individual bases in the DNA often do not directly affect gene expression, but in many cases can still be useful for locating disease-associated genes or for diagnosing patients. Numerous methodologies have been established to identify, or &#8220;genotype&#8221;, SNPs.JoVE&#8217;s introduction to SNP Genotyping begins by discussing what SNPs are and how they can be used to identify disease-associated genes. Several common SNP genotyping methods are then examined, including direct hybridization, PCR-based methods, fragment analysis, and sequencing. Finally, we present several examples of how these techniques are applied to genetic research today.

SNP Genotyping and GWAS

Genotipizzazione SNP

Single nucleotide polymorphisms, or SNPs, are the most common form of genetic variation in humans. These differences at individual bases in the DNA often ...

Cytogenetics

Cytogenetics is the field of study devoted to chromosomes, and involves the direct observation of a cell&#8217;s chromosomal number and structure, together known as its karyotype. Many chromosomal abnormalities are associated with disease. Each chromosome in a karyotype can be stained with a variety of dyes to give unique banding patterns. More recent techniques, including comparative genomic hybridization and fluorescence in situ hybridization (FISH), allow for detecting specific chromosomal features or abnormalities.This video will begin by examining the principles of these classical and modern cytogenetics techniques. This is followed by an examination of a general protocol for performing FISH. Finally, several examples of how karyotyping can be applied to various medical applications are presented.

Cytogenetics, Karyotyping and FISH

Citogenetica

Cytogenetics is the field of study devoted to chromosomes, and involves the direct observation of a cell&#8217;s chromosomal number and structure, ...

An Overview of Gene Expression

Gene expression is the complex process where a cell uses its genetic information to make functional products. This process is regulated at multiple stages, and any misregulation could lead to diseases such as cancer. This video highlights important historical discoveries relating to gene expression, including the understanding of how distinct combinations of DNA bases encode the amino acids that make up proteins. Key questions in the field of gene expression research are explored, followed by a discussion of several techniques used to measure gene expression and investigate its regulation. Finally, we look at how scientists are currently using these techniques to study gene expression.

Gene Expression Research: Concepts and Techniques

Una panoramica dell'espressione genica

Gene expression is the complex process where a cell uses its genetic information to make functional products. This process is regulated at multiple ...

Expression Profiling with Microarrays

Microarrays are important tools for profiling gene expression, and are based on complementary binding between probes that are attached to glass chips and nucleic acids derived from samples. Using these arrays, scientists can simultaneously evaluate the expression of thousands of genes. In addition, the expression profiles of different cells or tissue types can be compared, allowing researchers to deduce how the expression of different genes change during biological processes, and thus gain insight into how the genes may function in pathways or networks.Here, JoVE explains the principles behind microarrays. This is followed by a general protocol for performing a microarray experiment, and a brief introduction to analyzing microarray data. We end on a discussion of how scientists are currently using microarrays, for example to compare gene expression between different cell types derived from cancerous and non-cancerous tissues, to study important biological problems.

Profilatura delle espressioni con Microarray

Microarrays are important tools for profiling gene expression, and are based on complementary binding between probes that are attached to glass chips and ...

