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.

Verified step by step guidance

1. Understand the basic principle of DNA base pairing: In a DNA double helix, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G).
2. Examine option a: 5′-AGCT-3′ with 5′-TCGA-3′. Check if each base pairs correctly according to the base pairing rules: A with T, G with C, C with G, and T with A.
3. Examine option b: 5′-GCGC-3′ with 5′-TATA-3′. Check if each base pairs correctly: G with C, C with G, G with C, and C with G. Notice that T does not pair with G, and A does not pair with C.
4. Examine option c: 5′-ATGC-3′ with 5′-GCAT-3′. Check if each base pairs correctly: A with T, T with A, G with C, and C with G.
5. Determine which pair(s) follow the correct base pairing rules and conclude whether one or more pairs can form a normal double helix of DNA.

Key Concepts

DNA Structure

DNA is composed of two strands forming a double helix, where each strand consists of nucleotides with bases adenine (A), thymine (T), cytosine (C), and guanine (G). The strands are antiparallel, meaning they run in opposite directions, which is crucial for base pairing and the overall structure of DNA.

Base Pairing Rules

In DNA, base pairing follows specific rules: adenine (A) pairs with thymine (T) via two hydrogen bonds, and cytosine (C) pairs with guanine (G) via three hydrogen bonds. These complementary base pairs are essential for the stability and replication of the DNA double helix.

Antiparallel Orientation

The antiparallel orientation of DNA strands means one strand runs in the 5′ to 3′ direction, while the complementary strand runs in the 3′ to 5′ direction. This orientation is necessary for proper base pairing and the formation of the double helix, ensuring the strands align correctly for replication and transcription.