Viruses

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Viruses





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Capsid: the outer protein layer that surrounds the genetic material of a virus o Surrounds DNA and RNA o Protects nucleic acids from digestive enzymes o Allows viron to attach to host cell o Proteins to penetrate host cell membrane Tail Sheath: like a needle o Penetrates the cell wall o Injects viral nucleic acid into host cell Tail Fiber: like landing gear o Helps attach to cell surface Genome: the complete sequence of the DNA or RNA of an organism o Single-stranded or double-stranded nucleic acids

Major differences between viruses and bacteria:  Viruses are not organisms  Without a host cells, they cannot: o Sustain themselves o Reproduce o Synthesize proteins – uses host cells’ RNA o Can’t generate or store energy o They also require basic building materials from host cells’  Viruses are generally classified by the organisms they infect  Smaller than bacteria cell o About 50 nm (10-9) – 1000x smaller than bacteria o No organelles  Only protein and DNA/RAN (genome) o Viruses only attack one type of cell (has to be a protein match between virus and outside surface of host cell) *Virus: a structure that contains strands of DNA/RNA, surrounded by a protective protein code; it cannot live independently outside of cells.*

Lytic cycle:  Step 1: Attachment o Proteins on the virus connect to the protein receptors on host cell’s membrane  Step 2: Entry o The virus injects genetic material into the host cell  Step 3: Replication o Virus breaks down host cell’s DNA and takes over host cell’s activities o The host cell creates more viral DNA/RNA and proteins, as well as viral “body” parts  Step 4: Assembly o The host cell assembles new viral particles  Step 5: Lysis and Release o The host cell bursts open and releases 100-200 new viral particles Lysogenic cycle:  Cycle with dormancy – latent infection  Viral DNA becomes a part of the host cell’s chromosome  As host cells undergoes mitosis, both viral DNA and host cell DNA is being replicated  Daughter cells end up with the latent genome as well  In many cases, the genes are not activated until later  Activation results in a continuation of the lytic cycle Retrovirus:  Any RNA virus that inserts a DNA copy of its genome into the host cell in order to replicate  RNA: o No thymine o Usually single-stranded o Extra oxygen on sugar  Step 1: cell membrane and viral membrane fuse  Step 2: an enzyme called reverse transcriptase changes single stranded RNA into single-stranded DNA; reverse transcriptase has poor proof-reading and therefore there are many random mutations  Step 3: RNA, now single-stranded DNA, is once again reverse transcribed into double-stranded DNA  Step 4: the new double-stranded DNA is inserted into the host cell’s chromosome by going through the nuclear membrane  Step 5: messenger RNA (just RNA) is made by new double-stranded DNA and is sent out of the nucleus  Step 6: messenger RNA makes viral proteins using ribosomes in rough ER  Step 7: two messenger RNA and viral proteins bud off of cell’s surface, creating new virus  Step 8: the proteins are digested and a new virus matures

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