From ScienceDaily:
To make the iconic, twisted double helix that accounts for the diversity of life, DNA rules specify that G always pairs with C, and A with T.
But, when it’s all added up, the amount of G+C vs A+T content among species is not a simple fixed percentage or, standard one-to-one ratio.
For example, within single-celled organisms, the amount of G+C content can vary from 72 percent in a bacteria like Streptomyces coelicolor while the protozoan parasite that causes malaria, Plasmondium falciparum, has as little as 20 percent.
In single-celled eukaryotes, yeast contain 38 percent G+C content, plants like corn have 47 percent, and humans contain about 41 percent.
The big question is, why?
…
Now that they’ve shown the G+C composition correlation, it has opened up the door to many more questions, and answers that remain elusive.
“One question is, ‘why does the mutation spectrum change so dramatically across species'”? asked Lynch. “Species don’t have the same mutation spectrum. There are species whose mutation profiles are more AT rich and others more GC rich. We still don’t know the mechanisms behind such divergence in the mutational spectrum.”
They may be due to simple differences in chemistry and biophysics. One general force that may be of relevance is DNA stability, driven by the chemistry of the DNA letters. The forces that keep the DNA ladder intact are called hydrogen bonds. G:C pairs involve three hydrogen bonds, whereas, A:T pairs involve only two.
“The prevailing thought is that more G:C content adds to genome stability,” said Lynch.
Another possibility is during reproduction, when the DNA strands intertwine from each parent to make a fertilized egg, mismatches can occur in the base pairing, leading to mistakes that DNA proofreading enzymes have to fix later on. Sometimes, a G can get changed to an A, or a T becomes a C, converting genes during this mismatch repair process.
“That’s generally thought to be biased towards Gs and Cs,” said Lynch. Paper. (paywall) – Hongan Long, Way Sung, Sibel Kucukyildirim, Emily Williams, Samuel F. Miller, Wanfeng Guo, Caitlyn Patterson, Colin Gregory, Chloe Strauss, Casey Stone, Cécile Berne, David Kysela, William R. Shoemaker, Mario E. Muscarella, Haiwei Luo, Jay T. Lennon, Yves V. Brun, Michael Lynch. Evolutionary determinants of genome-wide nucleotide composition. Nature Ecology & Evolution, 2018; DOI: 10.1038/s41559-017-0425-y More.
Som of us are putting our money on physics-related stability rather than random mutations as the model.
See also: The “deteriorating” Y chromosome features new genes
Jumping’ Genes!: A quarter of cow DNA came from reptiles?
and
Why the genome must be a product of intelligent design