Why isn't it possible for adenine to pair up with guanine or cytosine?

Answer 1

Lets look at it this way:

Thymine and adenine are pieces of a jigsaw puzzle that matches. Cytosine and Guanine are also pieces of jigsaw puzzle that matches. A cytosine cannot match with thymine because the shape of cytosine will only match with the shape of a guanine.

To look at it in another way:

Thymine(uracil in RNA) and cytosine are pyrimidines. Pyrimidines are single-ringed organic base that will only attach to purines, or adenine and guanine by hydrogen bond. Because cytosine and thymine are both pyrimidines, it cannot form a hydrogen bond with each other.

Answer 2

There are two reasons:

1. Shape- The structure of adenine cannot allow it to bond with guanine. It's just impossible. Same goes for thymine and cytosine. It just isn't physically possible.

2. Hydrogen Bonds- Adenine shares 2 hydrogen bonds with thymine, and cytosine shares 3 hydrogen bonds with guanine. If Adenine tried to bond with guanine, there would be an "incomplete bond," since one wants to make 3 hydrogen bonds and the other wants to make 2.

Hope that helps! The second reason is the one I'd more likely use in class or for homework.

Actually it is possible for them to pair differently, but it happens less often than once in a trillion pairings and is one of the causes for mutations. Also it is the pattern of available hydrogen bonds (2 vs. 3) that gives the bases the appearance of having different shapes at the molecular level (like a key in a lock).

Answer 3

The biggest reasons is that the structure of adenine does not allow for binding to cytosine and such a complex would be highly energetically unfavorable. The two types of base pairs form different numbers of hydrogen bonds, AT forming two hydrogen bonds, and GC forming three hydrogen bonds. An A attempting to bind to a C (or G to a T) would not even be possible much like the wrong sized key does not fit into the lock. It is important to note that the hydrogen bonding between the base pairs merely provides specificity of the pairing, not stability. Therefore the instability of an AC complex would likely come not from the incorrect hydrogen bonding (although that would prevent A and C from ever getting together in the first place), but rather the contribution to the destabilization of the stacking interactions if the AC complex was to ever come into form.

Answer 4

adenine and thymine form two hydrogen bonds and they have a structure that allows them form those bonds.

guanine and cytosine form three hydrogen bonds, also they have structures that complement each other.

Answer 5

Between A-T, there are two hydrogen bonds made. If A were to pair with C or G there would be an extra hydrogen, making the DNA unstable. C-G have three hydrogen bonds, making them stable.