Chapter 4, Problem 1Q

Summary Introduction

To analyze:

The ratio of purine to pyrimidine for the given set of deoxyribonucleic acid (DNA), observation of a pattern, and the relation of this pattern to the DNA structure.

Given:

The composition of DNA is different in different organisms. The composition of DNA bases in some organisms is tabulated as follows in Table 1:

Table 1: The composition of DNA bases in some organisms

Organism and the tissue from which DNA is extracted	The composition of DNA base (%)
	Adenine	Guanine	Cytosine	Thymine
Herring sperm	27.8	22.2	22.6	27.5
Rat bone marrow	28.6	21.4	21.5	28.4
Human sperm	30.7	19.3	18.8	31.2
Escherichia coli	26.0	24.9	25.2	23.9
Yeast	31.3	18.7	17.1	32.9

Introduction:

Purines are the heterocyclic compounds, which contain carbon, hydrogen, and nitrogen atoms. They contain a double ring system. Pyrimidine is also heterocyclic compounds, but they contain a single ring. The concentration of both purines and pyrimidine are important to define the genetic code of an individual. The DNA consist of two types of purines, that are, guanine (G) and adenine (A) (remember ‘i’ in purine, adenine, and guanine), and two types of pyrimidine, that are, thymine (T) and cytosine (C) (remember ‘y’ in cytosine, thymine, and pyrimidine).

Explanation of Solution

Each purine is bonded to a pyrimidine with the help of hydrogen bonds. The adenine is bonded to thymine by two hydrogen bonds and cytosine is bonded to guanine by three hydrogen bonds. The rule of bonding of a pyrimidine with a purine is called as the Chargaff’s rule. The number of purines is always equal to the number of pyrimidines.

The ratio of purine to pyrimidine is calculated by dividing the total amount of purine with the total amount of pyrimidines and it can be seen in Table 2 given below:

Table 2: Ratio of purine to pyrimidine

Organism and the tissue from which DNA is extracted	Purines	Pyrimidines	Ratio
	A+G	C+T
Herring sperm pyrimidines ratio	$\begin{array}{l}27.8+22.2=50.0\hfill \end{array}$	$\mathrm{22.2.6}+27.5=50.1$	$\frac{50.0}{50.1}=1$
Rat bone marrow	$28.6+21.4=50.0$	$21.5+28.4=49.9$	$\frac{50.0}{49.9}=1$
Human sperm	$30.7+19.3=50$	$18.8+31.2=50$	$\frac{50}{50}=1$
Escherichia coli	$26.0+24.9=50.9$	$25.2+23.9=49.1$	$\frac{50.9}{49.1}=1.04$
Yeast	$31.3+18.7=50$	$17.1+32.9=50$	$\frac{50}{50}=1$

From Table 2, it is clear that the ratio of purine to pyrimidine is always equal to 1. This means that the DNA always occurs as a double helix structure in which one purine is always bonded to one pyrimidine.

Conclusion

Thus, it can be calculated from Tables 1 and 2 that each organism has a different composition of DNA bases but the ratio of purines to pyrimidine is always equal to 1 indicating that DNA is a double helix structure, in which each purine is hydrogen bonded to a pyrimidine.