Chapter 5: Viruses and Their Multiplication - Exam 2

Microbiology - Exam 2

Virus

Genetic element that can multiply only in a living (host) cell; not living or on tree of life

Obligate intracellular parasite

Needs a host cell for energy, metabolic intermediates, and protein synthesis.

Virion

Extracellular form of a virus; fully assembled and transmits between hosts.

Capsid

Protein shell that surrounds the genome of a virus.

Naked virus

Virus with no other layers; common in plant and bacterial viruses.

Enveloped virus

Has an outer layer of phospholipid bilayer and viral proteins from the host membrane.

Nucleocapsid

Combination of nucleic acid and protein in enveloped viruses.

Virulent (lytic) infection

Virus replicates and destroys host cell.

Lysogenic infection

Virus integrates its genome into host DNA, altering the host genetically.

Virus genome types

Can be DNA or RNA, single- or double-stranded—never both.

Capsomere

Individual protein molecules forming the capsid.

Helical symmetry

Rod-shaped virus structure (e.g., Tobacco mosaic virus).

Icosahedral symmetry

Spherical virus with 20 faces and 12 vertices (e.g., HPV).

Complex virus

Has multiple structural parts (e.g., bacteriophage T4).

Fibrils

Hairlike structures that attract hosts and increase infection rates.

Apical pore / Stargate

Portals for releasing viral genome into host cell.

Enveloped virus structure

Contains lipoprotein membrane and surface glycoproteins; uncommon in plant/bacterial viruses.

Lysozyme

Enzyme that breaks down cell wall for viral entry and release.

Neuraminidase

Breaks glycoproteins/glycolipids to help virus exit cell (e.g., influenza).

RNA replicase

RNA-dependent RNA polymerase.

Reverse transcriptase

RNA-dependent DNA polymerase (retroviruses).

Plaque assay

Method for detecting/counting viruses by clear zones on host cell lawns.

Titer

Number of virus particles per volume of fluid or number of antibodies.

Viral replication cycle steps

Attachment → Penetration → Synthesis → Assembly → Release.

Eclipse phase

Genome replication and protein synthesis occur; no virions seen.

Maturation phase

Viral nucleic acids are packaged into capsids.

Latent period

Eclipse + maturation phase.

Burst size

Number of virions released from one infected cell.

T4 bacteriophage receptors

Bind to LPS carbohydrates on E. coli.

T4 penetration process

Tail fibers attach, retract, and inject viral DNA; capsid stays outside.

Bacterial defenses

Toxin-antitoxin systems and CRISPR protect against phage infection.

Restriction endonucleases

Enzymes that cut foreign DNA at specific sites.

Methylated viral DNA

Protected from restriction enzyme cleavage.

Early proteins (T4)

Needed for DNA replication and modifying host enzymes.

Middle proteins (T4)

Form head and tail structures.

Late proteins (T4)

Lysozymes and other enzymes for virion release.

Prohead

Empty bacteriophage head precursor.

Packaging motor

Uses ATP to pump viral DNA into the capsid.

Lytic cycle

Virus replicates and kills host.

Temperate virus

Can switch between lytic and lysogenic cycles.

Prophage

Viral DNA integrated into host chromosome.

Induction

Trigger (e.g., stress) that activates lytic cycle from lysogeny.

Viroplasm

Membrane-bound viral “factory” in eukaryotic cells.

Eukaryotic virus entry

Whole virion enters via fusion or endocytosis.

Uncoating

Removal of capsid to release viral genome.

Budding

Virus exits cell by taking part of the host membrane as envelope.

Virulent infection (animal)

Host cell lysis and death.

Latent infection (animal)

Viral genome present but dormant (provirus).

Persistent infection

Continuous slow release of virions; cell survives.

Transformation

Conversion of normal cell into a tumor cell.

Plant virus traits

Mostly RNA genomes, not enveloped, wide host range.

Plant virus transmission

Occurs via wounds or vectors like insects, nematodes, or fungi.

Vectors

Organisms that carry viruses between hosts.

Movement proteins (plants)

Allow virus to move through plasmodesmata to infect entire plant.