BIOL 1407 - Viruses

virus characteristics

composed of DNA or RNA enclosed in a protein and sometimes a lipid bilayer, no organelles, do not metabolize, cannot reproduce on own

virus envelope

Surrounds the capsid

Helps the virus enter the cell

Includes glycoproteins

four virus shapes

filamentous, icosahedral, enveloped, head and tail

virus core

The center of the virus, consisting of nucleic acid, either DNA or RNA but not both, may be single or double stranded, circular or linear, one piece or segmented

positive sense

describes the genome of an RNA virus containing RNA that serves directly as the transcript for viral protein production

negative sense

describes the genome of a virus in which the genome is not directly transcribed

RNA viruses

Use an RNA-dependent RNA-polymerase to transcribe their mRNA, enzyme makes more errors, RNA viruses mutate more frequently

baltimore classification

based on morphology, genetics, and production of mRNA to classify viruses

Steps of viral infection

attachment, entry, replication, assembly, egress (release)

attachment of virus

-involves very specific binding between virion's attachment proteins and receptors on host's cell surface, virus capsid proteins or envelope glycoproteins

- receptors on host are actually there for some other function, but virus co-opts them

-virus evolves to fit host's receptors

-virus species specific or even cell type specific

Entry of virus

bacteriophage enters host cell naked

eukaryotic viruses enter by endocytosis or fusion with cell membrane if enveloped

Replication and assembly of virus

depends on viral genome, DNA genome transcribes mRNA then duplicates DNA, RNA genome makes complementary RNA to transcribe then copies RNA, RNA retrovirus uses reverse transcriptase to transcribe DNA from RNA then DNA incroporates into host genome and directs synthesis

Modes of egress (release)

lysis-results in death of host cell

budding-uses cell membrane resulting in enveloped virions

exocytosis-virions leave in vesicles

bacteriophages

Viruses that specifically infect bacteria.

Regressive hypothesis

Viruses are cells that lost some of the replicative and metabolic traits over time

progressive hypothesis

viruses originated from genetic material that gained the ability to replicate and be transmitted semi-autonomously, ex: retrovirus

Virus first hypothesis

states that viruses predate cellular hosts. Viruses originated in a precellular world and were the first self-replicating entities

Prions

very small, contain no nucleic acids, cause fatal neurodegenerative disease (mad cow disease), not destoyed by cooking

Viriods

only known to infect plants, can replicate in cell, do not manufacture any proteins, can cause crop failure

obelisks

small circles of RNA, only known from sequence data, found on microbiome genomes, affects unknown

lytic cycle

the virus rapidly replicates, destroying the host cell in the process (lysis) to release new virus particles.

lysogenic cycle

in the lysogenic cycle, the viral DNA integrates into the host's genome, becoming a prophage, and replicates along with the host cell without immediately destroying it