THE GENETICS OF BACTERIA

Bacteria Are Prokaryotes

Electron Micrograph of E. coli

The Three Domains of Life

Genetic Variation Introduced By

• Mutation

• Conjugation

• Transduction

• Transformation

What is a bacterial genome like?

• The chromosome is a loop of DNA in the nucleoid region of the bacterial cell.

• The DNA is not associated with histone proteins and doesn’t contain introns.

• Typical size is about 4000-5000 genes consisting of approx. 4-5 Mbp.

• Some bacteria contain plasmids: Small loops of DNA with a few to several dozen genes. (Plants and Yeast also have plasmids)

Mutation Provides Genetic Variation in Bacteria

• Bacteria reproduce asexually by binary fission.• E.coli reproduces every 20 min. in log phase. 1 x

108 bacteria in 12 hrs.• The chance of a spontaneous mutation occurring

in a gene is about 1 in 10 million• 2 x 1010 bacteria are produced in the human colon

each day.• The result: 2,000 mutated bacteria each day

Replication of Bacterial Chromosome

Transformation

• Alteration of a bacterial cell’s genotype by the uptake of naked, foreign DNA from the surrounding environment.

• Facilitated by special proteins on the cell wall of some bacteria.

• High concentrations of Ca2+ can stimulate E. coli to take up plasmid DNA.

• Used extensively in biotechnology.

Transduction - Generalized

• Phage viruses degrade bacterial chromosome during the lytic cycle.

• Capsids incorporate bacterial DNA instead of phage DNA.

• Phage infects another bacterium and injects DNA from previous bacterium.

• DNA is incorporated into the homologous region of 2nd bacterium. (Generalized Transduction.)

Transduction - Specialized

• Temperate viruses are lysogenic.

• Temperate viruses incorporate their DNA into the bacterial DNA as prophage.

• Prophage exits from bacterial DNA taking adjacent bacterial DNA with it.

• Virus infects another bacterium, inserts new DNA into infected cell as a prophage.

Conjugation

• Conjugation is a process of DNA exchange between two bacteria.

• Transfer is through temporary connecting tubes called pili (plural).

• Chromosomal DNA or plasmid DNA can be exchanged.

• Transfer is one way: “male” to “female”.

• F plasmid or factor required to make pili.

Conjugation

Plasmids• Small circles of DNA

• Self-replicating

• F plasmids can undergo reversible incorporation into bacterial chromosome.

• Episome = a genetic element that can act alone or as part of bacterial chromosome.

• R plasmids contain antibiotic resistance genes; as many a 10.

Transposons “Jumping Genes”

• Transposable elements – move from one location to another in a cell’s genome.

• Genetic elements may move by Cut-and-paste transposition.

• Replicative transposition – copies of transposon insert elsewhere without being lost from original site.

• Shown to exist in eukaryotic cells by Dr. Barbara McClintock.

E.Coli OPERONS ARE COMPOSED OF 4 PARTS

• Promoter – Region where RNA polymerase can bind

• Operator – Region past promoter that can block RNA polymerase

• Regulatory Gene – Produces a protein that can produce an active or inactive repressor protein

• Structural Gene(s) – Code for enzymes that direct the production of particular end product. Situated down-stream from operator.

The trp Operon (Active)A repressible operon

The trp Operon (Repressed)

The lac Operon (Repressed)An inducible operon

The lac Operon (de-repressed)

Positive Feedback Control

• The lac operon won’t work efficiently unless glucose is in very short supply.

• cAMP accumulates when glucose is low

• cAMP receptor protein, CRP, is an allosteric activator of transcription

• When CRP binds cAMP, it becomes active and increases RNA polymerase binding to the lac operon. Transcription increases.

Feedback Continued…

• If glucose levels increase, cAMP becomes scarce.• CRP becomes inactive and disengages from the

lac promoter.• Even if lactose (allolactose) is present, transcription

of the lac operon is slow.• Dual control exists for lac operon: negative control

by lac repressor and positive control by CRP• Negative Control of Genes = Operons switched off

by the active form of the repressor.

Lactose Present & Glucose Low

Lactose Present & Glucose High

Positive Feedback

• Gene regulation is said to be positive when the activator molecule interacts directly with the genome and switches the gene on.

• Active CRP is an activator.