Bacterial Genetics

Elements of the bacterial genome

Mechanisms of genetic exchange

chromosome

plasmids

bacteriophages

insertion sequences

transposons

Most bacteria are haploid (one copy of the chromosome), so mutations are common

Mutations

can change a bacterial gene so that the protein is no longer affected by an antibiotic, yet still retains its function

Completely new genes are not constructed by a single point mutation

transformation - uptake of naked DNA

transduction - bacteriophage as vectors

conjugation - plasmids moved by cell-cell contact

The recipient cell must be competent for uptake of DNA

natural competence

artificial competence (lab only)

Only certain bacteria are naturally transformable

Bacteriophage (phage) = virus of bacteria.

lytic - always lyse (kill) host bacterial cell

temperate - can stably infect and coexist within bacterial cell (lysogeny) until a lytic phase is induced or just lyse immediatley

lysogeny

phage genome = prophage

bacterial cell = lysogen

lysogenic conversion - phage encodes observable function

Specialized Transduction Example

  1. prophage integrates into the bacterial genome
  1. prophage may drag along a piece of the bacterial genome into the next integration site
  1. That sequence is added into the next bacteria's genome, changing their genome

Generalized transduction

  1. When a phage lyses the host bacterial cell, it normally packages phage genome into the capsid
  1. Sometimes the capsid is accidentally filled with random pieces of bacterial genome
  1. When the capsid injects to a new cell, the new genes can recombine into the recipient's genome and cause change

Virulence and antibiotic resistance genes can be moved by generalized transduction

image

conjugative

mobilizable

non-transmissible

can't move by conjugation

plasmid can move itself from donor cell to recipient cell

plasmid can not move itself, but can be moved

Mechanism

  1. Pilus forms
  1. cell-cell contact
  1. copying plasmid DNA and transfer of copy into recipient cell

more common method of sharing antibiotic resistance genes than phages - can move multiple at once

Move DNA to other places within one cell

Move DNA to other places within one cell

Transposase recognizes inverted repeats and moves the IS

Cut and paste

Gene encoding an observable function flanked by two copies of an insertion sequence - can especially transfer antibiotic resistance

Know the difference!

Phase variation and Antigenic variation

Phase Variation

Binary switch - something goes from on to off, or A to B

Antigenic variation

Caused by a flipping of DNA

Change in sequence with multiple possible outcomes

Cassette model

silent copies of the gene of interest are recombined to a site where the cassettes (genes) can be expressed