Question

The DNA of many organisms contains a substantial fraction of modified or unusual bases. In human DNA, for example, approximately 5% of cytosine residues are modified to 5-methylcytosine after DNA replication. Human DNA contains 31% thymine.

What percent of human DNA is 5-methylcytosine?

Answer 1

Genes consist of specific sequences of DNA (deoxyribonucleic acid) nucleotides otherwise called “bases,” which carry the message encoding for the synthesis of a protein. There are four such bases, namely, adenine (A), thymine (T), guanine (G), and cytosine (C), which are the building blocks of DNA’s double helical structure.

Chargaff proposed that, in a double stranded DNA, the ratio of purines to pyrimidines is always 1: 1.

Thus, if the percent of thymine is 31%, the percent of adenine is also 31%. The remaining 38% consists of guanine and cytosine (each 19%). Thus, the percent of cytosine is 19%.

It is given that, about 19% of cytosine is modified to 5-methylcytosine , means 0.95% of human DNA 5- methylcytosine.