Which of the following pairs of base sequences could form a short stretch of a normal double helix of DNA? (A) 5′-AGCT-3′ with 5′-TCGA-3′
(B) 5′-GCGC-3′ with 5′-TATA-3′
(C) 5′-ATGC-3′ with 5′-GCAT-3′
(D) All of these pairs are correct

The correct option is C

Explanation:

First of all, we need to know that a DNA molecule can have the following bases; Adenine (A), Thymine (T), Guanine (G) and cytosine (C). These bases have the ability to bind to each other on complementary strands in the following way; adenine can only bind to thymine and vice versa while guanine can only bind to cytosine and vice versa.

It also be noted that when one strand of the DNA runs from 5' to 3', the complementary strand runs from 3' to 5', hence they run anti-parallel to each other.

So, the only option sequence that meets the criteria discussed above is option C,

5'-ATGC-3'

3'-TACG-5'

The illustration above shows how the bases will be on the complimentary strands when they run anti-parallel to each other. And they can also be written as 5′-ATGC-3′ (this is read the way it is written) and 5′-GCAT-3′ (this is read from the back as in from 3' to 5')

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