The Molecular Basis of Inheritance: Structure and Replication of DNA

The Molecular Basis of Inheritance

Deoxyribonucleic Acid (DNA): Structure and Composition

DNA is the hereditary material in all living organisms, encoding the instructions for life. Its structure and replication are central to understanding genetics and molecular biology.

• DNA is a polymer of nucleotides, each consisting of:

  • One five-carbon sugar (deoxyribose)

  • One phosphate group

  • One of four nitrogenous bases: adenine (A), guanine (G), thymine (T), or cytosine (C)

• Sugar-phosphate backbone: Alternating sugar and phosphate units form the backbone, running in a 5’ to 3’ direction.

• Nitrogenous bases:

  • Purines: Adenine and Guanine (double-ring structure)

  • Pyrimidines: Cytosine and Thymine (single-ring structure)

Example: A DNA strand with 15% adenine will also have 15% thymine, due to complementary base pairing.

Discovery of DNA Structure

• X-ray Crystallography: Maurice Wilkins and Rosalind Franklin used this technique to reveal DNA's helical structure.

• Watson and Crick (1958): Built the double helix model, using Chargaff’s Rules for base pairing:

  • A pairs with T via 2 hydrogen bonds

  • G pairs with C via 3 hydrogen bonds

Additional info: Chargaff’s Rules state that in any DNA sample, the amount of adenine equals thymine, and guanine equals cytosine.

Origins of Replication

DNA replication begins at specific sequences called origins of replication, where the double helix is unwound to form replication bubbles.

• Each bubble has two replication forks (Y-shaped regions where DNA is unwound and copied).

Proteins Involved in DNA Replication

• Helicase: Unwinds the DNA double helix.

• Single-stranded binding proteins: Stabilize unwound DNA strands.

• Topoisomerase: Relieves strain ahead of the replication fork by cutting and rejoining DNA strands.

• Primase: Synthesizes short RNA primers to initiate DNA synthesis.

• DNA Polymerase: Adds nucleotides to the 3’ end of the primer, synthesizing new DNA.

Synthesizing a New DNA Strand

• DNA polymerases can only add nucleotides to an existing 3’ end; they cannot start synthesis de novo.

• The initial nucleotide strand is a short RNA primer (5–10 nucleotides), synthesized by primase.

• Elongation rate: ~500 nucleotides/sec in bacteria, ~50/sec in human cells.

Leading and Lagging Strands

• Leading strand: Synthesized continuously in the 5’ → 3’ direction toward the replication fork.

• Lagging strand: Synthesized discontinuously as short Okazaki fragments, also in the 5’ → 3’ direction, but away from the fork.

Bacterial DNA Replication: Key Enzymes and Steps

Step	Enzyme	Function
1	DNA Polymerase III	Copies DNA by adding nucleotides
2	DNA Polymerase I	Replaces RNA primers with DNA
3	DNA Ligase	Seals gaps between DNA fragments

Proofreading and Repairing DNA

• DNA polymerases proofread and correct errors during replication.

• Mismatch repair: Other enzymes correct base-pairing errors missed by DNA polymerase.

• Nucleotide excision repair: Nucleases remove and replace damaged DNA segments.

• DNA can be damaged by chemicals, radiation, or spontaneous changes.

Significance of Mutations

• Even after proofreading, some errors (mutations) persist and may be inherited.

• Mutations are the source of genetic variation, essential for natural selection.

• Some mutations result in recessive traits.

Chromosomes: Structure and Organization

• Chromosomes are structures made of DNA and associated proteins, carrying genetic information.

• Human cells have 46 chromosomes; if stretched, the DNA would be about 2 meters long.

Histones and Nucleosomes

• Histones: Proteins that organize and package DNA in eukaryotic chromosomes.

• Nucleosome: A segment of DNA wound around a core of histone proteins, forming the basic unit of chromatin structure.

Additional info: The packaging of DNA into nucleosomes allows efficient storage and regulation of gene expression in eukaryotic cells.