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Chapter 20: Problem 12

Why are high rates of mutation observed in regions of DNA that contain methylcytosine?

Short Answer

Expert verified
High rates of mutation are observed in regions of DNA that contain methylcytosine because of its inherent chemical instability, which makes it prone to deamination. The resulting thymine, a normal DNA base, is not recognized as an error by DNA's repair machinery during replication, leading to a higher mutation rate.

Step by step solution

01

Understand methylation process in DNA

Methylation in DNA involves the addition of a methyl group to the cytosine ring, resulting in methylcytosine. This process is often involved in the regulation of gene expression. In humans and other mammals, this frequently occurs at CpG sites, where a cytosine is followed by a guanine.
02

Explain the role of methylcytosine in mutation

Methylcytosine is chemically unstable and can deaminate, or lose an amino group, to become thymine. While the cell's proofreading machinery typically detects and corrects most of these errors during DNA replication, some still slip through. This is one of the reasons why methylated regions, or CpG islands, often have a higher mutation rate compared to other regions.
03

Link high mutation rate and methylcytosine

Because methylcytosine is prone to deamination and the resulting thymine is a normal DNA base, DNA replication machinery cannot recognise this as a mutation and repair it. Therefore, the resulting mismatch during replication leads to a permanent transition mutation from CG to TA pair. Consequently, regions of the DNA with high methylcytosine content would show higher mutation rates, resulting in genetic variation.

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Most popular questions from this chapter

The chromosome of a certain bacterium is a circular, double-stranded DNA molecule of \(5.2 \times 10^{6}\) base pairs. The chromosome contains one origin of replication, and the rate of replication-fork movement is 1000 nucleotides per second. (a) Calculate the time required to replicate the chromosome. (b) Explain how the bacterial generation time can be as short as 25 minutes under extremely favorable conditions.

Describe two methods that can be used to repair pyrimidine dimers in \(E\). coli.

Damage to a single strand of DNA is readily repaired through a variety of mechanisms, while damage to bases on both strands of DNA is more difficult for the cell to repair. Explain.

Why does homologous recombination occur only between DNAs with identical, or almost identical, sequences?

E. coli uses several mechanisms to prevent the incorporation of the base uracil into DNA. First, the enzyme dUTPase, encoded by the dut gene, degrades dUTP. Second, the enzyme uracil \(N\)-glycosylase, encoded by the ung gene, removes uracils that have found their way into DNA. The resulting apyrimidinic sites have to be repaired. (a) If we examine the DNA from a strain carrying a mutation in the dut gene, what will we find? (b) What if we examine the DNA from a strain in which both the dut and ung genes are mutated?
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