Chapter 3, Problem 1QC

Summary Introduction

To determine:

The relationship between $\text{\%R and \%Y}$ in a double stranded DNA molecule.

Concept introduction:

DNA (deoxyribonucleic acid) is the nucleic acid that carries the hereditary information. It is found in the nucleus of the cell and in some semi-autonomous cell organelles. It is a polynucleotide made up of four different nucleotides. The only difference between the nucleotides is the nitrogenous base attached to them. These are categorized into purines (adenosine and guanine) represented as R and pyrimidines (cytosine and thymine) represented as Y.

Explanation of Solution

In the DNA molecule, the purines and pyrimidines attached to each other through hydrogen bonds. Adenosine (A) and thymine (T) bond to each other by two hydrogen bonds and cytosine (C) and guanine (G) by three hydrogen bonds.

The relation between $\text{\%R and \%Y}$ can be established by Chargaff’s rule. Different organisms have different nucleotide sequence that is what differentiates them. However, A always binds to T and C always binds to G. The rule says that the ratio of purines to pyrimidines is always one which can be represented as:

$\frac{\text{(A+G)}}{\text{(T+C)}}\text{=1}$

If purines (A and G) are represented as R and pyrimidines (T and C) are represented as Y then:

$\begin{array}{c}\frac{\text{(A+G)}}{\text{(T+C)}}\text{=}\frac{\text{R}}{\text{Y}}\text{=1}\\ \text{R=Y}\end{array}$

If $\text{R=Y}$, then their percentages will also be equal to which can be written as:

$\text{R\%=Y\%}$

Conclusion

The relationship between $\text{\%R and \%Y}$ is that they are equal, $\text{R\%=Y\%}$.