Bacterial Genetics

Prokaryotic Cell• Circular (and

naked) double stranded DNA

• Bacteria have very short generation spans (ex. E.coli divides every 20 minutes)

Plasmid – small circular DNA found in some bacteria

– Contain a few accessory genes (e.g. antibiotic resistance)

– NOT required for bacterial cell reproduction or survival

Side Note

Bacterial Cell Reproduction• Binary fission – asexual (production of offspring

from one parent) process for prokaryotic cell division

• Each fission results in two daughter cells each with 1 copy of the original chromosome

Why bother?• Bacteria rapidly reproduce which leads to

increase in genetic diversity (due to mutations that may or may not confer advantages to offspring)– 9 million mutations per day per human host– Beneficial mutations allow bacteria to continue

to survive in host and contribute to evolution of bacterial populations

Mechanisms for gene transfer:

• Mutations cannot account for all bacterial diversity

• Bacteria have three mechanisms for exchanging genetic information between themselves:– 1. Transformation– 2. Transduction– 3. Conjugation

• These exchanges create new strains of bacteria

1. Transformation

• Bacterium takes up DNA from environment

2. Transduction

• Viruses (bacteriophage) transfer genes between bacteria

3. Conjugation

• Genes are directly transferred from one bacterium to another via “mating bridge”

Transposable Elements

• In addition to the transfer of genetic material (DNA) b/n bacterial cells, DNA of a single bacterium can undergo recombination via transposable elements

• Essentially, DNA “jumps” from one part of chromosome to another

Regulation of gene expression• Individual bacterium have to deal with

fluctuations in host environment so they have developed levels to control gene expression

• 2 types of genes in bacteria:– Regulatory genes – produce proteins that

control gene expression – Structural genes – produce proteins that do

something (e.g. break down lactose)

2 Levels for Metabolic Control1. Use allosteric control (e.g. feedback inhibition)

– Ex: Negative feedback to stop production of tryptophan after thanksgiving dinner

2. Regulate expression of genes using an operon system

Operons• Prokaryotic genes are grouped into functional

units called operons

3 parts to an operon

1. Operator – controls access of RNA polymerase to the promoter

2. Promoter – where RNA polymerase attaches to begin transcription of genes

3. Genes – code for expression of proteins related to one particular function (e.g. breaking down galactosidase)

Regulatory gene

• Located away from operon itself

• Produce repressor protein that is able to bind to operator and prevent RNA polymerase from attaching to promoter

2 types of operons

1. Repressible – normally on but can be inhibited (i.e. IS REPRESSIBLE)

– Usually anabolic (building essential molecules)– Repressible protein (coded for by the regulatory

gene) is inactive– If the essential molecule is present, it binds to the

repressible protein, activates it, and this activated protein binds to the operator and turns operon OFF (blocks RNA polymerase thus REPRESSED!)

2. Inducible – normally off but can be turned on (i.e. IS INDUCIBLE)

– Usually catabolic (breaking down food for energy)

– Repressible protein is active– Inducer (small molecule) binds to & inactivates

the repressible protein, preventing it from binding to the operator. Now RNA polymerase transcribes the genes (INDUCED!)

Lac Operon (No lactose present)

You drink milk (contains lactose) and…

Videos

• Lac Operon (inducible)

• Trp Operon (repressible)