describe the structure of a DNA molecule as a double helix, including: the importance of complementary base pairing between the 5′ to 3′ strand and the 3′ to 5′ strand (antiparallel strands), differences in hydrogen bonding between C–G and A–T base p

Structure of Nucleic Acids and Replication of DNA

1. The DNA Double Helix

DNA (deoxyribonucleic acid) exists as a right‑handed double helix. Each helix is composed of two long polymer chains of nucleotides that wind around a common axis.

Key features of the double helix:

• Two strands are antiparallel: one runs from the 5′ end to the 3′ end, the opposite runs from 3′ to 5′.
• The backbone of each strand consists of alternating deoxyribose sugars and phosphate groups linked by phosphodiester bonds.
• Inside the helix, nitrogenous bases pair by hydrogen bonding, forming the “rungs” of the ladder.

2. Antiparallel Orientation

Each nucleotide has a 5′‑phosphate group and a 3′‑hydroxyl group. In the double helix:

1. One strand is oriented 5′→3′ (the “sense” strand).
2. The complementary strand runs in the opposite direction, 3′←5′ (the “antisense” strand).

This antiparallel arrangement is essential for the complementary base‑pairing rules to be satisfied and for enzymes such as DNA polymerase to synthesize DNA in the 5′→3′ direction.

3. Complementary Base Pairing

Base pairing follows strict rules:

• Adenine (A) pairs with Thymine (T) via two hydrogen bonds.
• Cytosine (C) pairs with Guanine (G) via three hydrogen bonds.

These pairings are complementary because the pattern of hydrogen‑bond donors and acceptors on each base matches only its partner.

4. Hydrogen Bonding Differences

5. Phosphodiester Bonds

Within each strand, nucleotides are linked by phosphodiester bonds formed between the 3′‑hydroxyl group of one deoxyribose and the 5′‑phosphate group of the next. The reaction can be represented as:

\$\text{(deoxyribose)}n\text{-PO}4\text{- (deoxyribose)}{n+1} + \text{H}2\text{O} \rightarrow \text{(deoxyribose)}n\text{-O-P-O- (deoxyribose)}{n+1} + \text{H}^+ + \text{OH}^-\$

These covalent bonds give the DNA backbone its strength and resistance to chemical attack, while the hydrogen bonds between bases are relatively weak and can be broken during replication or transcription.

6. Summary of Key Points

• DNA is a double‑helix of two antiparallel strands.
• Each strand’s backbone is a chain of phosphodiester‑linked nucleotides.
• Complementary base pairing (A–T, C–G) holds the two strands together.
• C–G pairs have three hydrogen bonds; A–T pairs have two, influencing stability.
• The antiparallel orientation is crucial for accurate replication and transcription.