Viruses Chapter 19

Discovery of Viruses 1883 – Aldof Mayer Discovers tobacco mosaic disease can be transferred plant to plant 1893 – Dimitri Ivanovsky Used a filter small enough to remove bacteria Disease still passed through the sap to other plants Martinus Beirjerinck Carried out a series of experiments that showed the infectious agent could replicate only within the host

Viral Structure Extremely small Infectious particle Shape determines infection Capsid – viruses protein coat Contains either DNA or RNA In some viruses, protein coat surrounded by lipids Can form spikes used for attachment Used for identification-Enveloped, Helical, Polyhedral

Viral Replication Viruses lack ribosomes Can only replicate in their host Each virus can only a specific number of hosts – host range Usually identify by a lock and key mechanism Replication begins when the genome enters the cell Two types of infections Lytic cycle Lysogenic cycle

Lytic Cycle Cycle results in the death of the host cell Viral DNA enters and degrades host DNA Host cell machinery makes more viruses The host cell will rupture and release more phages that can infect Viruses that use only this cycle are virulent phages Bacteria protect themselves by using restriction enzymes

The Lysogenic Cycle Infection allows the host cell to live while viral genome replication takes place Viral DNA enters the cell One of the genes of the genome will silence most of the genes Viral DNA incorporates into host DNA Replication occurs with the viral DNA being passed on Environmental cues may trigger the lytic cycle Viruses that choose which cycle to use are called temperate phages

Envelopes Production in Viruses Animal viruses with envelopes use the it to enter cells Glycoproteins attach to receptors on the cell Virus enters the cell Viral genome is a template for RNA synthesis New viral genomes are made from the RNA strands RNA also acts as mRNA Vesicles transport envelope glycoproteins to the cell membrane Capsids are assembled around viral genomes Virus buds from the cell with glycoproteins in the membrane

RNA Viral Genetic Material Retroviruses – use RNA and reverse transcriptase Transcribes RNA into DNA Ex. HIV Virus RNA and proteins enter cell Reverse transcriptase makes DNA from RNA and then makes another DNA strand Double stranded DNA is incorporated into cell DNA RNA is transcribed from viral DNA for the new viral genome Just like an envelope, new viruses are made

Viral Diseases in Animals Viruses cause damage different ways Release of hydrolitc enzymes from lysosomes Cause infected cells to produce toxins Defense involves immune system and vaccine A harmless version of the pathogen Stimulates the immune system to mount a defense Very little can be done once infected – a few antivirals exist Acyclovir – blocks replication by inhibiting viral polymerases Azidothyidine (AZT) – slows HIV replication by interfering with synthesis of DNA by reverse transcriptase Viruses can lead to large outbreaks that result in an epidemic or pandemic

Viroids and Prions Viroids – small round RNA molecules that can infect plants Cause errors in plant regulatory systems Prions – proteins that cause a number of degenerative brain diseases m animals Misfolded proteins that when in the brain cause normal proteins to misfold Act very slowly – can incubate for up to 10 years Virtually indestructible