The Molecular Basis of Inheritance and Gene Expression: Study Guide

The Molecular Basis of Inheritance

Introduction

The molecular basis of inheritance explores how genetic information is stored, replicated, and transmitted in living organisms. This topic focuses on the structure and function of DNA, the experiments that established DNA as the genetic material, and the mechanisms of DNA replication and chromosome organization.

Key Historical Contributions

• Rosalind Franklin: Used X-ray crystallography to reveal the helical structure of DNA.

• James Watson & Francis Crick: Proposed the double helix model of DNA structure, explaining base pairing and antiparallel strands.

Experiments Demonstrating DNA as Genetic Material

1. Griffith's Transformation Experiment: Showed that a "transforming principle" could transfer genetic traits between bacteria.

2. Avery, MacLeod, and McCarty: Identified DNA as the "transforming principle" responsible for heredity.

3. Hershey-Chase Experiment: Used bacteriophages to confirm that DNA, not protein, is the genetic material.

Types and Functions of Nucleic Acids

• DNA (Deoxyribonucleic Acid): Stores genetic information; double-stranded.

• RNA (Ribonucleic Acid): Involved in protein synthesis; usually single-stranded.

Structural Differences: DNA vs. RNA

• Sugar: DNA contains deoxyribose; RNA contains ribose.

• Nitrogenous Bases: DNA: Adenine (A), Thymine (T), Cytosine (C), Guanine (G); RNA: Adenine (A), Uracil (U), Cytosine (C), Guanine (G).

• Strandedness: DNA is double-stranded; RNA is single-stranded.

Formation of Nucleic Acid Strands

• Condensation (Dehydration) Synthesis: Nucleotides are joined by phosphodiester bonds, releasing water.

• Directionality: Strands have a 5’ (phosphate) end and a 3’ (hydroxyl) end.

DNA Double Helix Structure

• Antiparallel Strands: Two strands run in opposite directions (5’ to 3’ and 3’ to 5’).

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

• Double Helix: Twisted ladder structure stabilized by base pairing and hydrophobic interactions.

DNA Replication

• Semiconservative Replication: Each new DNA molecule consists of one old and one new strand.

• Origin of Replication: Specific sequence where replication begins, forming a replication bubble.

• Replication Fork: Y-shaped region where new DNA strands are synthesized.

Key Enzymes and Proteins in DNA Replication

• Helicase: Unwinds the DNA double helix.

• Single-Strand Binding Proteins: Stabilize unwound DNA.

• Topoisomerase: Relieves supercoiling ahead of the replication fork.

• Primase: Synthesizes RNA primers to initiate DNA synthesis.

• DNA Polymerase III: Adds nucleotides to the growing DNA strand.

• DNA Polymerase I: Replaces RNA primers with DNA nucleotides.

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

Leading vs. Lagging Strand Synthesis

• Leading Strand: Synthesized continuously toward the replication fork.

• Lagging Strand: Synthesized discontinuously in short segments called Okazaki fragments, away from the fork.

• Okazaki Fragments: Short DNA segments on the lagging strand, later joined by DNA ligase.

Replication Challenges and Solutions

• RNA Primer Removal: DNA polymerase I replaces RNA primers with DNA, except at the 5’ end of the leading strand.

• Telomeres: Repetitive DNA sequences at chromosome ends; protect against loss of genetic information.

• Telomerase: Enzyme that extends telomeres in germ cells.

DNA Repair Mechanisms

• Proofreading: DNA polymerases correct errors during replication.

• Mismatch Repair: Enzymes fix incorrectly paired bases.

• Excision Repair: Damaged DNA is removed by nucleases and replaced.

Chromosome Structure and Organization

• Chromatin: DNA-protein complex; can be euchromatin (less condensed, active) or heterochromatin (highly condensed, inactive).

• Histones: Proteins around which DNA winds to form nucleosomes ("beads on a string").

• Levels of DNA Packing: 10 nm fiber (nucleosomes), 30 nm fiber (coiled nucleosomes), higher-order structures.

• Chromosome vs. Chromatid: Chromosome is a single DNA molecule; chromatid refers to one of two identical halves after replication.

