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).


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