Under normal circumstances, adenine does not pair up with guanine or cytosine; any other mismatches cannot occur. What 2 factors prevent a mismatch?

Expert Answers

An illustration of the letter 'A' in a speech bubbles

Adenine and Guanine are bothe purines. Purine-purine pairings are energetically unfavorable, since moelcules are too close to each othet. (This is the other way round for pyrimidines, they are too apart for stable bonding). Therefore because of this reason Adenine can't bond with Guanine. This is the first factor.

Adenine has only two hydrogen bonding sites whereas Cytosine has three hydrogen bonds. Therefore an Adenine-Cytosine paring is not energetically favorable. It can only bond with a base with two hydrogen sites. This is the second factor.

The only base that Adenine pairs with is Thymine which is a pyrimidine and has only two hydrogen bonding sites. Thus AT and GC paring rule comes into existence.

Approved by eNotes Editorial Team
An illustration of the letter 'A' in a speech bubbles

Adenine and guanine are two bases involved in the structure of DNA that are referred to as purine bases.  The other two bases involved in the construction and composition of DNA are thymine and cytosine; these two are referred to as pyrimidine bases.  That would be the first factor, that a purine must bond to a pyrimidine in the construction of a "rung" of the DNA double-helix ladder.  Adenine correctly bonds to thymine, while guanine correctly bonds to cytosine.  This pairing is referred to as "complementary base pairing".  The other factor that preserves the correct pairing of the complementary base pairs is the use of the existing DNA strand as a template to build new strands of DNA.  The original strand unwinds and breaks its bonds right down the middle, each side having a base sequence to serve as a template.  New bases are assembled according to the specified order of the existing DNA strand, using complementary base pairing.  When complete, there are now two strands of DNA, where there was originally only one.

Approved by eNotes Editorial Team