Key Terms Table

Term	Definition
Transformation	Change in genotype and phenotype due to assimilation of external DNA
Bacteriophage	Virus that infects bacteria
Chargaff’s Rules	In DNA, %A = %T and %C = %G
Okazaki Fragment	Short DNA fragment on lagging strand
Telomere	Repetitive DNA at chromosome ends
Histone	Protein for DNA packaging

Gene Expression: From Gene to Protein

Introduction

Gene expression is the process by which information from a gene is used to synthesize functional gene products, typically proteins. This involves two main stages: transcription and translation, governed by the central dogma of molecular biology.

Central Dogma and Genetic Code

• Central Dogma: DNA → RNA → Protein

• Triplet Code: Three-nucleotide sequences (codons) in mRNA specify amino acids.

• Genetic Code Properties: Redundant (multiple codons for one amino acid), unambiguous (each codon specifies one amino acid), nearly universal.

Gene Structure and Function

• Gene: DNA region encoding a functional product (protein or RNA).

• Protein Structure: Polypeptide chains folded into specific 3D shapes; enzymes are proteins that catalyze reactions.

Transcription: DNA to RNA

• Location: Nucleus (eukaryotes)

• Enzyme: RNA polymerase synthesizes RNA from DNA template.

• Promoter: DNA sequence where RNA polymerase binds (includes TATA box).

• Steps:

  1. Initiation: Transcription factors and RNA polymerase assemble at promoter.

  2. Elongation: RNA polymerase synthesizes RNA in 5’ to 3’ direction.

  3. Termination: RNA polymerase stops at terminator sequence; RNA transcript released.

RNA Processing (Eukaryotes)

• Primary Transcript: Initial RNA copy (pre-mRNA).

• Processing Steps:

  1. Addition of 5’ cap (modified guanine nucleotide)

  2. Addition of poly-A tail (adenine nucleotides) at 3’ end

  3. RNA splicing: Removal of introns, joining of exons

  4. Export of mature mRNA to cytoplasm

• Spliceosome: Complex of snRNPs and proteins that removes introns.

• Alternative Splicing: Allows one gene to code for multiple proteins.

Translation: RNA to Protein

• Location: Cytoplasm at ribosomes

• Key Players: mRNA (messenger), tRNA (transfer), rRNA (ribosomal)

• Ribosome Structure: Large and small subunits; sites: A (aminoacyl), P (peptidyl), E (exit)

• Steps:

  1. Initiation: Small ribosomal subunit binds mRNA; initiator tRNA binds start codon (AUG).

  2. Elongation: tRNAs bring amino acids; peptide bonds form; ribosome moves along mRNA (A → P → E sites).

  3. Termination: Stop codon reached; release factor binds; polypeptide released.

• Polyribosomes: Multiple ribosomes translating one mRNA simultaneously.

Genetic Code Relationships

• DNA Triplet: Template for mRNA codon.

• mRNA Codon: Specifies amino acid.

• tRNA Anticodon: Base-pairs with mRNA codon.

Mutations and Their Effects

• Point Mutation: Change in a single nucleotide (substitution, insertion, deletion).

• Silent Mutation: No change in amino acid sequence.

• Missense Mutation: Changes one amino acid.

• Nonsense Mutation: Introduces a stop codon.

• Frameshift Mutation: Alters reading frame (insertion/deletion not in multiples of three).

• Mutagen: Physical or chemical agent causing mutations.

Summary Table: Types of RNA and Their Functions

RNA Type	Function
mRNA	Conveys genetic information from DNA to ribosome
tRNA	Brings amino acids to ribosome during translation
rRNA	Structural and catalytic component of ribosomes

Protein Folding and Modification

• Polypeptide Folding: Newly synthesized polypeptides fold into functional proteins, sometimes with help from chaperone proteins.

• Post-translational Modifications: May include cleavage, addition of carbohydrates, lipids, or phosphate groups.

Additional info:

• Beadle and Tatum’s experiments with Neurospora crassa led to the "one gene-one enzyme" hypothesis, foundational for understanding gene function.

• Chromatin remodeling is essential for regulating gene expression by altering DNA accessibility.