Describe the structure of DNA. How is DNA replicated?

Structure of DNA

DNA (Deoxyribonucleic Acid) is a double-stranded helical molecule that carries genetic information. Its structure consists of three main components:

1. Phosphate Group: Forms the backbone of the DNA strand, providing structural stability.
2. Deoxyribose Sugar: A five-carbon sugar that links phosphate groups via phosphodiester bonds, forming the sugar-phosphate backbone.
3. Nitrogenous Bases: Four types of bases are attached to the sugar:
   • Purines: Adenine (A) and Guanine (G) (double-ring structure).
   • Pyrimidines: Thymine (T) and Cytosine (C) (single-ring structure). The two strands are held together by hydrogen bonds between complementary bases:
   • Adenine (A) pairs with Thymine (T) via two hydrogen bonds.
   • Cytosine (C) pairs with Guanine (G) via three hydrogen bonds. The strands run antiparallel (one 5' → 3' and the other 3' → 5').

The double-helix structure was proposed by James Watson and Francis Crick (1953), based on X-ray crystallography data by Rosalind Franklin.

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DNA Replication

DNA replication is a semi-conservative process where each original strand serves as a template for a new complementary strand, resulting in two identical DNA molecules. It occurs during the S-phase of the cell cycle and involves three main steps:

1. Initiation

• Helicase unwinds the DNA double helix, breaking hydrogen bonds between bases, forming a replication fork.
• Single-Strand Binding Proteins (SSBs) stabilize the unwound strands.
• Topoisomerase relieves supercoiling ahead of the replication fork.

2. Elongation

• DNA Polymerase III synthesizes new DNA strands in the 5' → 3' direction (only).
  • On the leading strand, synthesis is continuous.
  • On the lagging strand, synthesis is discontinuous, forming Okazaki fragments (short DNA segments).
• Primase lays down a short RNA primer to initiate DNA synthesis.
• DNA Polymerase I removes RNA primers and fills gaps with DNA nucleotides.
• Ligase seals the gaps between Okazaki fragments via phosphodiester bonds.

3. Termination

• In prokaryotes, replication ends at specific termination sites.
• In eukaryotes, multiple replication forks work simultaneously, and telomerase extends telomeres to prevent chromosome shortening.

Key Enzymes Involved:

DNA replication ensures genetic fidelity with high accuracy, though errors (mutations) may occur and are corrected by proofreading mechanisms of DNA polymerase.