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Question 556
Biochemistry → General
Classification
Question
SSBAQuestion Header
A DNA fragment contains 30 purine bases in one strand. After formation of its fully complementary strand, how many pyrimidine bases must be present in the complete double-stranded fragment as a direct consequence of complementary base pairing?
Question Stem
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Options
A
15
B
30
C
45
D
60
E
The number cannot be determined
Explanation
Each purine base in one DNA strand pairs with a pyrimidine base in the complementary strand. Therefore the 30 purines specified in the original strand require 30 complementary pyrimidines. The numbers of any additional pyrimidines in the original strand and their complementary purines are not needed for this specific count.
Option Validity
A) Complementary pairing requires one pyrimidine for each of the 30 specified purines, not one for every two purines.
C) Thirty specified purines generate 30 complementary pyrimidines, not 45.
D) This doubles the number of complementary pyrimidines required by the 30 specified purines.
E) The number of pyrimidines complementary to the 30 specified purines is directly determined by one-to-one complementary pairing.
Further Reading
Saxena - MRCOG-1 Basic Sciences - Biochemistry, printed p.128.