Lecture_geneticsII

Microbial Genetics Overview

Part II

• Focus on the mechanisms underlying microbial genetics and the implications of mutations, gene transfer, and recombination.

Mutations

What Happens When Things Go Wrong?

• HBB Gene Mutations:

  • Original sequence: GGACTCCTC

  • Example of a point mutation: HbSS

    • Change: GGACACCTC

    • Amino Acids: PRO -> VAL

  • Point mutations can lead to various disorders, including Cystic Fibrosis and Huntington's Disease.

Point Mutations

• Nonsense Mutation:

  • Removes the amino acid sequence and creates a stop codon.

  • Example: CAGCAGCAG (normal) vs. CAG repeats (mutated).

• Missense Mutation:

  • Alters a single amino acid.

  • Example: A change from cysteine to tyrosine can alter neuronal function.

Definitions

• Mutation: Structural change in DNA base sequences. Can alter the resultant proteins.

• Mutagen: Agents that induce mutations.

• Wild Type: The typical form of organisms found in nature.

Causes of Mutations

• Spontaneous Mutations: Occur naturally without external influence.

• Induced Mutations: Caused by environmental factors (mutagens).

• Mutation Rate: Approximately 1 in 10^8 nucleotides in bacteria.

  • Reversion: A new mutation that reverses an old one.

Effects of Mutations in Organisms

1. Point Mutation/Base Substitution: Affects single bases, which can lead to:

   • Missense mutation: Amino acid change

   • Nonsense mutation: Stop codon generation

   • Silent mutation: No change in protein function

2. Insertion/Deletion: Changes reading frame leading to:

   • Frameshift mutation: Entire downstream sequence altered.

Chemical Mutagens

• Alter Base Structures:

  • Example: Nitrous acid (A to G transitions).

• Nucleoside Analogs: Compete with normal bases during replication.

  • Example: 2-Aminopurine replaces adenine but can pair with cytosine.

• Intercalating Agents: Inserts itself between DNA base pairs, potentially causing mutations.

  • Example: Ethidium bromide.

Radiation Effects

1. Ionizing Radiation: Breaks DNA molecules (e.g., X-rays).

2. Nonionizing Radiation: Causes thymine dimers (e.g., UV light).

Thymine Dimers

• Result from adjacent thymines cross-linking due to UV radiation, altering DNA structure.

DNA Repair Mechanisms

• DNA Polymerase: Has proofreading capabilities to correct errors during DNA replication.

• Repair Methods for Thymine Dimers:

  1. Light Repair: Uses photolyases in the presence of light.

  2. Nucleotide Excision Repair: Enzymatic removal of damaged sections.

Ames Test

• A method for assessing the mutagenic potential of chemicals using Salmonella His-145 strains. Positive results indicate increased mutation rates.

Genetic Transfer and Recombination

Gene Transfer Methods

• Vertical Gene Transfer: From parent to offspring.

• Horizontal Gene Transfer: Through mechanisms such as Transformation, Conjugation, and Transduction.

Transformation Example

• Griffith's Experiment: Demonstrated transformation using smooth (S) and rough (R) strains of Streptococcus pneumoniae.

Conjugation Mechanism

• Occurs through a sex pilus enabling transfer of gene material between bacteria (F+ to F- cells).

Transduction

• Mechanism of gene transfer involving bacteriophages that incorporate bacterial DNA.

Plasmids

• Circular, self-replicating DNA molecules aside from chromosomal DNA that may carry beneficial genes, including antibiotic resistance.

Antibiotic Resistance Factors

• Key causes include over-prescribing, incomplete courses, and agricultural use of antibiotics.

Regulation of Gene Expression

Operon Model

• Operon: Regulatory unit comprising an operator, promoter, and genes whose expression it controls.

• Example: Lac Operon

  • Regulates lactose metabolism based on glucose levels and lactose presence.

Growth Rate of E. coli

• E. coli growth shows variances based on carbon source availability (glucose vs. lactose).

Summary of Topics

• Key Areas Covered:

  • Replication, Transcription, Translation, Mutations, Genetic Recombination, Gene Regulation, Movement of Genetic Information (Transformation, Conjugation, Transduction).