Chapter 13 The 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 (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