Compare the structure of purines and pyrimidines. Can you find out why the distance between two polynucleotide chains in DNA remains almost constant?
Purines (Adenine and Guanine) and Pyrimidines (Cytosine, Uracil and Thymine) are the two types of nitrogenous bases found in nucleic acids.
The distance between two polynucleotide chains in DNA remains almost constant because always a purine comes opposite to a pyrimidine during base pairing. Adenine forms two hydrogen bonds with Thymine, and Guanine bonds with Cytosine through three hydrogen bonds. This complementary pairing of a larger purine with a smaller pyrimidine generates approximately uniform distance between the two strands of the helix.
Explanation
This answer addresses both parts of the question. First, it lists the purines (Adenine, Guanine) and pyrimidines (Cytosine, Uracil, Thymine) as mentioned in the context. Second, it explains the constant distance using the key concept from the textbook: the specific base pairing rule where a purine always pairs with a pyrimidine (A with T, G with C), which maintains uniform spacing between the DNA strands. The answer uses bold terms for key concepts and stays within the 80-120 word limit appropriate for a 2-mark question.
Solution Steps
Step 1: Identify purines (Adenine, Guanine) and pyrimidines (Cytosine, Uracil, Thymine) from the context
Step 2: Extract the explanation for constant distance - purine always pairs with pyrimidine
Step 3: Mention specific base pairing (A-T with 2 H-bonds, G-C with 3 H-bonds) and conclude that this generates uniform distance between strands