Question

The following ratio is generally constant for a species

• A. A + G / C + T
• B. T + C / G + A
• C. G + C / A + T
• D. A + C / T + G

Answer 1

The Correct Option is C

The question asks which ratio is generally constant for a species, focusing on nucleotide composition in DNA. This involves understanding the Chargaff's rules, which are foundational in molecular biology.

Chargaff's rules state that in any given DNA sample, the amount of adenine (A) is approximately equal to the amount of thymine (T), and the amount of guanine (G) is approximately equal to the amount of cytosine (C). Moreover, the total purines (A + G) will equal the total pyrimidines (C + T).

Let us analyze the given options to identify which ratio remains constant:

1. The ratio \(\frac{A + G}{C + T}\) is not constant because it compares total amounts of purines and pyrimidines, which are equal, resulting in a ratio close to 1 but not inherently indicative of a species-specific constant.
2. The ratio \(\frac{T + C}{G + A}\) does not consistently follow any particular rule of constancy across species.
3. The ratio \(\frac{G + C}{A + T}\) is known as the G+C content and is indeed a species-specific characteristic. While the A=T and G=C rule holds, the proportion of G+C relative to A+T can vary between different organisms, making it a distinctive and often constant trait for a species.
4. The ratio \(\frac{A + C}{T + G}\) similarly does not follow a rule leading to a constant value across species.

Given these examinations, the ratio \(\frac{G + C}{A + T}\) is generally considered constant for a species due to its relation to G+C content, which is often used to differentiate species and study evolutionary relationships.

Therefore, the correct answer is:

G + C / A + T