Chapter 16: The Molecular Basis of Inheritance – DNA Structure and Replication

Chapter 16: The Molecular Basis of Inheritance

Introduction

This chapter explores the discovery, structure, and replication of DNA, the molecule responsible for inheritance in living organisms. It covers key experiments, molecular structure, and the mechanisms by which genetic information is faithfully transmitted from one generation to the next.

DNA: The Genetic Material

Properties and Function

• DNA is a double-stranded, coiled molecule found in the nucleus of eukaryotic cells.

• It serves as the "Code of Life" by controlling synthesis of proteins, which determine cellular characteristics.

• DNA is used to pass genetic information to the next generation.

Key Experiments Identifying DNA as Genetic Material

• Griffith's Transformation Experiment: Demonstrated that harmless bacteria could be transformed into pathogenic forms by an unknown heritable substance.

• Avery, MacLeod, and McCarty (1940s): Showed that DNA, not protein, was the "transforming substance" responsible for inheritance.

• Hershey and Chase (1952): Used bacteriophage T2 and radioactive labeling to confirm that DNA, not protein, is the hereditary material.

Table: Summary of Key Experiments

Experiment	Key Finding
Griffith	Transformation of bacteria by heritable substance
Avery, MacLeod, McCarty	DNA is the transforming substance
Hershey & Chase	DNA, not protein, is genetic material in phages

Structure of DNA

Nucleotide Composition

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

  • a nitrogenous base (Adenine, Thymine, Guanine, Cytosine)

  • a deoxyribose sugar

  • a phosphate group

• Erwin Chargaff's Rules:

  • DNA composition varies between species.

  • In any species, the number of purines (A, G) equals the number of pyrimidines (T, C).

Double Helix Model

• Rosalind Franklin & Maurice Wilkins (1953): X-ray diffraction revealed the double helix structure.

• Watson and Crick (1953): Built the ball-and-stick model of DNA, showing the double helix and base pairing.

Base Pairing and Molecular Geometry

• Purine + Purine: Too wide for the helix.

• Pyrimidine + Pyrimidine: Too narrow.

• Purine + Pyrimidine: Width matches X-ray data; correct base pairing is A-T and G-C.

Table: DNA Base Pairing

Base Pair	Bonding
Adenine (A) - Thymine (T)	2 hydrogen bonds
Guanine (G) - Cytosine (C)	3 hydrogen bonds

Antiparallel Structure

• DNA strands run in opposite directions (5' to 3' and 3' to 5').

• 5' end: Exposed phosphate group attached to 5' carbon of sugar.

• 3' end: Exposed hydroxyl group attached to 3' carbon of sugar.

DNA Replication

Basic Principle

• DNA replication is semiconservative: each new DNA molecule consists of one parental strand and one newly synthesized strand.

• Base pairing to a template strand ensures accurate copying.

Steps of DNA Replication

1. Initiation:

   • Replication begins at origins of replication.

   • Helicase unwinds the double helix.

   • Single-strand binding proteins stabilize unwound strands.

   • Topoisomerase relieves strain ahead of the replication fork.

   • Primase synthesizes short RNA primers.

2. Elongation:

   • DNA polymerase adds nucleotides to the 3' end of the primer, synthesizing new DNA in the 5' to 3' direction.

   • Leading strand: Synthesized continuously toward the replication fork.

   • Lagging strand: Synthesized discontinuously away from the fork in Okazaki fragments.

   • DNA ligase joins Okazaki fragments.

3. Termination:

   • RNA primers are removed and replaced with DNA.

   • Ligase seals gaps between fragments.

Table: Key Enzymes in DNA Replication

Enzyme	Function
Helicase	Unwinds DNA helix
Topoisomerase	Relieves supercoiling
Primase	Synthesizes RNA primer
DNA Polymerase	Adds nucleotides to growing DNA strand
DNA Ligase	Joins DNA fragments
Single-strand binding proteins	Stabilize unwound DNA

Antiparallel Elongation

• DNA polymerases can only add nucleotides to the free 3' end of a growing strand.

• New DNA strands elongate in the 5' to 3' direction.

Chromosome Structure

DNA Packaging

• DNA is packed with proteins into chromatin.

• Euchromatin: Less condensed, transcriptionally active.

• Heterochromatin: Highly condensed, transcriptionally inactive.

Proofreading and Repairing DNA

DNA Repair Mechanisms

• Mismatch repair: Enzymes correct errors in base pairing.

• Nucleotide excision repair: Damaged DNA (e.g., thymine dimers from UV exposure) is cut out and replaced.

• DNA polymerase fills in missing nucleotides; DNA ligase seals the strand.

Table: Steps in Nucleotide Excision Repair

Step	Description
Damage recognition	UV causes thymine dimer formation
Excision	Nuclease cuts out damaged section
Replacement	DNA polymerase fills in gap
Sealing	DNA ligase seals the strand

Telomeres

• Telomeres are special nucleotide sequences at the ends of eukaryotic chromosomes.

• They postpone, but do not prevent, the shortening of DNA molecules during replication.

Summary

• DNA is the hereditary material, as demonstrated by classic experiments.

• Its double helix structure allows for accurate replication and transmission of genetic information.

• Multiple enzymes and proteins coordinate the complex process of DNA replication.

• Repair mechanisms maintain genetic integrity, and telomeres protect chromosome ends.

Key Equations:

• Base pairing: $[A] = [T], [G] = [C]$

• Semiconservative replication: $\text{New DNA} = \text{1 parental strand} + \text{1 new strand}$

Additional info: These notes expand on the provided slides with definitions, tables, and context for exam preparation.