You measure the concentration of DNA in a preparation of nucleic acid using a di

Question

You measure the concentration of DNA in a preparation of nucleic acid using a diphenylamine assay. You also measure the absorbance of the preparation at 260 nm. Another assay indicates that there is no RNA contaminating the preparation. Two days later, you continue your study by measuring absorbance of the preparation again. The absorbance has increased significantly. A quick check reveals that the DNA concentration has not changed. There is no microbial contamination in the preparation that may account for this seeming oddity. What has happened?

You isolate some DNA and monitor its dimensions. This particular DNA has a greater number of base pairs per turn of the helix than is normally the case. What is a possible explanation?

Why DNA has Thymine instead of Uracil as its pyrimidine base?

Explanation / Answer

Ans: DNA has Thymine instead of Uracil as its pyrimidine base because the presence in DNA of thymine rather than uracil was an enigma for many years. Both bases pair with adenine. The only difference between them is a methyl group in thymine in place of the C-5 hydrogen atom in uracil.The existence of an active repair system to correct the deamination of cytosine provides a convincing solution to this puzzle. Cytosine in DNA spontaneously deaminates at a perceptible rate to form uracil. The deamination of cytosine is potentially mutagenic because uracil pairs with adenine, and so one of the daughter strands will contain an U-A base pair rather than the original C-G base pair. This mutation is prevented by a repair system that recognizes uracil to be foreign to DNA. This enzyme, uracil DNA glycosylase, is homologous to AlkA. The enzyme hydrolyzes the glycosidic bond between the uracil and deoxyribose moieties but does not attack thymine-containing nucleotides. The AP site generated is repaired to reinsert cytosine. Thus, the methyl group on thymine is a tag that distinguishes thymine from deaminated cytosine. If thymine were not used in DNA, uracil correctly in place would be indistinguishable from uracil formed by deamination. The defect would persist unnoticed, and so a C-G base pair would necessarily be mutated to U-A in one of the daughter DNA molecules. This mutation is prevented by a repair system that searches for uracil and leaves thymine alone. Thymine is used instead of uracil in DNA to enhance the fidelity of the genetic message. In contrast, RNA is not repaired, and so uracil is used in RNA because it is a less-expensive building block.

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