Point mutations are divided into two categories: transitions and transversions. Since point mutations only occur during DNA replication (an integral process of both mitosis and meiosis), the mutated nitrogenous bases only appear on the newly synthesized DNA strands, rather than on the parental strands that serve as templates during DNA replication.

A transition occurs when a would-be purine is replaced by another purine, or a would-be pyrimidine is replaced by another pyrimidine. In contrast, a transversion occurs when a would-be purine is replaced by a pyrimidine, or vice versa. Because the two strands of a DNA molecule always follow the strict rule of complementary base-pairing, there must be some impact on chemical structures of the DNA molecule when either a transition or a transversion occurs.

Specifically, under normal circumstances, an adenine forms two hydrogen bonds with its complementary thymine, and a cytosine forms three hydrogen bonds with its complementary guanine. When a transition occurs, such as when a would-be thymine is replaced by a cytosine, would hydrogen bonds still form between the two bases? And how many hydrogen bonds will there be? Same questions for when a transversion occurs, such as when a would-be thymine is replaced by a guanine.

Another relevant point to ask is the distance between the two DNA strands that constitute a DNA molecule. The rule of complementary base-pairing helps to maintain equal widths for each complementary base pair. Thus, an A-T pair and a C-G pair are of the same width. But when a transition occurs, would the new A-C combination have a different width? Same question for when a tranversion occurs.

Thanks to anyone who answers my questions.

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    $\begingroup$ Although this is certainly in the realm of chemistry I think that it can better fit in Biology SE. They can already know the answer without much drawing and thinking of molecular models. $\endgroup$
    – Alchimista
    Feb 15, 2021 at 10:11
  • $\begingroup$ Here is a paper doi.org/10.1093/nar/gkv254 $\endgroup$
    – Karsten
    Feb 19, 2021 at 11:55

1 Answer 1


When Point mutations occur in one strand of DNA, irregular base-pair emerge. One of the main features of this irregular state is that the width of the double-strand differs from the standard form(usually larger width as the new, irregular base pairings are weak) in that specific area. In other words, the portion of DNA which has the defective base-pair bulges out or protrudes leading to the asymmetry of DNA.(analogous but not the same as Thymine dimers) Most of the times the DNA repair system finds this and deals with it in the correct way. I do not know about the exact number of hydrogen bonds that appear in these situations but perhaps it obeys either Watson-Crick, Hoogsteen, or Wobble rules.


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