Viruses and Bacterial Genomes

Viruses

Viruses

• Nearly all forms of life have viruses that infect them

• They vary in structure, replication methods, and their target hosts

• Much about virus origins and evolution are unknown

Basic Structure of Viruses

• Tiny; much smaller than a bacteria About 20-250 nanometers in diameter

• Acellular

• Nucleic acid core enclosed in a protein coat or capsid

• May have an outer envelope of proteins and phospholipids derived from the host

• May contain enzymes and other additional proteins

Types of Viruses

Shapes classified into four groups

• Filamentous - many plant viruses; tobacco mosaic virus (TMV)

• Isometric (icosahedral) - poliovirus and herpesvirus

• Enveloped - many animal viruses; HIV

• Head and Tail - infect bacteria; bacteriophages

Viral Genomes

• Tend to be small

• Only genes that encode proteins the virus can't get from the host

• May use DNA or RNA

• May be single or double-stranded; linear or circular DNA viruses direct the host cell's replication of the viral genomes to transcribe and translate into viral proteins

• RNA viruses have the enzyme reverse transcriptase that replicate RNA into DNA (cDNA)

• Retroviruses

• More likely to make copy errors and mutations in RNA viruses occur more frequently

Host Specificity

• Many viruses use glycoproteins to attach to hosts

• Attach to molecules called viral receptors on the host cell

• (glycoproteins on virus attach to viral receptors on host)

• Normally found on cell and have their own function

• Viruses have evolved means to attach to these cell receptors for their own replication

Viral Infections

• Viruses must attach - be taken inside - manufacture proteins and copy genome - and find a way to escape

• Infect only certain species of hosts and only certain cells within that host

• Based on receptor proteins, immune response, and gene expression

Host Cell and Viral Infections

• Viral replication causes damaging changes to host cells

• May change cell function or destroy cell

• Infected cells may die through lysis or apoptosis

• Some cells may live for a period after viruses are released, but won't function normally due to damage

• Symptoms of viral disease result from immune response and cell damage

Steps of a Viral Infection

1. Attachment

• binds to specific receptor on host

2. Entry

• Genome may enter naked without capsid, fuse envelope with cell membrane, or by endocytosis

3. Replication and Assembly

• Depends on viral genome; DNA or RNA

4. Release of new viruses

• Infect adjacent cells

Lytic Cycle (Bacteriophage)

• Kills the host cell by causing it to lyse

• Injects genetic material into host and uses it to produce new viral proteins and make copies of DNA/RNA

• New viruses are assembled and break open (LYSE) host cell to release new viruses

Lysogenic Cycle (Bacteriophage)

• Viral genetic material is injected into host cell and is incorporated into host cell genome

• Now called a prophage

• Viral genome is copied every time host cell reproduces

• Latency - viruses exist in host cells without causing damage or symptoms

• Viral genome eventually exits the host genome and initiates lytic cycle

• Usually due to environmental triggers

Oncogenic Viruses

Can cause cancer by interfering with the regulation of the host cell cycle by interfering with genes or gene expression

• HPV and cervical cancer

• Hepatitis B and liver cancer

• T-cell leukemia and lymphomas

Vaccines and Treatment

Vaccinations - intended to prevent outbreaks by building immunity

• May be prepared using live viruses, killed viruses or molecular subunits of the virus

Antiviral drugs - used to control and reduce symptoms

• May inhibit the virus by blocking the actions of proteins

• Have limited success in curing viral disease

Other Strange Infections

Viroids - tiny, naked, circular molecules of RNA that infect plants

Prions - misfolded proteins that convert normal proteins in the brain to prion version

• Causes many degenerative brain diseases "Mad cow disease," Kuru, and Creutzfeldt-Jakob disease

• Spread by the consumption of meat, nervous tissue or internal organs between members of the same species

Bacterial Genomes

• DNA in nucleoid region; no nucleus

• Single, double-stranded, circular chromosome

• Have no introns

• Has an origin of replication

• May also contain plasmids

• Small, self-replicating, disposable circles of DNA with small # of genes

Plasmids (circular DNA strand in bacteria)

May provide for the expression of beneficial phenotypes, but not required for survival and reproduction

May be exchanged between different bacteria Examples:

• F plasmids - required for the production of sex pili used in conjugation; bacteria either F+ or F- Exchange of plasmid transfers the ability to conjugate

• R plasmids - contain genes for antibiotic resistance

Reproduction and Recombination

Reproduce by binary fission (asexual)

• Copy DNA and split into two identical cells Begin replication at a single ori site and make DNA in both directions around the circle

Genetic diversity/recombination accomplished by:

• Mutations

• Transformation - uptake of foreign DNA; often plasmids

• Transduction - gene transfer by phages

• Conjugation - one-way direct transfer of genes (plasmids) between two bacteria through a pilus (pili)

Bacterial Gene Expression

Bacteria genes contain operons

• All genes needed to produce proteins in the same biochemical pathway encoded together

• Series of genes are turned on or off together One promoter for the whole series

Three parts:

• Operator - "on-off switch;" around promoter region and controls access of RNA polymerase

• Promoter - binding location for RNA polymerase

• Genes they control - entire sequence of DNA for the pathway

Operon Control

Repressors - proteins that can bind to an operator and block RNA polymerase, preventing transcription

• Can also be regulated by corepressors, which help the repressor, inducers which, inactivate the repressors, and activators, which make it easier for RNA polymerase to bind

Operon Examples

trp Operon - E.coli can either ingest or make tryptophan (an amino acid)

• Have a series of five genes used to synthesize the amino acid

• If tryptophan is present in the environment, a repressor binds and the genes are turned off (trp operon isnt needed)

• If tryptophan availability is low, transcription is initiated because the repressor dissociates (trp operon is turned on)

Operon Examples

lac Operon - An inducible operon that involves both activators and repressors if glucose isn't present E. coli may use other sugars like lactose

• The operon encodes genes need to acquire and process lactose

• For the operon to be activated, glucose must be very low and lactose must be present

• Transcribing the genes without these conditions would be wasteful for the cell