What percent of nucleotides in a double stranded DNA molecule are cytosine and thymine? On an exam my professor claimed that it's 50% but I believe that it varies ... Please clarify this and cite your source. He obviously has poorly worded and vague questions.

There are 4 nucleotides that make up DNA and neither one is preferred so As and Gs make up 50% and Cs and Ts make up 50% of the population of DNA nucleotides (i.e each nucleotide makes up 25% of the population). There isn't much to source, it's basic maths.

Last edited by ReetikaSuri (2nd Feb 2016 10:44:19)

Yes, for humans the amount of purine bases equals the amount of pyrimidine bases, so A + G (or C + T) is about 50%. As far as I am aware there is some overall nucleotide bias in a variety of genomes, where e.g., there are high (greater than 50%) and low G/C content bacteria, and the human genome is less than 50% G/C. It is not necessarily a 1:1:1:1 ratio of A:C:G:T (or 1:1 for A:G; or 1:1 for C:T).

[for a basic reference see Table 2.2, p43 “Genetics: Analysis of Genes and Genomes 7th Ed, Hartl & Jones, 2009]

Last edited by Steve Lolait (2nd Feb 2016 14:51:17)