Genetics of Viruses and Bacteria

Viral structure Virus: “poison” (Latin); infectious

particles consisting of a nucleic acid in a protein coat (there are MANY, MANY types of viruses)

Composition of virus Capsid: protein shell that encloses

the viral genome (the protein subunits are called capsomeres)

DNA or RNA that is inserted into infected cells

Examples of viruses

Virus structure (cont.)Other accessories for viruses/virus

types:Membranous envelope that

allows a virus to “fool” a cell membrane and allow the virus to enter the cell (viral envelope)

Bacteriophage (phage): viruses that are able to infect bacteria

General features of viral reproduction Viruses are intracellular parasites

They need a host cell to reproduce They lack enzymes, ribosomes and all

other machinery needed to make proteins

Viruses can only infect a limited range of cells (host range) This is why diseases are usually

species or tissue specific

Lytic Cycle The lytic cycle is a viral reproductive strategy that

results in the death of the host cell Attachment: virus binds to a specific receptor

site on the outer membrane Injection: the viral DNA/RNA is inserted into the

cell membrane Synthesis: the viral DNA directs the production of

viral proteins and the synthesis of viral nucleotides

Assembly: the synthesized viral material is assembled

Release: the viral particles are released from the organism, thereby destroying the host cell

Virulent virus (phage reproduction only by the lytic cycle)

Lytic cycle

Lysogenic Cycle Genome replicated w/o destroying the host cell Very similar to the lytic cycle Key differences:

Genetic material of virus becomes incorporated into the host cell DNA by recombination (uses crossing-over)at a specific chromosomal loci

The incorporated viral DNA is known as a prophage

Once the prophage synthesizes its material, it circulates in the cell

Temperate virus (phages capable of using the lytic and lysogenic cycles)

May give rise to lytic cycle

Lysogenic cycle

Animal Viruses Viruses that infect animals are extremely

varied They can be double stranded or single

stranded They can be made of DNA or RNA They can have an outer membrane (viral

envelope) or not PURPOSE: The reason for the extreme

variability in viral composition is to enter cells and utilize their reproductive machinery

Retroviruses (class of RNA VirusesRetroviruses: a class of RNA

virus that can use an RNA template to transcribe its nucleotides into the DNA template

Uses an enzyme called reverse transcriptaseOne deadly example of a

retrovirus is HIVThis is the virus that leads to

the disease known as AIDS

Retrovirus (HIV)

HIV (cont.)Unlike a prophage in bacteria,

the integrated viral DNA (provirus) is a permanent part of the cells genotype

The cell will continue to synthesize the virus for the life of the cell

How do we fight viruses?Viruses are extremely damaging

They utilize our own cellular machinery to produce, infect and destroy our own cells

With the creation of vaccines (harmless variants of pathogenic microbes), we can condition our body to destroy the infection before it can result in illness

Why do we still have viruses? With the advent of vaccination, a lot of

diseases have become extinct (polio or small pox)

Yet, viruses have a high level of mutation They are constantly changing to “fool” your

bodies immune system Even the influenza virus (flu) mutates every

year so that you must get a new flu vaccine each season

Also we do not understand enough about some viruses to create a vaccine

Viroids and prions Viroids: tiny, naked

circular RNA that infect plants; do not code for proteins, but use cellular enzymes to reproduce; stunt plant growth

Prions: “infectious proteins”; “mad cow disease”; trigger chain reaction conversions; a transmissible protein

Bacterial genetics Nucleoid: region in

bacterium densely packed with DNA (no membrane)

Plasmids: small circles of DNA (separate from bacterial genome)

Reproduction: binary fission (asexual)

Bacterial DNA-transfer processes Transformation: genotype alteration by the

uptake of naked, foreign DNA from the environment

Transduction: phages that carry bacterial genes from 1 host cell to another Generalized: random transfer of host cell

chromosome Specialized: incorporation of prophage DNA

into host chromosome Conjugation: direct transfer of genetic material;

cytoplasmic bridges; pili; sexual

Bacterial PlasmidsSmall, circular, self-replicating

DNA separate from the bacterial chromosome

F (fertility) Plasmid: codes for the production of sex pili (F+ or F-)

R (resistance) Plasmid: codes for antibiotic drug resistance

Transposable elements Transposable elements: nucleotide

sequences that can move from one site in a chromosome or plasmid to another site

Insertion sequence: (only in bacteria) can move one gene from one site to another

Transposons: transposable genetic element; piece of DNA that can move from location to another in a cell’s genome (chromosome to plasmid, plasmid to plasmid, etc.); “jumping genes” This allows genetic information to be

incorporated or passed on to other bacteria

Incorporation of a plasmid

Operons (the basic idea) For many proteins, there is a segment of DNA where

all of the necessary genes are grouped together Therefore, you only need a single promoter site

where RNA polymerase can begin to transcribe the DNA code

Near the promoter site is a stretch of DNA that controls whether RNA polymerase can bind. This is called the operator

The promoter site, the operator and the stretch of DNA that codes for the protein(s) is called the operon

Operons (the trp operon) An example of an operon is the tryptophan (trp) operon in E. coli that

produces the amino acid, trp The way it works

Trp operon is usually ‘on’ . . . RNA polymerase has access to the promoter

To stop the production of trp, the operon has to be turned ‘off’ A protein called the trp repressor binds to the operator and

blocks the attachment of RNA polymerase This repressor protein is specific to the trp operator site and stops

transcription The trp repressor is the product of another regulatory gene with

its own operon When trp is absent, the repressor is inactive and the production of

trp proceeds normally When trp is present in higher concentrations, it acts as a

corepressor It binds with the repressor protein and “activates” it so that it can

bind to the operator and turn off transcription

Repressible operon The trp operon is called a repressible

operon This means that the trp operon is

usually in the “on” condition . . . it can transcribe the DNA normally

Transcription can only be inhibited when trp binds with the repressor protein This allows the repressor protein to

bind to the operator and prevent transcription

Inducible operon In an inducible operon, the operon is

usually “off” It is not possible to transcribe the DNA There must be some sort of signal

(molecule) that can turn the operon on An example of an inducible operon is

the lactose (lac) operon

Operons (the lac operon) In E. coli, the enzyme beta-galactosidase is needed to

break lactose into glucose and galactose Normally, E. coli does not have a large amount of this

enzyme present The operon to create beta-galactosidase is normally in

the “off” position A regulatory gene, lacI, creates a repressor protein

that is normally bound to the operator of the lac operon

When lactose is present, it will bind to the repressor protein and inactivate it Since this molecule is needed to start DNA

transcription, it is called an inducer