Rapid Amplification of cDNA Ends

Source: Pablo Sanchez Bosch2, Sean Corcoran2 and Katja Br&#252;ckner1,2,3 
1Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research 
2Department of Cell and Tissue Biology,&#160; 
3Cardiovascular Research Institute, University of California San Francisco, San Francisco, CA, USA&#160; &#160;&#160;
Rapid Amplification of cDNA Ends (RACE) is a technique that allows amplification of full-length cDNA from mRNA by extending to the 3&#8217; or 5&#8217; end, even without prior knowledge of the sequence (Frohman et al., 1988). In contrast to regular PCR, it uses only one specific PCR primer and a second non-specific primer that will indiscriminately bind to most mRNAs. Depending on whether the area to be amplified is on the 3&#8217; or 5&#8217; end of the mRNA, the second primer is chosen to bind the polyA tail (3&#8217; end) or a synthetic linker added to all transcripts (5&#8217; end). These primer combinations are known to yield &#8220;one-sided&#8221; or &#8220;anchored&#8221; PCR due to PCR amplification using the known sequence of one side- 3&#8217; or 5&#8217;(Ohara et al., 1989). The approach allows the capture of gene-specific rare mRNAs that otherwise would be hard to detect, e.g. because of their relatively low expression or unknown complete sequence (Frohman et al., 1988).
RACE is initiated with a reverse transcription step (RT) to synthetize single stranded complimentary DNA (cDNA) from mRNA. This is followed by two consecutive PCRs that amplify the cDNA fragments from a gene of interest. To perform RACE, the sequence of the gene of interest must be at least partially known, as it is needed to design the gene-specific primers (GSPs; Frohman, 1994; Liu and Gorovsky, 1993). As the second primer pair is a generic primer that will anneal to all transcripts present in the sample, the specificity of the PCR reactions is reduced. RACE is therefore ideally performed with two nested PCRs to lower the chances of amplifying non-specific transcripts (Figure 1). Depending on the direction of amplification relative to the GSP, the technique is categorized as 5&#8217; or 3&#8217; RACE (Figure 1).&#160;
3&#8217; RACE (Figure 1A) takes advantage of the poly(A) tail present in mRNAs as a generic binding site for the non-specific primer. This simplifies the technique, as one can synthetize cDNA by using oligo (dT) primers. GSPs are located in the 5&#8217; region of the transcripts. This RACE variant allows the detection of transcript variants and different 3&#8217; UTRs (Scotto Lavino et al., 2007a). In 5&#8217; RACE (Figure 1B), GSPs are located in the 3&#8217; region of the gene. To be able to use a non-specific primer that binds to all transcripts, an adapter that serves as generic primer binding site is attached to the 5&#8217; RNA ends. To attach the adapter to the mRNA, the 5&#8217; cap that protects mRNAs against exonucleases and promotes translation must be removed (Bird et al., 2016). Opposed to 3&#8217; RACE, 5&#8217; RACE helps finding differential 5&#8217; splice variants and alternative 5&#8217; UTRs (Scotto Lavino et al., 2007b).
Here we will perform 3&#8217; RACE to identify and isolate different transcript variants using the known 5&#8217; sequence (Figure 2A) encoded by the Drosophila dSmad2 (Smox) gene. dSmad2, a fly Smad protein, is an important transducer of Activin-&#946; signaling, a pathway of the TGF-&#946; superfamily. dSmad2 regulates multiple cellular processes, such as cell proliferation, apoptosis and differentiation (Upadhyay et al., 2017). As differentially spliced transcripts of dSmad2 may have different functions in the adult fly, a first step in exploring these potential functions is to assess all transcript variants of dSmad2 at this developmental stage.

Amplificazione rapida delle estremità del cDNA

Source: Pablo Sanchez Bosch2, Sean Corcoran2 and Katja Br&#252;ckner1,2,3
1Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research
...

RNA-Seq

Among different methods to evaluate gene expression, the high-throughput sequencing of RNA, or RNA-seq. is particularly attractive, as it can be performed and analyzed without relying on prior available genomic information. During RNA-seq, RNA isolated from samples of interest is used to generate a DNA library, which is then amplified and sequenced. Ultimately, RNA-seq can determine which genes are expressed, the levels of their expression, and the presence of any previously unknown transcripts.Here, JoVE presents the basic principles behind RNA-seq. We then discuss the experimental and analytical steps of a general RNA-seq protocol. Finally, we examine how researchers are currently using RNA-seq, for example, to compare gene expression between different biological samples, or to characterize protein-RNA interactions.

Next-Gen Transcriptomics Using RNA-Seq

Sequenziamento dell'RNA (RNA-Seq)

Among different methods to evaluate gene expression, the high-throughput sequencing of RNA, or RNA-seq. is particularly attractive, as it can be performed ...

An Overview of Epigenetics

Since the early days of genetics research, scientists have noted certain heritable phenotypic differences that are not due to differences in the nucleotide sequence of DNA. Current evidence suggests that these &#8220;epigenetic&#8221; phenomena might be controlled by a number of mechanisms, including the modification of DNA cytosine bases with methyl groups, the addition of various chemical groups to histone proteins, and the recruitment of protein factors to specific DNA sites via interactions with non-protein-coding RNAs.In this video, JoVE presents the history of important discoveries in epigenetics, such as X-chromosome inactivation (XCI), the phenomenon where an entire X-chromosome is silenced in the cells of female mammals. Key questions and methods in the field are reviewed, including techniques to identify DNA sequences associated with different epigenetic modifications. Finally, we discuss how researchers are currently using these techniques to better understand the epigenetic regulation of gene function.

Epigenetics, lncRNAs and Epigenetic Code

Una panoramica dell'epigenetica

Since the early days of genetics research, scientists have noted certain heritable phenotypic differences that are not due to differences in the ...

DNA Methylation Analysis

Methylation at CpG dinucleotides is a chemical modification of DNA hypothesized to play important roles in regulating gene expression. In particular, the methylation of clusters of methylation sites, called &#8220;CpG islands&#8221;, near promoters and other gene regulatory elements may contribute to the stable silencing of genes, for example, during epigenetic processes such as genomic imprinting and X-chromosome inactivation. At the same time, aberrant CpG methylation has been shown to be associated with cancer.In this video, the biological functions and mechanisms of DNA methylation will be presented, along with various techniques used to identify methylation sites in the genome. We will then examine the steps of bisulfite analysis, one of the most commonly used methods for detecting DNA methylation, as well as several applications of this technique.

Analisi di metilazione del DNA

Methylation at CpG dinucleotides is a chemical modification of DNA hypothesized to play important roles in regulating gene expression. In particular, the ...

