Problem 7 - Campbell Biology

Question

Which of the following pairs of base sequences could form a short stretch of a normal double helix of DNA? $$\begin{array}{l}{\text { (A) } 5^{\prime}-\mathrm{AGCT}-3^{\prime} \text { with } 5^{\prime}-\mathrm{TCGA}-3^{\prime}} \\ {\text { (B) } 5^{\prime}-\mathrm{GCGC}-3^{\prime} \text { with } 5^{\prime}-\mathrm{TATA}-3^{\prime}} \\ {\text { (C) } 5^{\prime}-\mathrm{ATGC}-3^{\prime} \text { with } 5^{\prime}-\mathrm{GCAT}-3^{\prime}} \\ {\text { (D) All of these pairs are correct. }}\end{array}$$

Step-by-Step Solution

Answer

Pairs (A) and (C) could form a short stretch of a normal double helix of DNA. Pair (B) cannot.

Step 1: Understand the Laws of Base Pairing

In DNA, adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C).

Step 2: Evaluate Pair (A)

For the sequence 5'-AGCT-3', the complementary sequence using base pairing rules should be 3'-TCGA-5'. Convert this to the format provided: 5'-TCGA-3'. This pair matches the given pair in (A). Thus, (A) appears correct.

Step 3: Evaluate Pair (B)

For the sequence 5'-GCGC-3', the complementary sequence using base pairing rules should be 3'-CGCG-5'. Convert this to the format provided: 5'-CGCG-3'. This does not match the given pair in (B) as 5'-TATA-3'. Thus, (B) is incorrect.

Step 4: Evaluate Pair (C)

For the sequence 5'-ATGC-3', the complementary sequence using base pairing rules should be 3'-TACG-5'. Convert this to the format provided: 5'-GCAT-3'. This pair matches the given pair in (C). Thus, (C) appears correct.

Step 5: Conclusion

Only pairs (A) and (C) obey the base pairing rules for forming a normal double helix of DNA. Hence, the correct answer should not be 'all of these pairs'.

Key Concepts

Complementary Base Pairing

In DNA, complementary base pairing is the rule that ensures each base of one DNA strand is paired with a specific base on the opposite strand. This is critical for DNA replication and transcription. DNA bases are adenine (A), thymine (T), guanine (G), and cytosine (C). Complementary base pairing means that Adenine always pairs with Thymine, and Guanine always pairs with Cytosine. This systematic pairing is what allows DNA to form its stable double helix structure and carry genetic information reliably. Without this precise system, the DNA replication process would lead to numerous errors.

Adenine-Thymine Pairing

Adenine (A) and Thymine (T) are one of the two complementary base pairs in the DNA double helix. These bases are held together by two hydrogen bonds, which make this pairing reasonably strong and stable. When examining DNA strands, if you see Adenine on one strand, you can correctly predict that Thymine resides opposite to it on the complementary strand. In the context of the exercise, if one sequence is 5'-AGCT-3', its complementary strand sequence should be 3'-TCGA-5' or written in the same format, 5'-TCGA-3'. This principle is foundational for understanding DNA replication and the integrity of genetic information.

Guanine-Cytosine Pairing

Guanine (G) pairs with Cytosine (C) in the DNA double helix. This pair is connected by three hydrogen bonds, making it more stable compared to the Adenine-Thymine (A-T) pairing. This additional hydrogen bond contributes to the overall stability of the DNA molecule, especially in regions with high G-C content. For example, in the problem provided, the sequence 5'-ATGC-3' pairs with 5'-GCAT-3' because A pairs with T, and G pairs with C. Correct pairing of G and C is crucial for maintaining the DNA structure and for accurate processes like DNA transcription and repair.

Double Helix Structure

The DNA double helix structure is perhaps one of the most iconic images in biology. This structure is formed by two strands of DNA that wind around each other, with the bases on each strand pairing in the middle, connected by hydrogen bonds. Each turn of the helix contains about ten base pairs. The double helix structure allows DNA to be incredibly compact while still holding vast amounts of information. This compact structure makes DNA stable, which is essential for its function as a long-term storage of genetic information. Understanding this structure helps explain why incorrect base pairing, such as in step 4 of the exercise, can disrupt the helical structure and lead to mutations or replication errors.