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