The Genetics of Viruses & Bacteria

Ch. 15

Warm up: What do the highlighted terms mean? What do the highlighted terms mean?

1. She was 2 weeks overdue, so her doctor gave her some medication to induce labor.

2. The uprisings in Singapore were repressed by government forces.

Bacterial Diversity

Ch. 15

The bacterial chromosome One double stranded

circular DNA molecule DNA is tightly coiled into a

nucleoid Many bacteria also have

plasmids

E. coli has ~ 4.6 million base pairs (4400 genes)

Bacterial replication Bacteria divide by binary

fission DNA is copied first Very fast

Genetic variation in bacteria

Since bacterial reproduction is asexual, how do bacteria become genetically diverse and thus adapt to changing environments?

1. Mutation - rare But…that can be as many as 9 million new mutants each day

in a single host because they reproduce so quickly and are so numerous

2. genetic recombination - combining DNA from 2 sources

This occurs through A.) Transformation B.)Transduction C.)Conjugation

Transformation The uptake of naked, foreign DNA from

the surrounding environment that alters the bacterial genotype and phenotype

Transduction Occurs when a phage (virus)

carries bacterial genes from one cell to another

Sometimes when the phage is being formed, it picks up pieces of the bacterial DNA

When it infects a new cell, this DNA is transferred

Conjugation Bacterial “sex” The direct transfer of genetic material between

2 cells that are temporarily joined The “male” has a sex pilus that joins the 2 cells

together Coded for by an F factor gene in the DNA

Either on the main chromosome or in a plasmid F factor plamids can integrate into the chromosome

and back out again• These are called episomes

The F plasmid Made up of ~ 25 genes Cells with the plasmid called F+ cells

They donate DNA during conjugation The F+ cell donates one parental strand,

keeping the other, so it stays F+

R plasmids Confer resistance to certain antibiotics

Many R plasmids, like F plasmids, also have genes that encode for a sex pilus, making sharing of the plasmids possible

Quick think: Explain why there is so much bacterial diversity

Regulation of gene Regulation of gene expression in bacteriaexpression in bacteria

Bacteria can adapt to new conditions

Transfer of genes, through plasmids, transposons, etc, can help a population of bacteria adapt and survive over time

This is natural selection What about the individual What about the individual

bacteria? How does it adapt bacteria? How does it adapt to changing conditions?to changing conditions?

How individual bacteria adapt By controlling metabolism

Allows the bacteria to respond to and adapt to its environment

They can turn on genes when needed and turn off genes when not needed

Saves energy Can you think of examples of

how you and I respond to and adapt to our environment?

How bacteria regulate their metabolism

1. Feedback inhibition (aka negative feedback) - the end product of a pathway turns off the pathway when the end product levels are high

Here tryptophan is the end product of this metabolic pathway. When it gets built up, it shuts down the pathway by inhibiting the first enzyme

2. Regulation of gene expression (aka regulation of the DNA)

Cells can adapt by turning on or off certain genes as needed

Here the buildup of tryptophan shuts down the production of the enzymes necessary for making tryptophan

Operons - how genes are turned on & off

Operon = 1. Promoter region where RNA pol binds to DNA to being transcription2. Operator region – like the on/off switch for that stretch of DNA3. Genes for the enzymes the bacteria needs

This whole thing together is an operon

Some operons are ALWAYS turned on

The operator is always open so DNA mRNA proteins

UNLESS

The cell is in an environment where the proteins are not needed

If enough of the protein is already present in the environment, the protein acts as a repressor repressor molecule molecule BLOCKS the operator

Now the cell will NOT do DNAmRNAproteins

(it doesn’t need to. There are already plenty of those proteins present!)

This is an example of a Repressible Operon The trp operon trp operon is a repressible operon

It is usually on but can be repressed when the trp repressor protein binds to tryptophan and they block the operator

An inducible operon

Some operons are ALWAYS turned offoff The operator is always

blocked so no DNA mRNA proteins

UNLESS

The cell is in an environment where those proteins are needed

We call this an inducible inducible operonoperon

If the bacteria is in the presence of sugars, it will UNBLOCK the operator that codes for the enzymes to digest those sugars.

The presence of the sugar INDUCES the operon.

Inducible vs repressible enzymes

Inducible enzymes Usually function in catabolic

(breakdown) pathways

Repressible enzymes Usually function in anabolic

(buildup) pathways

Make a model Use the materials provided to make a model of an

operon Include:

DNA Promoter region on DNA Operator on DNA RNA polymerase mRNA Genes 1, 2, 3 on DNA Enzymes (proteins) 1, 2, 3 made from genes 1, 2, 3 Repressor molecule to block operator Repressor or an inducer molecule to change shape of

repressor molecule

Explain your model and use it to demonstrate both types of operons

1. Identify all key parts

2. Show how the operon works when it is “on”

3. Show how the operon can be turned “off”