Question

The ratio constant for a species is
A. $\dfrac{{A + G}}{{C + T}}$
B. $\dfrac{{T + C}}{{G + A}}$
C. $\dfrac{{A + C}}{{T + G}}$
D. $\dfrac{{G + C}}{{A + T}}$

Answer 1

In a stoichiometric pairing of purines and pyrimidines in a double-stranded DNA molecule, the amount of both the types of nitrogenous bases are equal.

Step by step answer:
Chargaff’s base-pair rule states that the DNA from any cell of an organism must have a $1:1$ ratio of purines and pyrimidines. This rule was named after an Austrian-American biochemist Erwin Chargaff at Columbia University. This rule suggests that whatever amount of adenine (A) and thymine (T) in an organism remains the same. Similarly, the amount of cytosine (C) and guanine(G) remains the same.
According to this rule, the molar ratios of- $[A] = [T]$ and $[C] = [G]$ , and the $\dfrac{{[C + G]}}{{[A + T]}}$ should be less than one or unity. The value of $[C + G]$ is always less in an organism.
This ratio varies with the species but is usually constant in a species. He also stated that adenine
always pairs with thymine whereas cytosine always pairs with guanine. He proposed two rules that
are as follows-
The first rule, states that the double-stranded molecule of DNA has observed percentage base
equality. This rule forms the basis of the Watson and Crick model.
The second rule, states that each of the DNA strands observes an approximately equal quantity of
adenine and thymine and cytosine and guanine.
Hence, the correct option is (D) $\dfrac{{G + C}}{{A + T}}$

Note: The amount of adenine-thymine and cytosine-guanine is equal because of the process of base pairing. Base pairs have an equal number of nitrogenous bases because adenine always pairs with thymine and cytosine pairs with guanine.