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Question 568
Genetics → General
Classification
Question
SSBAQuestion Header
What percentage of all bases in the sample is guanine?
Question Stem
Analysis of a double-stranded DNA sample shows that adenine accounts for 30% of all its bases. The sample contains only the four usual DNA bases.
Options
A
20%
B
10%
C
30%
D
40%
E
70%
Explanation
In double-stranded DNA, adenine pairs with thymine and guanine pairs with cytosine. Adenine and thymine therefore each account for 30%, together making 60%. The remaining 40% is divided equally between guanine and cytosine, giving 20% guanine.
Option Validity
B) The remaining 40% must be divided between two equally represented bases, guanine and cytosine, giving 20% rather than 10% each.
C) Thymine, not guanine, must match the 30% adenine proportion.
D) 40% is the combined guanine-plus-cytosine proportion, not the guanine proportion alone.
E) 70% includes thymine, guanine and cytosine together; it is not the guanine proportion.
Further Reading
Saxena R. Textbook for MRCOG-1: Basic Sciences in Obstetrics and Gynaecology. 1st ed. Jaypee Brothers Medical Publishers; 2016. Chapter 8: Genetics — Deoxyribonucleic Acid. Printed page 254.