Question

What rule is used to join the free nucleotides to the exposed bases of the DNA?

Answer 1

The nucleotides are made up of nitrogen bases, a phosphate group and a sugar base. The joining of nucleotides to the exposed part of DNA takes place between the nitrogen bases like purine and pyrimidine by double bond or triple bond.

Complete answer:
Joining of nucleotides in free bases or exposed bases of DNA takes place at the time of replication and transcription by base-pairing rule. The base-pairing rule is the pairing formed in DNA between the purine and pyrimidine. According to this rule adenine always pairs with thymine while cytosine always pairs with guanine. When the two DNA strands remain parallel, the space between them remains uniform and the presence of two purines makes the double helix wide while the pyrimidine makes the strand narrow. Due to base-pairing, space between two DNA strands becomes 20 Å and for two nitrogen bases, it is 11 Å. This space is sufficient for the bonding between one purine and one pyrimidine. There are two hydrogen bonds between adenine and thymine while pairing while three hydrogen bonds form between cytosine and guanine. In the case of transcription, where RNA is formed then the thymine is replaced by Uracil and transcription takes place between adenine and Uracil only in the RNA strand.

Note:
When the nucleotide is joined with the exposed part of DNA during replication they always join in an antiparallel manner, where one end of the strand lies opposite to the other strand. This type of base pairing is also useful in the case of complementary base pairing.