Unit 5: Classification and Unit 5: Classification and KingdomsKingdoms

VirusesViruses

VirusesViruses VirusesViruses: infectious particles made up of : infectious particles made up of

nucleic acidnucleic acid, , protein coatprotein coat, and sometimes , and sometimes lipidslipids

Viruses, continuedViruses, continued

Nucleic Acid can Nucleic Acid can either be either be DNADNA or or RNARNA

Nucleic Acid is Nucleic Acid is protected by a protected by a surrounding surrounding protein protein coatcoat or a or a capsidcapsid..

Viruses, continuedViruses, continued They bind to They bind to specificspecific membrane proteins on host. membrane proteins on host. They are They are much smallermuch smaller than than bacteriabacteria and and can pass can pass

through filtersthrough filters.. Viruses- Viruses- obligate intracellular parasitesobligate intracellular parasites must have a must have a

host cell!host cell!– Cannot make their own proteinsCannot make their own proteins

NOTNOT living living– They They don’t meet all eight characteristics of lifedon’t meet all eight characteristics of life. .

– Why? Why? Can’t reproduce on their ownCan’t reproduce on their ownDon’t grow and developDon’t grow and develop

Viral InfectionViral Infection

Most viruses are highly specific to the cells Most viruses are highly specific to the cells they infectthey infect– Plant viruses only infect plantsPlant viruses only infect plants– Animal viruses only infect certain species of Animal viruses only infect certain species of

animalsanimals– Bacterial viruses Bacterial viruses (called (called bacteriophagesbacteriophages) only ) only

infect bacteriainfect bacteria

5 Basic Steps of Virus Replication5 Basic Steps of Virus Replication

1.1. AttachmentAttachment – Virus attaches to host – Virus attaches to host

2.2. Invasion Invasion (Penetration)– Injects DNA or RNA(Penetration)– Injects DNA or RNA

3.3. ReplicationReplication & & SynthesisSynthesis – hijacks host machinery to – hijacks host machinery to make new DNA/RNA & new capsidsmake new DNA/RNA & new capsids

4.4. AssemblyAssembly – puts viral DNA/RNA into capsids, – puts viral DNA/RNA into capsids, envelopesenvelopes

5.5. ReleaseRelease – New Viruses Lyses (break out of) cell, – New Viruses Lyses (break out of) cell, releasing more viruses to infect new cells releasing more viruses to infect new cells

Viral Life CyclesViral Life Cycles Lytic CycleLytic Cycle::

– virus injects virus injects nucleic acid nucleic acid

– virus makes copies virus makes copies of itself of itself

– it causes it causes host cell host cell to to lyselyse, or “burst,” , or “burst,” releasing copiesreleasing copies

Viral Life CyclesViral Life Cycles

Lysogenic Cycle:Lysogenic Cycle:– Virus injects nucleic acidVirus injects nucleic acid

– Viral nucleic acid turns into a Viral nucleic acid turns into a prophageprophage ( (plasmidplasmid) by ) by embedding itself in the host embedding itself in the host cell’s DNAcell’s DNA

– Host cell with viral nucleic Host cell with viral nucleic acid replicates and divides acid replicates and divides normallynormally

– In In unfavorable conditions, unfavorable conditions, the virus enters a lytic cyclethe virus enters a lytic cycle

Virus DangersVirus Dangers

Viruses cause yellow fever, rabies, Viruses cause yellow fever, rabies, poliopolio, , AIDSAIDS (HIV), (HIV), influenzainfluenza (the flu) , (the flu) , HerpesHerpes, , HepatitisHepatitis & the even the & the even the commoncommon coldcold

You cannot “cure” a viral infection; you can You cannot “cure” a viral infection; you can only treat symptoms and prevent it with only treat symptoms and prevent it with vaccines.vaccines.– VaccinesVaccines – – a preparation of a weakened or a preparation of a weakened or

killed virus that is injected into the body to killed virus that is injected into the body to increase immunity (i.e. it causes your body to increase immunity (i.e. it causes your body to make more white blood cells).make more white blood cells).

Examples of Viruses…Examples of Viruses…

RetrovirusRetrovirus: uses RNA; must copy genetic : uses RNA; must copy genetic info backwards (ex: HIV info backwards (ex: HIV AIDS AIDS virusvirus))

PrionsPrions: protein infectious particles similar : protein infectious particles similar to viruses, only they do not have nucleic to viruses, only they do not have nucleic acids (i.e. No DNA or RNA)acids (i.e. No DNA or RNA)– might cause might cause Mad Cow DiseaseMad Cow Disease