Chapter 13, Problem 15STP

Summary Introduction

To list and explain:

The importance of purine and pyrimidine bases

Introduction:

Watson and Crick built a model of the double helix for DNA that conformed to other scientists’ research. Together they solved the structure of DNA. They suggested that DNA was a double helix formed by two strands of nucleotides twisted around each other.

Answer to Problem 15STP

The purine bases are adenine and guanine.

The pyrimidine bases are thymine and cytosine.

The purines and the pyrimidines form complementary base pairs in the structure of DNA. This maintains a uniform distance between the two nucleotide chains and also help in semi- conservative replication of DNA.

Explanation of Solution

According toWatson and Crick’s model of DNA, there are two chains of nucleotides that are held together by the nitrogenous bases. The bases are joined together by weak hydrogen bonds.There are two types of bases; purines and pyrimidines. Adenine and guanine are purines while thymine and cytosine are pyrimidine bases.

The basesform pairs with one purine base from one chain hydrogen bonded to onepyrimidine base from the other chain. This ensures a consistent distance between the two nucleotide chains. The number of purine bases is also equal to the number of pyrimidine bases in a sample of DNA. Hence, A=T and C=G. Also, C+T= A+G.

The hydrogen bonds can form only between specific bases, so the bases on one strand determine the bases on the other strand. So adenine on one strand will pair with only thymine on the other strand, and guanine will pair with cytosine. These base pairs are called complementary base pairs. This specific base pairing is responsible for the copying mechanism in DNA. During replication each new strand formed is complementary to the original strand due to specific base pairing mechanism. The replication of DNA is semi- conservative.