Virus

Virus

Infectious particles of genes packaged in a protein coat, not considered living.

Genome

The genetic material of a virus:

• DNA or RNA

• single-stranded

• double-stranded

• linear or circular

Capsid

Protein shell that encloses the viral genome

• Capsomere

• Rod-shaped

• Polyhedral

• Complex

Capsomere

Protein subunit of a capsid

Rod-shaped

Helical virus

Polyhedral

Icosahedral virus (20 sides)

Complex

Icosahedral head + helical tail

Envelope

Membrane derived from host cells that surrounds many animal viruses.

• Phospholipids + membrane proteins from host

• Glycoproteins + enzymes from virus

Obligate intracellular parasite

can only replicate inside a host cell (has to live in a cell)

• Viruses lack enzymes for making macromolecules

Host range

Limited number of species that a virus can infect

Host specificity

Evolutionary recognition of receptor proteins on host cells

• Lock + Key fit of viral proteins to host receptor

Bacterial virus

Also known as a phage, a virus that infects bacteria

Lytic cycle

Replicative cycle that ends in the death of the host cell

• Attachment

• Entry

• Synthesis

• Assembly

• Release

Virulent

A virus that only uses the lytic cycle, such as the T4 phage.

Attachment

Virus uses tail fibers to binds to specific receptors on the host cell

Entry

Virus injects its DNA into the host cell leaving the empty capsid outside

Synthesis

Viral genome/ proteins are produced using host components

Assembly

Virus genomes packaged into capsids (automatic)

Release

Viral enzymes break down the bacterial cell wall causing the cell to burst and release viral particles

Lysogenic cycle

Replicative cycle that incorporates viral DNA into the host genome without immediately destroying the host cell

• Attachment

• Entry

• Prophage

• Replication

Temperate

A virus that uses a lysogenic replicative cycle, such as λ phage

Prophage

Viral DNA integrated into host DNA

Replication

Normal reproduction of the host cell that also copies the viral genome

• Subsequent (following) generations continue to carry infection

• Viral genes can be latent: dormant in host

• Viral genes can be expressed: change host phenotype

  • Ex. Normal gut E.coli turning into pathogenic (food poisoning) E.coli

• Can be triggered by environmental changes

  • Virus replicates and escapes when host is threatened

Latent viral genes

Viral genes that remain dormant in the host

Expressed viral genes

Viral genes that can change the host phenotype, such as turning normal E. coli into pathogenic E. coli.

Genetic diversity

Not all cells in a population will have the exact surface receptor to match virus (naturally immunity)

Restriction enzymes

Bacterial enzymes that recognize foreign DNA entering and cuts it

CRISPR-Cas system

Long-term, targeted defense mechanism in bacteria against specific viruses

• Piece of viral DNA incorporated into CRISPR region of bacterial genome

• Cas nucleases search out and cut up any DNA sequences identified in CRISPR region

Evolutionary arms race

Ongoing evolution of bacteria and viruses to outsmart each other's defenses

• Bacteria evolve to avoid virus

  Virus evolve to beat defenses

Enveloped virus

Have envelope from host cell - animal virus

Naked virus

Does not have an envelope - animal virus

Reverse transcriptase

Copies viral RNA into DNA

Provirus

Viral DNA integrated in host genome

Vaccine

Harmless derivative of a pathogen that stimulates the immune system to mount a defense

Viroid

Infectious nucleic acid, no protein coat

Prion

Infectious self-replicating protein, no nucleic acid

Plasmids

Small circular DNA that can be shared between bacteria

Transposons

DNA segments that can jump to different sections of the cell’s genome