The Molecular Basis of Inheritance (Chapter 16) – Study Notes

The Molecular Basis of Inheritance

Introduction

The molecular basis of inheritance refers to how genetic information is stored, transmitted, and expressed in living organisms. This chapter explores the discovery that DNA is the hereditary material, its structure, and the mechanisms by which it is replicated and maintained.

Hereditary Information and DNA

DNA as the Genetic Material

• Hereditary information is encoded in DNA and exists in all cells of the body.

• This DNA programming directs the development of biochemical, anatomical, physiological, and (to some extent) behavioral traits.

The Search for the Genetic Material

Scientific Inquiry and Early Experiments

• T. H. Morgan's group proved that alleles are located on chromosomes, making DNA and protein candidates for genetic material.

Evidence That DNA Can Transform Bacteria

• Frederick Griffith (1928) discovered transformation: a change in genotype and phenotype due to assimilation of foreign DNA.

• Griffith's experiment with Streptococcus pneumoniae (S and R strains) showed that heat-killed pathogenic cells could transform harmless cells into pathogenic ones.

Experiment	Result
Living S cells	Mouse dies
Living R cells	Mouse healthy
Heat-killed S cells	Mouse healthy
Mixture of heat-killed S cells and living R cells	Mouse dies; living S cells recovered

DNA or Proteins? The Role of Viruses

Bacteriophage Experiments

• Bacteriophages (viruses that infect bacteria) were used to determine whether DNA or protein is the genetic material.

• Alfred Hershey and Martha Chase (1952) showed that DNA, not protein, is the genetic material of phage T2.

Batch	Label	Result
1	Radioactive sulfur ($^{35}$S) labels protein	Radioactivity found in liquid (not in cells)
2	Radioactive phosphorus ($^{32}$P) labels DNA	Radioactivity found in pellet (inside cells)

Additional Evidence: DNA Structure and Diversity

DNA Composition and Chargaff's Rules

• DNA is a polymer of nucleotides, each with a nitrogenous base, a sugar (deoxyribose), and a phosphate group.

• Erwin Chargaff (1950) found that DNA composition varies between species.

• Chargaff's rules: In any species, the amount of adenine (A) equals thymine (T), and the amount of guanine (G) equals cytosine (C).

Base	Pairs With
Adenine (A)	Thymine (T)
Guanine (G)	Cytosine (C)

Building a Structural Model of DNA

X-ray Crystallography and the Double Helix

• Maurice Wilkins and Rosalind Franklin used X-ray crystallography to study DNA structure.

• Franklin's images revealed that DNA is helical and consists of two strands forming a double helix.

• Watson and Crick built models showing two antiparallel sugar-phosphate backbones with nitrogenous bases paired in the interior.

Base Pairing and the Double Helix

• Pairing a purine (A, G) with a pyrimidine (T, C) results in a uniform width, consistent with X-ray data.

• Adenine (A) pairs only with thymine (T); guanine (G) pairs only with cytosine (C).

• The double helix is stabilized by hydrogen bonds between base pairs.

Base Pair	Bond Type
A-T	2 hydrogen bonds
G-C	3 hydrogen bonds

DNA Replication

Base Pairing to a Template Strand

• The two DNA strands are complementary; each serves as a template for building a new strand.

• During replication, the parent molecule unwinds, and two new daughter strands are synthesized based on base-pairing rules.

Semiconservative Model of Replication

• Watson and Crick proposed the semiconservative model: each daughter DNA molecule consists of one parental strand and one new strand.

• Meselson and Stahl's experiment using nitrogen isotopes ($^{15}$N and $^{14}$N) confirmed the semiconservative model.

Model	First Replication	Second Replication
Conservative	Heavy and light bands	Heavy and light bands
Semiconservative	Intermediate band	Intermediate and light bands
Dispersive	Intermediate band	Single intermediate band

DNA Replication: Mechanism and Enzymes

Initiation and Origins of Replication

• Replication begins at origins of replication, where DNA strands separate, forming a replication bubble.

• Eukaryotic chromosomes have multiple origins; prokaryotes typically have one.

• Replication proceeds in both directions from each origin.

Key Enzymes and Proteins

• Helicase: Unwinds the DNA double helix at the replication fork.

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

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

• Primase: Synthesizes a short RNA primer to provide a starting point for DNA polymerase.

• DNA polymerase: Adds nucleotides to the 3' end of the primer, synthesizing new DNA.

• DNA ligase: Joins Okazaki fragments on the lagging strand.

Leading and Lagging Strands

• DNA polymerase synthesizes the leading strand continuously toward the replication fork.

• The lagging strand is synthesized discontinuously, forming Okazaki fragments that are later joined by DNA ligase.

Antiparallel Elongation

• DNA strands are antiparallel: one runs 5' to 3', the other 3' to 5'.

• DNA polymerase can only add nucleotides to the 3' end, so synthesis occurs in opposite directions on the two strands.

Proofreading and Repair

DNA Polymerase and Repair Mechanisms

• DNA polymerases proofread newly synthesized DNA, correcting errors.

• Mismatch repair enzymes fix errors in base pairing.

• Nucleotide excision repair removes and replaces damaged DNA segments.

Replicating the Ends of DNA: Telomeres

Telomeres and Telomerase

• Linear eukaryotic chromosomes have telomeres at their ends, which protect genes from erosion during replication.

• Telomerase is an enzyme that extends telomeres in germ cells, preventing loss of essential genetic information.

• Shortening of telomeres is associated with aging; telomerase activity is often found in cancer cells.

Summary Table: Key DNA Replication Enzymes

Enzyme/Protein	Function
Helicase	Unwinds DNA at replication fork
Single-strand binding protein	Stabilizes single-stranded DNA
Topoisomerase	Relieves strain ahead of fork
Primase	Synthesizes RNA primer
DNA polymerase	Synthesizes new DNA strand
DNA ligase	Joins Okazaki fragments

Key Equations and Concepts

• Base pairing: $A = T$, $G = C$

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

Conclusion

The discovery of DNA as the genetic material and the elucidation of its structure and replication mechanisms are foundational to modern biology. Understanding these processes is essential for further study in genetics, molecular biology, and biotechnology.