Bacterial Genetics Review

Bacterial Genetics Notes

Learning Objectives

  • Understand the structure and replication of genomes.
  • Learn the function of genes related to transcription and translation.
  • Know how genotype relates to phenotype.
  • Understand the steps of replication, transcription, and translation.

Bacterial Genomes

  • Composed of:
    • Chromosomal DNA
    • Extra-chromosomal DNA: Plasmids
    • Plasmids can enter or exit bacterial cells without altering the species.
    • Carry genes essential for survival in specific environments, including:
      • Virulence genes (disease properties)
      • Antibiotic resistance genes
      • Genes for conjugation pili
      • Toxin genes
      • Enzymes

Structure of DNA

  • DNA is a polymer of nucleotides.
  • Key findings by Watson and Crick in the 1950s:
    • Chargaff's Rules:
    • Amount of Adenine (A) = Thymine (T)
    • Amount of Guanine (G) = Cytosine (C)
    • Rosalind Franklin's X-ray crystallography revealed the X-pattern of DNA, indicating a double helix.

DNA Replication

  • Overview: Replication makes an exact copy of DNA.

    • Semiconservative Replication:
    • Each new DNA molecule consists of one old (parent) strand and one new strand.
    • The parent strand serves as a template.

  • Key Vocabulary:

    • Origin of Replication: A region rich in A and T where replication begins.
    • Helicase: Enzyme that unwinds the DNA by breaking hydrogen bonds.
    • Replication Bubble: Formed when DNA is unwound; contains replication forks where the DNA is still paired.
    • Replication Fork: The area at the edges of the replication bubble.

  • Leading and Lagging Strand Synthesis:

    • Antiparallel Elongation: DNA polymerases can only add nucleotides to the free 3' end.
    • Leading Strand is synthesized continuously.
    • Lagging Strand is synthesized discontinuously, forming Okazaki fragments.
Leading Strand Synthesis Steps:
  1. Helicase unwinds DNA at the origin, forming the replication bubble.
  2. Single Strand Binding Proteins prevent the strands from re-annealing.
  3. Topoisomerase prevents tangling ahead of the replication fork.
  4. Primase adds an RNA primer for DNA pol III to initiate replication.
  5. DNA Polymerase III synthesizes new DNA, proofreads, and links nucleotides.
  6. DNA Polymerase I replaces RNA primers with DNA nucleotides.
Lagging Strand Synthesis Characteristics:
  • DNA is produced in fragments (Okazaki fragments).
  • Each fragment requires an RNA primer, removed and replaced with DNA by DNA Pol I.
  • Ligase connects Okazaki fragments, creating a continuous strand.

Central Dogma of Genetics

  • Explains how genetic information flows from DNA to RNA to protein.
Transcription Process

  • Transcription: DNA to RNA, occurring in three phases:

    1. Initiation: RNA polymerase binds to DNA.
    2. Elongation: RNA polymerase synthesizes RNA by adding nucleotides.
    3. Termination: RNA polymerase disassociates from the DNA when a termination signal is reached.

  • Prokaryotic vs. Eukaryotic Transcription:

    • Prokaryotic occurs in the cytoplasm; Eukaryotic in the nucleus.
    • Eukaryotic pre-mRNA is processed (introns removed, exons joined) and modified (5' cap and 3' poly-A tail added).

Translation Process

  • Translation: Converting mRNA sequences into polypeptides (proteins).
  • Key Players:
    • mRNA (messenger RNA):
    • Carries genetic instructions from DNA for protein synthesis.
    • tRNA (transfer RNA):
    • Carries amino acids to the ribosome; each has an anticodon complementary to mRNA codon.
    • Ribosomes:
    • Site of protein synthesis; composed of rRNA and proteins.
Translation Steps:
  1. Initiation: Start codon (AUG) pairs with tRNA in the ribosome's P site.
  2. Elongation: Ribosome shifts, and amino acids are linked by peptide bonds.
  3. Termination: Stop codon reached, release factors disassemble the translation complex.
Genetic Variability in Bacteria
  • Bacteria reproduce asexually yet exhibit variability through:
    1. Mutations: Changes in DNA sequence impacting phenotype.
    2. Horizontal Gene Transfer: Exchange of genetic material between organisms.
    3. Regulation of Gene Expression: Adjusting the expression of genes based on cellular needs.
Types of Mutations:
  • Point Mutations: Substitution of a single nucleotide, leading to silent, missense, or nonsense mutations.
  • Insertion/Deletion Mutations: Addition or loss of nucleotides can cause frameshift mutations affecting protein structure.
Horizontal Gene Transfer Mechanisms:
  • Transformation: Uptake of DNA from the environment.
  • Transduction: Gene transfer via bacteriophages.
  • Conjugation: Direct transfer between bacterial cells through contact.

Gene Regulation

  • Operons: Groups of genes regulated together.
    • Include:
    • Promoter: Start site for transcription.
    • Operator: Site where a repressor binds to prevent transcription.