Post on 12-Jan-2016
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Chapter 13The Genetics of Viruses and Prokaryotes
Biology 101
Tri-County Technical College
Pendleton, SC
Viral Components
Are obligate intracellular parasites Outside living host cell exist as individual
particles called VIRIONS Have central core of either DNA or RNA but
never both Surrounded by CAPSID composed of
capsomeres (one or more proteins)
Components, cont.
Animal viruses may have ENVELOPE derived from host cell’s plasma membrane
Virions lack cell wall and ribosomes of bacteria
Therefore, they are unaffected by antibiotics VIROIDS are infectious genetic material
Describing Viruses
Can be described by whether the genome is DNA or RNA
Whether the nucleic acid is single-stranded or double-stranded
Whether the shape of the virion is simple or complex crystal
Whether the virion is surrounded by a membrane (envelope)
Obligate Intracellular Parasites Viruses NOT classified as “living” Whole viruses NEVER arise directly from
preexisting viruses Develop and reproduce ONLY within cells of
specific hosts Cells of animals, plants, fungi, protists, and
prokaryotes (both bacteria and archaea) serve as hosts
Parasite, cont.
Use host’s synthetic machinery to reproduce themselves
Usually destroy host cell in process How quickly is extremely variable depending
on replication cycle utilized Host cell releases progeny viruses which
then infect new hosts
Replication Cycles
Virus that infects bacteria called bacteriophage (phage for short)
Virus that reproduces only via LYTIC cycle is called a VIRULENT virus
Once phage has injected its nucleic acid into host, that nucleic acid takes over host cell’s machinery
Viral genome contains promoter sequence that attracts host RNA polymerase
Cycles, cont.
EARLY GENES code for proteins that shut down host transcription, stimulate viral genome replication, and stimulate LATE GENE transcription
Nuclease enzymes digest host’s chromosomes, providing nucleotides for synthesis of viral genomes
In late stage, viral late genes code for proteins of viral capsid and for those that lyse host cell to release new viriions
Lytic Cycle Visual
Cycles, III
LYSOGENIC virus called TEMPERATE virus Lysogenic bacteria contain noninfective entity
called a PROPHAGE Prophage can remain inactive within host
genome through many cell divisions Cell becomes stressed or damaged,
prophage released from inactive state and lytic cycle proceeds
Enveloped Cycle(s)
**Influenza virus’s glycoproteins (on capsid) bind to receptors on host cell’s plasma membrane
Virus enters cells by endocytosis Viral and vesicle membranes fuse, capsid
breaks down, and viral RNA is released Viral RNA makes mRNA via viral RNA-
dependent polymerase
Enveloped cycles, cont.
Viral mRNA is translated into viral proteins (capsid and envelope)
Virion is assembled Envelope glycoproteins made on host ER
and transported to cell membrane via Golgi apparatus
New viruses assemble by budding and are released from cell
Influenza Visual
Enveloped III
HIV is a retrovirus (reverse transcriptase) Attaches to host cell at membrane protein
CD4 Viral envelope fuses with host’s plasma
membrane, capdis breaks down, and viral RNA released into cell
Viral RNA uses reverse transcriptase to make complementary DNA (cDNA)
Enveloped IV
Viral RNA degrades Second DNA strand is synthesized cDNA enters nucleus and is integrated into
host chromosome forming PROVIRUS Upon activation, proviral DNA transcribed into
viral RNA which is exported to cytoplasm In cytoplasm, viral RNA translated into
proteins using host ribosomes
Enveloped V
Viral proteins, new capsids, RNA, and envelopes are assembled
Assembled virus buds from plasma membrane
Spend some quality time on Overheads 13. 4 (influenza) and 13.5 (HIV)
Enough said
HIV Visual
Lytic vs Lysogenic
In animal cells, lytic and lysogenic cycles cause differing pathologies lytic infections of plasma cells by the Epstein-Barr virus
(EBV) occur in mononucleosis latent infections of B cells by EBV predispose the person
to lymphoma
lytic infections by human papilloma virus (HPV) cause genital warts
latent infections by some strains of HPV lead to cervical cancer
Vectors and more…
VECTOR is an agent like an insect that carries a pathogen affecting another species
Can also refer to plasmid or virus that carries an inserted piece of DNA into bacterium for cloning purposes in recombinant DNA technology
Viruses that infect plants must pass through cell wall as well as PM
Vectors, cont.
Insect vector penetrates cell wall with its proboscis allowing virions to move from insect to plant
HORIZONTAL transmission refers to spread from plant to plant
VERTICAL transmission refers to spread from parent to offspring
Can be either vegetative or sexual reproduction
The good and the bad…
Phages using lytic cycle destroy their hosts so might be useful in treating bacterial infections
D’Herelle used phages to control infection of chickens by Salmonella gallinarium
Phage protected group did not get the bacterial disease
Also used phage treatment successfully with plague-causing bacteria and with infectious cholera
Prions
“Proteinaceious infective particle” Are simply abnormal proteins Transmissible spongiform encephalopathies
(TSEs) Include scrapie, mad cow disease, and kuru
Prions
molecules of a normal body protein that have changed their three-dimensional configuration
PrPC The normal protein is called PrPc (for cellular) is a glycoprotein normally found at the cell surface inserted in
the plasma membrane has its secondary structure dominated by alpha helices is easily soluble is easily digested by proteases is encoded by a gene designated (in humans) PRNP located
on our chromosome 20
Prions, cont.
PrPSC The abnormal, disease-producing protein is called PrPSC (for scrapie) has the same amino acid sequence as the normal protein; that
is, their primary structures are identical but its secondary structure is dominated by beta conformation is insoluble in all but the strongest solvents is highly resistant to digestion by proteases When PrPSC comes in contact with PrPC, it converts the PrPC
into more of itself (even in the test tube). These molecules bind to each other forming aggregates It is not yet clear if these aggregates are themselves the cause of
the cell damage or are simply a side effect of the underlying disease process