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From what I know only the base pairs A and T/U, or G and C can occur due to hydrogen bonding. So if a base substitution mutation occurs, say an A is replaced by a C on the strand, does it affect only one of the strands on the DNA or does it changes the base pair entirely? Thanks.

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Consider a stretch of DNA

A T C C T C
T A G G A G

Let's say that during replication a nucleotide get switched

A G C C T C
T A G G A G

Now, this does not work. The cellular machinery will find this location and try to solve it. It will either resolve it as

A T C C T C
T A G G A G

and no mutation would have happened. Or it will solve it as

A G C C T C
T C G G A G

and a mutation just happened.

You might want to read about DNA repair mechanisms.

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A base substitution mutation often occur during replication of DNA. Remember that the template strand is methylated, in order to understand who will be the new synthetized strand between both.

Having this scenario, with a nucleotide substitution mutation in the 3rd position from 5' , the 5'-3' strand is the fresh synthetized.

The DNA polymerase will trait just the 5'-3' strand as something to work on : if a mutation happens the proof-reading activity of the DNA polymerase will try to cut off the wrong base, and to re-synthetize this piece of the strand.

5'- A G G T T C G -3' --> new strand
3'- T C A A A G C -5' --> template

If it isn't able to figure the mutation, it'll be fixed in the next replication process, so the mutation will be fixed into the considered DNA.

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