Chapter 13: Molecular Basis of Inheritance

Overview: Life’s Operating Instructions

The discovery of DNA's structure revolutionized our understanding of heredity. DNA contains the instructions for the development and functioning of all living organisms, and its accurate replication is essential for life.

• Double-Helical Model: Introduced by James Watson and Francis Crick in 1953, based on data from multiple researchers.

• Hereditary Information: DNA directs biochemical, anatomical, physiological, and some behavioral traits.

• DNA Replication: Ensures all cells inherit identical genetic information.

13.1 DNA is the Genetic Material

Evidence for DNA as Genetic Material

DNA is a polymer composed of nucleotides, each containing a nitrogenous base, a pentose sugar (deoxyribose), and a phosphate group. Multiple lines of evidence established DNA as the molecule of heredity.

• Nitrogenous Bases: Adenine (A), Guanine (G), Thymine (T), Cytosine (C).

• Chargaff’s Rules:

  • DNA base composition varies between species.

  • Within a species, %A = %T and %G = %C.

  • Purines (A, G) pair with pyrimidines (T, C), resulting in uniform width of base pairs.

Building the Structural Model of DNA

The structure of DNA was determined through scientific inquiry and collaboration.

• Rosalind Franklin: Used X-ray crystallography to reveal DNA’s helical structure, major and minor grooves, and uniform width.

• Watson and Crick: Integrated Franklin’s data and Chargaff’s rules to propose the double helix model.

• Antiparallel Strands: Sugar-phosphate backbones run in opposite directions (5’ to 3’).

• Base Pairing: A pairs with T, C pairs with G via hydrogen bonds.

13.2 Many Proteins Work Together in DNA Replication & Repair

Base Pairing to a Template Strand

DNA replication relies on the complementarity of its two strands. Each strand serves as a template for the synthesis of a new strand, ensuring genetic fidelity.

• Semiconservative Replication: Each daughter DNA molecule contains one old strand and one new strand.

• Parent Molecule: Unwinds to allow new strands to be built according to base-pairing rules.

DNA Replication: A Closer Look

Replication is a highly accurate and rapid process, involving numerous enzymes and proteins.

• Replication Origins:

  • Prokaryotes: Single origin on circular chromosome, two replication forks.

  • Eukaryotes: Multiple origins on linear chromosomes, many replication forks.

  • Replication bubbles form and forks move in opposite directions.

• Initiation Enzymes:

  • Helicase: Untwists and separates DNA strands at replication forks.

  • Single-Strand Binding Proteins: Stabilize separated strands.

  • Topoisomerase: Relieves strain ahead of replication fork.

Synthesizing a New Strand

DNA polymerases synthesize new DNA by adding nucleotides to an existing chain, using a template strand.

• RNA Primers: Short RNA sequences synthesized by primase to provide a starting point.

• DNA Polymerases:

  • Add nucleotides to the 3’ end of the primer.

  • Require a template strand and primer.

  • Proofread and ensure correct base pairing.

  • Elongation rate: ~500 nucleotides/sec (bacteria), ~50 nucleotides/sec (humans).

  • Form phosphodiester bonds via dehydration synthesis.

• Primer Removal: RNA primers are degraded and replaced with DNA.

Antiparallel Elongation

DNA strands are synthesized in an antiparallel manner, leading to distinct replication mechanisms for each strand.

• Leading Strand: Synthesized continuously toward the replication fork; requires one primer.

• Lagging Strand: Synthesized discontinuously away from the fork, forming Okazaki fragments.

• Okazaki Fragments: Short DNA segments joined by DNA ligase.

• Primer Replacement: DNA polymerase replaces RNA primers with DNA; DNA ligase seals gaps.

Comparison of Leading and Lagging Strand Synthesis

Proofreading and Repairing DNA

Accurate DNA replication is vital for cell survival. Multiple mechanisms ensure fidelity and repair damage.

• Proofreading: DNA polymerases correct errors during synthesis.

• Mismatch Repair: Other enzymes scan DNA post-replication to fix mismatches.

• DNA Damage: Caused by chemicals, radiation, or spontaneous changes.

• Nuclease: Cuts out damaged DNA; excision repair replaces it with correct bases.

• Nucleotide Excision Repair: Removes and replaces damaged DNA segments (e.g., UV-induced damage in skin cells).

DNA Repair Mechanisms

Replicating the Ends of DNA Molecules

Linear DNA presents unique challenges during replication, particularly at chromosome ends.

• Telomeres: Repetitive nucleotide sequences at chromosome ends (e.g., TTAAGGG repeated 100–1,000 times).

• Function: Protect genes from erosion during replication; act as buffer zones.

• Telomere Shortening: Occurs with each cell cycle; linked to aging.

• Telomerase: Enzyme that extends telomeres in germ cells; not active in most somatic cells but active in some cancer cells, contributing to cellular immortality.

• Cancer Therapy: Telomerase is a target for potential treatments.

Key Terms and Concepts

• DNA Polymerase: Enzyme that synthesizes DNA by adding nucleotides to the 3’ end.

• Helicase: Enzyme that unwinds the DNA double helix.

• Primase: Enzyme that synthesizes RNA primers.

• Ligase: Enzyme that joins Okazaki fragments on the lagging strand.

• Topoisomerase: Enzyme that relieves strain ahead of the replication fork.

• Telomere: Protective DNA sequence at chromosome ends.

• Telomerase: Enzyme that extends telomeres.

Important Equations

• Phosphodiester Bond Formation: DNA polymerase catalyzes the formation of covalent bonds between nucleotides.

• Chargaff’s Rule: For double-stranded DNA:

Example: DNA Replication in Human Cells

During cell division, human cells replicate their DNA at a rate of approximately 50 nucleotides per second, ensuring that each daughter cell receives a complete set of genetic instructions. The process involves multiple origins of replication, numerous enzymes, and strict proofreading mechanisms to maintain genetic integrity.

Additional info: DNA replication and repair are fundamental to cell division, genetic inheritance, and the prevention of mutations that can lead to disease. Understanding these processes is essential for fields such as genetics, molecular biology, and biotechnology.