In molecular genetics, nucleic acid hybridization techniques are based on which fundamental fact about DNA strands and their nucleotide bases?

Introduction / Context:
Nucleic acid hybridization is a powerful set of techniques used in molecular biology to detect specific DNA or RNA sequences. Examples include Southern blotting, Northern blotting, and many probe based assays. These methods rely on the ability of single stranded nucleic acids to find and bind to complementary sequences. This question asks which basic property of DNA underlies nucleic acid hybridization.

Given Data / Assumptions:

• DNA consists of two antiparallel strands forming a double helix.
• Nucleotide bases in DNA follow specific base pairing rules.
• Nucleic acid hybridization uses labeled probes to detect complementary sequences.
• We assume standard Watson Crick base pairing A T and G C.

Concept / Approach:
The essential concept behind hybridization is complementarity. When double stranded DNA is heated, the hydrogen bonds between complementary bases break, and the strands separate. When conditions are cooled and made favorable, single stranded DNA can base pair again with any complementary strand available. A labeled probe will hybridize with its matching target sequence due to specific base pairing between complementary nucleotides. While strand separation by heating is important practically, the deeper principle is complementary base pairing, without which specific hybridization would not occur.

Step-by-Step Solution:
Step 1: Recall that DNA strands in a double helix are held together by hydrogen bonds between complementary bases, A pairing with T and G pairing with C. Step 2: Understand that in hybridization experiments, DNA or RNA is often denatured by heating so that strands separate into single strands. Step 3: Recognize that single stranded probes with known sequences are then allowed to anneal to target nucleic acids under conditions where only complementary base pairing is stable. Step 4: Evaluate option C, which mentions that DNA strands can be separated. Although this is a practical step, separation alone does not explain why specific sequences find each other. Step 5: Evaluate option D, which focuses on pairing between complementary bases. This is the central reason that a probe can recognize and bind only to its matching sequence among many possibilities. Step 6: Conclude that nucleic acid hybridization is fundamentally based on specific complementary base pairing.

Verification / Alternative check:
Consider a typical Southern blot. Genomic DNA is cut by restriction enzymes, run on a gel, and transferred to a membrane. A labeled single stranded DNA probe is then added. The probe will only bind to regions on the membrane where sequences complementary to its own sequence are present. Non complementary regions will not form stable base pairs and the probe will wash away. This specificity arises solely from complementary base pairing. Methods such as DNA microarrays and in situ hybridization also rely on the same principle.

Why Other Options Are Wrong:
Option A states that a chromosome is composed of complementary strands, which is partly true for individual DNA molecules but is not the key reason for hybridization specificity. Option B suggests that all cells have DNA as their only genetic material, which is false since some viruses use RNA and this idea still does not explain hybridization. Option C mentions that DNA strands can be separated, which is important for preparing single strands but does not by itself cause specific binding. Option E is incorrect because DNA strands can reassociate under suitable conditions, and this ability is what makes hybridization possible.

Common Pitfalls:
Students sometimes confuse the technical steps of an experiment with the underlying principle. Denaturation and renaturation are steps, but the reason that hybridization is specific lies in the base pairing rules. Others may think that having DNA at all is enough, forgetting that hybridization can also involve RNA RNA or DNA RNA pairs, again based on complementarity.

Final Answer:
Nucleic acid hybridization is based on the fundamental fact that specific pairing between complementary nucleotide bases occurs.