Study Guide: Molecular Genetics, DNA Structure, Replication, and the Genetics of Viruses and Bacteria

Exam 2 Study Guide: Molecular Genetics and Microbial Genetics

Overview

This study guide covers key concepts in molecular genetics, including the structure and replication of DNA, the central dogma of molecular biology, gene expression, and the genetics of viruses and bacteria. It is designed to help students prepare for a college-level General Biology exam.

1. DNA as the Genetic Material

Historical Experiments and Evidence

• Hypotheses Before DNA: Early scientists debated whether proteins or DNA carried genetic information.

• Key Experiments:

  • Griffith's Experiment: Demonstrated transformation in bacteria, suggesting a 'transforming principle.'

  • Avery, MacLeod, and McCarty: Identified DNA as the transforming principle.

  • Hershey-Chase Experiment: Used bacteriophages to show DNA, not protein, is the genetic material.

• Model Organisms: Bacteriophages (viruses that infect bacteria) were crucial in these discoveries.

Additional info: These experiments laid the foundation for understanding DNA as the molecule of heredity.

2. Structure of DNA

Determining DNA Structure

• Double Helix: DNA is a double-stranded helix with antiparallel strands.

• Polynucleotide: DNA is a polymer of nucleotides.

• Nucleotide Structure: Each nucleotide consists of a phosphate group, deoxyribose sugar, and a nitrogenous base.

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

• Base Pairing: A pairs with T, C pairs with G (complementary base pairing).

• 5' and 3' Ends: DNA strands have directionality, with a 5' phosphate end and a 3' hydroxyl end.

Example: The structure of DNA allows for accurate replication and storage of genetic information.

3. DNA Replication

Mechanism and Enzymes

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

• Bidirectional Replication: Replication proceeds in both directions from the origin.

• Replication Origins: Eukaryotic chromosomes have multiple origins; prokaryotes typically have one.

• Replication Bubble and Fork: The region where DNA is unwound and replication occurs.

• Key Enzymes:

  • Helicase: Unwinds DNA.

  • DNA Polymerase: Synthesizes new DNA strands.

  • Primase: Synthesizes RNA primers.

  • Ligase: Joins Okazaki fragments on the lagging strand.

• Proofreading: DNA polymerase corrects errors during replication.

Equation: $\text{DNA}_{\text{parent}} \rightarrow \text{DNA}_{\text{daughter}} + \text{DNA}_{\text{daughter}}$

4. Central Dogma of Molecular Biology

Flow of Genetic Information

• Central Dogma: Information flows from DNA to RNA to protein.

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated into a polypeptide (protein).

Equation: $\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

5. Genes and the Genetic Code

Gene Structure and Expression

• Gene: A segment of DNA that codes for a functional product (usually a protein).

• One Gene-One Polypeptide Hypothesis: Each gene encodes a single polypeptide.

• Genetic Code: The set of rules by which nucleotide sequences are translated into amino acid sequences.

• Codon: A sequence of three nucleotides that specifies an amino acid.

• Redundancy: Multiple codons can code for the same amino acid.

Codon	Amino Acid
AUG	Methionine (Start)
UAA, UAG, UGA	Stop codons
UUU, UUC	Phenylalanine
Additional info: ...	... (see full codon table in textbook)

6. Transcription

Process and Regulation

• Initiation: RNA polymerase binds to promoter region.

• Elongation: RNA polymerase synthesizes RNA in the 5' to 3' direction.

• Termination: RNA synthesis ends at terminator sequence.

• Differences in Prokaryotes and Eukaryotes: Eukaryotic transcription involves RNA splicing, addition of 5' cap and 3' poly-A tail.

7. Translation

Protein Synthesis

• tRNA: Transfers amino acids to the ribosome during translation.

• Ribosome: Site of protein synthesis, composed of rRNA and proteins.

• Steps:

  1. Initiation

  2. Elongation

  3. Termination

• Polyribosome: Multiple ribosomes translating a single mRNA simultaneously.

8. Mutations

Types and Effects

• Silent Mutation: No change in protein function.

• Missense Mutation: Changes one amino acid.

• Nonsense Mutation: Introduces a premature stop codon.

• Frameshift Mutation: Insertion or deletion alters reading frame.

9. The Genetics of Viruses

Virus Structure and Replication

• Virus: An infectious particle composed of nucleic acid (DNA or RNA) and a protein coat.

• Phage: A virus that infects bacteria.

• Lytic vs. Lysogenic Cycle: Lytic cycle destroys host cell; lysogenic cycle integrates viral DNA into host genome.

• Animal Viruses: Can be DNA or RNA viruses; some are retroviruses (e.g., HIV).

• Emerging Viruses: Newly identified viruses that cause disease outbreaks.

• Prions: Infectious proteins causing neurodegenerative diseases.

10. The Genetics of Bacteria

Gene Transfer and Plasmids

• Gene Transfer Methods:

  • Transformation: Uptake of naked DNA from environment.

  • Transduction: Transfer by bacteriophages.

  • Conjugation: Direct transfer via cell-to-cell contact.

• Plasmids: Small, circular DNA molecules in bacteria; can carry antibiotic resistance genes.

• F Factor: A plasmid that enables conjugation.

Additional info: Understanding bacterial genetics is crucial for biotechnology and medicine, especially regarding antibiotic resistance.