Chapter 24 - Viruses and Sub-viral Agents

the genome of most plant viruses consists of ____________________________________

single-stranded RNA

lysogenic cycle

temperate viruses integrate into the host DNA -bacterial cells containing certain temperate viruses may exhibit new properties (lysogenic conversion)

human viruses cause: *most humans suffer from 2-6 viral infections each year*

-chickenpox -herpes simplex -mumps -rubella -rubeola -rabies -warts -infectious mononucleosis -influenza -viral hepatitis -AIDS -certain cancers

(of lysogenic cycles) integrated viral genome is called a _________________________

prophage (provirus) -when the bacterial DNA replicates, prophage also replicates

satellites (e.x. the agent that causes hepatitis D can reproduce only when the hepatitis B virus is present)

sub-viral agents that depend on co-infection of a host cell with a helper virus

discovered by Martinus Beikerinck (1898) -many characteristics of a living organism but *can only reproduce inside a cell* -infect all kinds of organisms, make up a large component of Earth's biomass -range in size from 20 to 300 nm

viruses

emerging diseases (AIDS, SARS, Ebola, etc.)

appear suddenly and are unknown to the human population

At what stage of a lytic infection are phage components put together to make new viruses?

assembly

virus genome

consists of 5,000 to more than 10,000 bases or base pairs

viruses responsible for viral diseases

pathogens *most viruses do not cause disease*

once a plant is infected, the ciruses spreads through the plant body by passing through __________________________________

plasmodesmata

factors that contribute to human viral infections:

human activity: -urbanization -global travel -war living conditions: -sanitation -nutrition -physical stress -health care -sexual practices *bioterrorism*

sub-viral agents

infective agents that are smaller and simpler than viruses (satellites, viroids, and prions)

_______________________________ are important vectors of plant disease; *viruses cannot penetrate plant cell walls unless the calls are damaged*

insects

viral penetration in animals

-some enveloped viruses fuse with the animal cell's plasma membrane -other viruses enter host cells by *endocytosis*

virus replication

-viral nucleic acid enters the cell and synthesizes components -viral components are assembled -viruses are released from the cell *2 types of virus reproductive cycles: lytic and lysogenic*

retroviruses

RNA viruses that have a DNA polymerase -reverse transcriptase transcribed viral RNA genome into DNA, which is integrated into host DNA by a viral enzyme -viral genes are transcribed by host RNA polymerases synthesizing copies of viral RNA -viral structural proteins are synthesized, the capsid is produced, and new viral particles are assembled

replication of viral DNA/protein synthesis is similar to host cell's normal processes

RNA viruses: RNA synthesis takes place with the help of an RNA-dependent RNA polymerase

enveloped viruses pick up ________________________________ as they exit the host cell

a fragment of host plasma membrane

What is the protein coat of a virus called?

capsid

viruses are _______________________________ in any of the 3 taxonomic domains

not classified -viruses can instead be classified on host range and physical characteristics *2 virus classification systems: Linnaean system: ICTV and the Baltimore classification system*

viruses are considered to be an _____________________________________; must depend on a living host to survive and replicate

obligate intracellular parasite

viruses infect all types of organisms (bacteria, archaea, posits, plants, fungi, and animals) some can even infect ________________________

other viruses

bioterrorism

the use of biological and chemical weapons in terrorist attacks

lytic reproductive cycles

the virus destroys the host cell -forces the infected host to use its metabolic machinery to produce viruses -viruses that have only lytic cycles are *virulent*, causing disease and often death

most viruses *cannot survive long outside a living host cell*

they must be transmitted from one animal to another -host range may be limited because attachment proteins on the surface of the virus are specific (e.x. poliovirus attaches to cells of the human digestive tract and motor neurons of the brain and spinal cord)

progressive hypothesis (escape hypothesis)

-viruses may have originated as mobile genetic elements (transposons or plasmids) that could have moved from one cell to another through damaged cell membranes -some viruses may have originated in animals cells others in plant cells or bacterial cells -supported by genetic similarity between some viruses and their host cells

What characteristic defines a virus?

viruses cannot metabolize.

enveloped viruses (e.x. HIV)

viruses surrounded by an outer envelope of plasma membrane derived from the host cell -while inside the host, the virus can synthesize proteins and insert them into the host's plasma membrane

biologists view viruses as nonliving particles because they:

-are non composed of cells -cannot carry on metabolic activities or reproduce on their own

animal viruses cause:

-hog cholera -foot-and-mouth disease -canine distemper -certain cancers

viruses can also be transmitted from one generation to the next through:

-infected seeds -asexual propagation

(viruses cause serious plant diseases) viral infections do not usually kill plants, but can:

-stunt their growth -cause changes in the shape of the foliage -cause spots, streaks or molted patters on leaves, flowers or fruits

regressive hypothesis (reduction hypothesis)

-viruses may have evolved from small, independent cells that were parasites in larger cells, and unneeded genes were gradually lost though evolution -supported by certain bacteria that are able to reproduce only inside host cells -explains why viruses are dependent on their hosts

capsid

outer protein coat of a virus -consists of protein subunits (capsomers) which determine the shape of the virus -capsomers can be *helical, polyhedral, or a combination*

virus-first hypothesis

-viruses predate or co-evolved with their current cellular hosts even before the life-forms assigned to the three domains diverged -viruses may have initially existed in a pre-cellular world as self-replicating units -supported by genetic similarities between some viruses that infect archaea and some viruses that infect bacteria

transmissible spongiform encephalopathies (TSEs)

a group of fatal degenerative brain diseases in birds and mammals -the infected brain appears to develop holes and become somewhat sponge-like

prions

a normally harmless protein (PrP) can sometimes fold into a different shape (prion) that causes disease -somehow induced other PrP molecules to misfold into the pathogenic form -can aggregate and accumulate in the brain and certain other tissues and cause fatal diseases (TSEs)

(outside of a host cell) a virus consists of. . .

a nucleic acid core surrounded by a protein coat (capsid) -contains either DNA or RNA, which can be single-stranded or double-stranded

most plant viruses have _______________________________, but *not envelopes*

capsids

viruses that do not have an outer envelope exit by ___________________________

cell lysis

re-emerging diseases (H1N1: evolves as it mutates and its RNA re-assorts in birds, swine, and humans COVID-19: RNA genome)

diseases that have been almost eradicated and then strike unpredictably causing an epidemic -sometimes in a new geographic area

coronavirus info

single-stranded RNA genome -contains 29,891 nucleotides, encoding for 9860 amino acids 82% nucleotide identity with human SARS-CoV -the new virus is called SARS-CoV-2 genomic analyses suggests that SARS-CoV-2 probably evolved from a *strain found in bats* family: Corono viridae subfamily: Orthocoronovirinae order: Nidovirale genus: Beta coronavirus sub genus: sarbecovirus