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.

• James Watson and Francis Crick (1953) introduced the double-helical model of DNA structure, building on the work of many scientists.

• Hereditary information in DNA directs the development of biochemical, anatomical, physiological, and some behavioral traits.

• DNA is reproduced in all cells during DNA replication.

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. The identification of DNA as the genetic material was supported by several key findings:

• Chargaff’s Rules (Erwin Chargaff, 1950):

  • DNA base composition varies between species.

  • Within a species, the percentage of adenine (A) equals thymine (T), and guanine (G) equals cytosine (C).

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

Building the Structural Model of DNA

The structure of DNA was determined through a combination of chemical analysis and X-ray crystallography.

• Rosalind Franklin used X-ray crystallography to reveal that DNA is helical, with major and minor grooves, and that the sugar-phosphate backbones are on the outside, running antiparallel (5' to 3' in opposite directions).

• Watson and Crick finalized the double helix model, showing that DNA consists of two antiparallel strands held together by complementary base pairing (A with T, C with G) via hydrogen bonds.

13.2 Many Proteins Work Together in DNA Replication & Repair

The Basic Principle: Base Pairing to a Template Strand

DNA replication relies on the complementarity of the two DNA strands. Each strand serves as a template for the synthesis of a new complementary strand.

• Semiconservative replication: Each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.

DNA Replication: A Closer Look

DNA replication is a highly accurate and efficient process involving many enzymes and proteins. The process is fundamentally similar in prokaryotes and eukaryotes, though more is known about bacterial replication.

Getting Started: Origins of Replication

• Origins of replication: Specific DNA sequences where replication begins.

• Prokaryotes: Single circular chromosome with one origin; replication proceeds in both directions, forming two replication forks.

• Eukaryotes: Multiple linear chromosomes with many origins; multiple replication forks and 'bubbles' form.

Enzymes Involved in Initiation

• Helicase: Unwinds and separates the DNA strands at the replication fork.

• Single-strand binding proteins: Stabilize and keep the separated DNA strands apart.

• Topoisomerase: Relieves strain ahead of the replication fork caused by unwinding.

Synthesizing a New Strand

• Primase: Synthesizes short RNA primers (~5–10 nucleotides) complementary to the DNA template, providing a starting point for DNA synthesis.

• DNA polymerases: Enzymes that add nucleotides to the 3' end of a preexisting chain, using the template strand for base pairing. They require an RNA primer and a DNA template.

• DNA polymerases also proofread and correct errors during synthesis.

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

• DNA nucleotides are joined by phosphodiester bonds via dehydration synthesis.

• After synthesis, RNA primers are degraded and replaced with DNA.

Antiparallel Elongation

• DNA strands are antiparallel; new DNA is synthesized only in the 5' to 3' direction.

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

• Lagging strand: Synthesized discontinuously away from the fork in short segments called Okazaki fragments; each fragment requires a new primer.

• RNA primers are replaced with DNA, and DNA ligase joins the fragments.

Proofreading and Repairing DNA

• DNA replication is highly accurate (error rate ~1 in 10 billion nucleotides).

• DNA polymerases proofread and correct mismatched bases during synthesis.

• Mismatch repair: Other enzymes scan DNA after replication to correct errors missed by polymerases.

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

• Nuclease enzymes remove damaged DNA segments; the gap is filled by DNA polymerase and sealed by DNA ligase (nucleotide excision repair).

• Example: Excision repair in skin cells corrects UV-induced damage.

Replicating the Ends of DNA Molecules

• Linear DNA (eukaryotes) cannot be fully replicated at the 5' ends, leading to progressive shortening with each cell division.

• Telomeres: Repetitive, non-coding nucleotide sequences at chromosome ends (e.g., TTAAGGG repeated 100–1,000 times) that protect genes from erosion.

• Telomeres act as a buffer but do not prevent shortening; their loss is associated with aging.

• Telomerase: An enzyme that extends telomeres in germ cells, preventing loss of essential genes in gametes. Not active in most somatic cells, but abnormally active in some cancer cells, contributing to their immortality.

• Telomerase is a target of ongoing cancer research.

Key Terms and Definitions

• DNA (Deoxyribonucleic Acid): The molecule that stores genetic information in all living organisms.

• Nucleotide: The building block of DNA, consisting of a nitrogenous base, a deoxyribose sugar, and a phosphate group.

• Antiparallel: Refers to the opposite orientation of the two DNA strands (5' to 3' and 3' to 5').

• Okazaki fragments: Short DNA fragments synthesized on the lagging strand during replication.

• Phosphodiester bond: The covalent bond that links nucleotides in a DNA strand.

• Telomere: The repetitive DNA sequence at the end of a eukaryotic chromosome.

• Telomerase: An enzyme that extends telomeres in germ cells.

Example: DNA Replication in Eukaryotes

• Replication begins at multiple origins along each chromosome.

• Replication forks form and move outward, synthesizing new DNA strands.

• Leading and lagging strands are synthesized simultaneously at each fork.

• After replication, proofreading and repair mechanisms correct errors.

Relevant Equations and Structures

• Phosphodiester bond formation:

• Base pairing:

• Directionality:

Table: Comparison of Leading and Lagging Strand Synthesis

Additional info: The above notes expand on the original outline by providing definitions, context, and examples to ensure a self-contained and comprehensive study guide for students preparing for exams on the molecular basis of inheritance.