biol 554 exam 1

a virion is/consists of

• complete infective form of a virus

  • a nucleid acid genome

  • protective protein coat (capsid)

  • lipid envelope (sometimes)

viruses are obligate intracellular parasites (requires being in host to replicate)

• replicate within another living cell

• genomes protected by capsid + lipid envelope + envelope protein

what are the three (or four?) domains of life?

• bacteria

• archaea

• eukarya

• acytota (biological infectious particles/entities)

what are some of the problems a virus must solve to live?

• lack of enzymes (nucleotide, aa, carbohydrate + lipid synthesis + protein syn. machinery)

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what is one thing found by studying viruses?

• oncogenes → understanding cancer is caused by mutation or unregulated expression

what is the overview of replication cycle of viruses?

• latent phase → lag before new infectious virions

• early stage → genes required for establishing infection and beginning genome replication

  • make tons of genomes → more structural proteins → more virions

• late stage → required for new virion formation

virus replication cycle?

1. binding to cell receptor

2. entry + uncoating

3. early gene expression

4. replication of viral genome

5. late gene expression

6. assembly of virions

7. exit

what are the baltimore classifications of viruses?

• RNA or DNA (not both)

• single or double stranded

• circular or linear

• +ssRNA → can be directly translated

• -ssRNA → needs RNA-dependent RNA polymerase (RDRP) protein in the virion

• dsRNA → need an RDRP

• dsDNA → need host RNA polymerase

• ssDNA → need host DNA polymerase

• retrovirus → have +ssRNA but use RDRP → dsDNA

• hepadnaviruses → partial ss/dsDNA made in host cell by reverse transcriptase

retrovirus

• use reverse transcription to go from RNA → DNA

what is the general structure of a virion

• capsid → contain composed of viral proteins

  • nucleocapsid → capsid with enclosed genome

  • some are surrounded by lipid bilayer = envelope

  • some capsids have icosahedral symmetry

what is the genetic economy?

• capsids → identical copies made from a small number of viral proteins → structural proteins → more virions

what is genome packaging?

• sequential = immature capsid formed → RNA genome is inserted via pore

• concerted = capsid is assembled around the DNA genome

• in both a packaging signal must be present and recognized

what is budding?

• interaction of capsid w cytoplasmic domain of aggregated glycoproteins → curves around capsid = budding

what are matrix proteins?

• structural proteins that link viral glycoproteins to nucleocapsid

how do virions get into cells?

• deliver viral genome from one cell to another → ensure replication of new virions from that genome

• entry = direct penetration → membrane fusion(only with enveloped) → endocytosis

what is required for non-enveloped virus entry + escape?

• enter through receptor-mediated endocytosis + endosome rupture

  • conformation change in capsid + formation of a channel in membrane (endosomal or plasma membrane)

• cell death or rupture of cellular membranes → escape host cells

what do enveloped viruses use for entry and exit?

• vesicular transport

• receptor-mediated endocytosis → fusion + fission of the envolope with plasma membrane

• outer membrane - release directly

• nuclear membrane, ER, endosome, golgi apparatus, any other membrane - release after transport through secretory pathways

how does plant viruses spread?

• mechanical damage to infect plant cells → drill for cement wall

• plasmodesmata (intercellular pores connecting adjacent plant cells allowing membrane and cytoplasmic continuity)

how does receptor-mediated endocytosis work?

• may occur using clathrin-mediated or non-clathrin → resulting in vesicles taking virions away from cell surface → delivered to early endosome → mature to become late endosomes + lysosomes

• may occur at lipid rafts or caveolae

• some also enter via macropinocytosis (drinking)

how are viral entry and pH linked?

• pH independent → receptor-mediated membrane fusion = direct release of RNP

• pH dependent → receptor mediated endocytosis → low pH triggers changes → endosomal membranes release RNP

what are attachment factors and receptors?

• serve only to bind virions

• bind to virions and play an ACTIVE role in mediating viral entry

what is host/species tropism and tissue tropism?

• appropriate receptor is important for which can get infected (host/species tropism)

• certain cell types can be infected (tissue tropism)

• flu virus → epithelial cells of respiratory = tropism is respiratory tract

what are fusion proteins?

• glycoproteins that mediate fusion → typically type I transmembrane proteins

• often activated by receptor binding or low pH to trigger exposure