3. We will use the figure below to explain how the DNA sequence can be \"read\"

Question

3. We will use the figure below to explain how the DNA sequence can be "read" from the outside of the helix. a) For each base pair, label all potential hydrogen bond donors as (D) and acceptors as (A) in both grooves b) Can you tell an AT base pair apart from all the others (TA, GC, and CG) from the side of the major groove? From the side of the minor groove? major groove major groove G N-H N-H minor groove major groove minor groove major groove H-N minor groove minor groove Recognizing the AT base pair from the major groove:

Explanation / Answer

a) the hydrogen bond are marked in this diagram by a pink/purple link between an H and an N or an O. The giver is always the group with the hydrogen. In this case the N-H group. And the reciever is the N alone or the Oxigen.

b) and c) you can always tell appart any of the base pairs from both the major and the minor groove. you just have to observe the groups that identifies each of the four bases:

For the T, from the major groove it can be seen that is the only base with a CH3 group, that is the easiest one to distinguish. The A has an H, an N and an NH2, G also has an H and an N but instead of an NH2 it has an O with double link (=O). And the C has two hydrogens and an NH2, it is similar to G but instead on an N is another H. So as you can see, from the major group each base can be identified even individually and even more when paired.

The minor group can be a little bit more tricky, C and T have the same groups, N and O with double link (=O), but the C can form a hydrogen bond because it is always linked to G. On the other hand, as long as you can distinguish G and A you will know that the other base is C or T, remember that G is always linked to C and A to T.

G and A are very similar but G has an NH2 and A has just an H in the same place.

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