Chromatin Immunoprecipitation

Histones are proteins that help organize DNA in eukaryotic nuclei by serving as &#8220;scaffolds&#8221; around which DNA can be wrapped, forming a complex called &#8220;chromatin&#8221;. These proteins can be modified through the addition of chemical groups, and these changes affect gene expression. Researchers use a technique called chromatin immunoprecipitation (ChIP) to better understand which DNA regions associate with specific histone modifications or other gene regulatory proteins. Antibodies are used to isolate the protein of interest, and the bound DNA is extracted for analysis. Here, JoVE presents the principles behind ChIP, discussing specific histone modifications and their relationship to gene expression and DNA organization. We then review how to perform a ChIP protocol, and explore the ways scientists are currently using this technique.

Immunoprecipitazione della cromatina

Histones are proteins that help organize DNA in eukaryotic nuclei by serving as &#8220;scaffolds&#8221; around which DNA can be wrapped, forming a complex ...

An Overview of Genetic Engineering

Genetic engineering &#8211; the process of purposefully altering an organism&#8217;s DNA &#8211; has been used to create powerful research tools and model organisms, and has also seen many agricultural applications. However, in order to engineer traits to tackle complex agricultural problems such as stress tolerance, or to realize the promise of gene therapy for treating human diseases, further advances in the field are still needed. Important considerations include the safe and efficient delivery of genetic constructs into cells or organisms, and the establishment of the desired modification in an organism&#8217;s genome with the least &#8220;off-target&#8221; effects. JoVE&#8217;s Overview of Genetic Engineering will present a history of the field, highlighting the discoveries that confirmed DNA as the genetic material and led to the development of tools to modify DNA. Key questions that must be answered in order to improve the process of genetic engineering will then be introduced, along with various tools used by genetic engineers. Finally, we will survey several applications demonstrating the types of experimental questions and strategies in the field today.

Genetic Engineering and Recombinant DNA Technology

Una panoramica dell'ingegneria genetica

Genetic engineering &#8211; the process of purposefully altering an organism&#8217;s DNA &#8211; has been used to create powerful research tools and model ...

Recombineering and Gene Targeting

One of the most widely used tools in modern biology is molecular cloning with restriction enzymes, which create compatible ends between DNA fragments that allow them to be joined together. However, this technique has certain restrictions that limit its applicability for large or complex DNA construct generation. A newer technique that addresses some of these shortcomings is recombineering, which modifies DNA using homologous recombination (HR), the exchange between different DNA molecules based on stretches of similar or identical sequences. Together with gene targeting, which takes advantage of endogenous HR to alter an organism&#8217;s genome at a specific loci, HR-based cloning techniques have greatly improved the speed and efficacy of high-throughput genetic engineering.In this video, we introduce the principles of HR, as well as the basic components required to perform a recombineering experiment, including recombination-competent organisms and genomic libraries such as bacterial artificial chromosomes (BAC). We then walk through a protocol that uses recombineering to generate a gene-targeting vector that can ultimately be transfected into embryonic stem cells to generate a transgenic animal. Finally, several applications that highlight the utility and variety of recombineering techniques will be presented.

Recombineering, Gene Targeting and BACs

Ricombinazione e gene targeting

One of the most widely used tools in modern biology is molecular cloning with restriction enzymes, which create compatible ends between DNA fragments that ...

Genome Editing

A well-established technique for modifying specific sequences in the genome is gene targeting by homologous recombination, but this method can be laborious and only works in certain organisms. Recent advances have led to the development of &#8220;genome editing&#8221;, which works by inducing double-strand breaks in DNA using engineered nuclease enzymes guided to target genomic sites by either proteins or RNAs that recognize specific sequences. When a cell attempts to repair this damage, mutations can be introduced into the targeted DNA region. In this video, JoVE explains the principles behind genome editing, emphasizing how this technique relates to DNA repair mechanisms. Then, three major genome editing methods&#8212;zinc finger nucleases, TALENs, and the CRISPR-Cas9 system&#8212;are reviewed, followed by a protocol for using CRISPR to create targeted genetic changes in mammalian cells. Finally, we discuss some current research that applies genome editing to alter the genetic material in model organisms or cultured cells.

Education Genetics

Science Education (Biologia avanzata)

Science Education (Biologia avanzata)

Science Education (Biologia avanzata)

Science Education (Biologia avanzata)

Science Education (Biologia avanzata)

Science Education (Biologia avanzata)

Science Education (Biologia avanzata)

Science Education (Biologia avanzata)

Science Education (Biologia avanzata)

Science Education (Biologia avanzata)

Science Education (Biologia avanzata)

Science Education (Biologia avanzata)

Science Education (Biologia avanzata)

Science Education (Biologia avanzata)

Science Education (Biologia avanzata)

Science Education (Biologia avanzata)