Viral and Bacterial Genomes

Review of Viruses

Are Viruses Alive?

• Contain genetic material (DNA or RNA)

• Cannot live outside of a cellular host

• Do not contain ribosomes• Do not carry out any type of

respiration• Are not made of cells

Virus-Host Specificity• Viruses have different shapes that

are specific to the attachment sites of their host cells

• Most viruses are species & cell specific

• Examples, adenovirus—respiratory cells; polio—nerve cells; HIV—T-cells

• Some viruses (such as rabies) can cross species

Viral Reproduction—Lytic vs. Lysogenic Cycles

• Lytic cycle– Attachment– Entry– Destruction of host cell DNA– Production of viral proteins & viral

DNA– Assembly of viruses– Lysis of Cell and Viral release

Lysogenic Cycle

• Attachment• Entry• Integration of Viral DNA in host

chromosome• Reproduction of host cell and viral DNA• “Activation” of viral DNA• Destruction of host cell DNA• Production of viral proteins & viral DNA• Assembly of viruses• Lysis of Cell and Viral release

Two Types of Viruses

• DNA Viruses• RNA Viruses

– RNA serves as mRNA– RNA serves as template for mRNA– RNA serves as template for DNA– These viruses are called

RETROVIRSES– HIV is an example of a RETROVIRUS

Reproduction of HIV

Reproduction of HIV

“Emerging Viruses”

• Viruses that cause NEW diseases• HIV• Ebola• Hantavirus• Nipah virus• Influenza Virus

Viroids

• Pieces of circular RNA that infect plants

• They reproduce and disrupt the metabolism of plant cells and stunt the growth of the entire plant

Prions

• An infectious protein molecule• Cause a number of degenerative

brain diseases including “mad cow disease”

Bacterial Genomes

DNA of Bacteria

• One circular chromosome• The dense region of the DNA is

called the nucleoid—not membrane bound

• Also, may have one or more plasmids

Bacterial Reproduction

• Bacteria reproduce by binary fission

• This asexual reproduction results in a colony of “clones”

• However, because of their fast proliferation (every 20 minutes for E. coli), rare mutations still result in significant bacterial diversity

Genetic Recombination in Bacteria

• Transformation• Transduction• Conjugation

Transformation

• Remember Fred Griffith!• Alteration of the bacterial cell’s

genotype by the uptake of naked, foreign DNA from the environment

Transduction

• Phages carry bacterial genes from one host cell to another

Conjugation

• Direct transfer of genetic material between two bacterial cells that are temporarily joined.

• The bacterial version of sex

Plasmids

• Small, circular, self-replicating DNA molecules separate from the bacterial chromosome.

• Has only a small number of genes• These genes not required for the

survival & reproduction of the bacterium

• However, they can give bacteria an advantage for survival

Most Famous Plasmids

• F plasmid—25 genes, most required for the sex pili

• R plasmids—contain genes conferring antibiotic resistance

Transposons

• A piece of DNA that can move from one location to another in a cell’s genome.

• Sometimes called “jumping genes”

• The process “scatters” genes throughout the genome

Metabolic Control of Gene Expression in Bacteria

1. Cells can vary the numbers of specific enzyme molecules made, i.e. they can regulate the expression of genes

2. Cells can adjust the activity of enzymes already present

Operons• E. coli synthesizes tryptophan in five steps,

each step catalyzed by a specific enzyme• The genes for the five enzymes are

clustered together on the bacterial chromosome

• A single promoter serves all five genes• Transcription gives rise to one mRNA

molecule that represents all five genes of the pathway with start and stop codons throughout

Operons

• A single “on-off” switch controls the whole cluster of related genes

• The switch is called the OPERATOR

• It is positioned within the promoter and controls access of RNA polymerase to the genes

• The operator plus promoter plus genes is called an OPERON

Control of Operons

• The Operator is normally “open” meaning the series of genes can be transcribed

• It can be turned off with a REPRESSOR—a protein that binds to the operator and blocks attachment of RNA polymerase

• This regulatory protein’s gene is located away from the operon

Control of Operons• The regulatory protein is transcribed

continuously and there are always some of these in the cell

• However, they are allosteric proteins• In their “normal” shape, they will not bind

to the operator• However, when the end product of the

operon pathway (i.e. tryptophan) attaches to the allosteric site, the shape changes and it will bind to the operator and switch it off

The lac Operon

The lac Operon