Ch. 10 - DNA Structure and Analysis

Klug - Concepts of Genetics 12th Edition

Klug12th EditionConcepts of Genetics ISBN: 9780135564776당신이 사용하는 게 아니라요?교과서 변경

Klug 12th Edition

Ch. 10 - DNA Structure and Analysis

문제 33

10장, 문제 33

During gel electrophoresis, DNA molecules can easily be separated according to size because all DNA molecules have the same charge-to-mass ratio and the same shape (long rod). Would you expect RNA molecules to behave in the same manner as DNA during gel electrophoresis? Why or why not?

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Recall that gel electrophoresis separates molecules primarily based on size because the molecules move through a gel matrix under an electric field, and their migration depends on their charge-to-mass ratio and shape.

Understand that DNA molecules have a uniform charge-to-mass ratio due to their consistent phosphate backbone and a relatively uniform shape (long rods), which allows size to be the main factor affecting their movement through the gel.

Consider the structure of RNA molecules: although RNA also has a phosphate backbone giving it a negative charge, RNA molecules often fold into complex secondary and tertiary structures, unlike the relatively uniform shape of DNA.

Because RNA molecules can have varied shapes and folding patterns, their effective size and shape during electrophoresis can differ, which affects their migration through the gel differently than DNA.

Therefore, RNA molecules do not behave exactly like DNA during gel electrophoresis because their variable shapes and structures influence their movement, making size separation less straightforward compared to DNA.

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Charge-to-Mass Ratio of Nucleic Acids

DNA molecules have a uniform negative charge due to their phosphate backbone, resulting in a consistent charge-to-mass ratio. This uniformity allows DNA fragments to separate primarily based on size during gel electrophoresis. RNA also has a phosphate backbone, but structural differences can affect its charge distribution.

Molecular Structure and Shape of DNA vs. RNA

DNA typically exists as a long, double-stranded helix with a relatively uniform rod-like shape, facilitating predictable migration in gels. RNA molecules are often single-stranded and can fold into complex secondary and tertiary structures, which influence their shape and mobility during electrophoresis.

Effect of Secondary Structure on Electrophoretic Mobility

RNA's ability to form intramolecular base pairing creates secondary structures like hairpins, altering its effective size and shape. These structures can slow or irregularly affect RNA migration in gels, making its separation less dependent solely on length compared to DNA.

관련 실천

교과서 질문

Electrophoresis is an extremely useful procedure when applied to analysis of nucleic acids as it can resolve molecules of different sizes with relative ease and accuracy. Large molecules migrate more slowly than small molecules in agarose gels. However, the fact that nucleic acids of the same length may exist in a variety of conformations can often complicate the interpretation of electrophoretic separations. For instance, when a single species of a bacterial plasmid is isolated from cells, the individual plasmids may exist in three forms (depending on the genotype of their host and conditions of isolation): superhelical/supercoiled (form I), nicked/open circle (form II), and linear (form III). Form I is compact and very tightly coiled, with both DNA strands continuous. Form II exists as a loose circle because one of the two DNA strands has been broken, thus releasing the supercoil. All three have the same mass, but each will migrate at a different rate through a gel. Based on your understanding of gel composition and DNA migration, predict the relative rates of migration of the three DNA structures mentioned above.

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교과서 질문

Newsdate: March 1, 2030. A unique creature has been discovered during exploration of outer space. Recently, its genetic material has been isolated and analyzed. This material is similar in some ways to DNA in its chemical makeup. It contains in abundance the 4-carbon sugar erythrose and a molar equivalent of phosphate groups. In addition, it contains six nitrogenous bases: adenine (A), guanine (G), thymine (T), cytosine (C), hypoxanthine (H), and xanthine (X). These bases exist in the following relative proportions:

A =T = H and C = G = X

X-ray diffraction studies have established a regularity in the molecule and a constant diameter of about 30 Å. Together, these data have suggested a model for the structure of this molecule.

Given the constant diameter of 30 Å, do you think that either (i) both H and X are purines or both pyrimidines, or (ii) one is a purine and one is a pyrimidine?

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교과서 질문

A =T = H and C = G = X

X-ray diffraction studies have established a regularity in the molecule and a constant diameter of about 30 Å. Together, these data have suggested a model for the structure of this molecule.

Propose a general model of this molecule. Describe it briefly.

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교과서 질문

A =T = H and C = G = X

X-ray diffraction studies have established a regularity in the molecule and a constant diameter of about 30 Å. Together, these data have suggested a model for the structure of this molecule.

What base-pairing properties must exist for H and for X in the model?

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교과서 질문

DNA and RNA are chemically very similar but are distinguished, in large part, by the presence of a 2'-OH group in RNA and a 2'-H group in DNA. Why do you suppose that both DNA and RNA have 3'-OH groups and we do not typically find nucleic acids within cells that have 3'-H groups?

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