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Apologies in advance for asking bit naive question. I have looked up for this concept a lot but didn't find anything. The wikipedia (I know its not authentic all the time, but I think works well for definitions atleast) page on Complementarity says:

The degree of complementarity between two nucleic acid strands may vary, from complete complementarity (each nucleotide is across from its opposite) to no complementary (each nucleotide is not across from its opposite) and determines the stability of the sequences to be together

This is quite new concept for me and how come we can have basepairing between opposite nucleotides if there isn't 100% complementarity? Any evidences behind it?

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This can happen when DNA separates after melting and then reanneals.

You likely will not see this in chromosomal DNA in the cell, but I would assume it can happen to RNA molecules sometimes as they are single stranded and can self-anneal.

Mostly though, we think about this issue when performing PCR or DNA/RNA hybridization assays. Shorter pieces of DNA will be more likely to bind and anneal incorrectly, especially if the DNA is rapidly cooled after it has been melted.

Rapid cooling does not give the DNA time to "test" the different combinations of complementarity and sometimes can get stuck in a place where there is very little complementarity.

You also need to remember that while A-T G-C hydrogen bonding is favored, bases are able hydrogen bond to other bases, it just makes the double stranded molecule less stable.

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