Viruses What is a virus? What is a virus? A small infectious agent - - PowerPoint PPT Presentation

Viruses

What is a virus?

What is a virus?

• A small infectious agent that reproduces only

within a host cell (obligate intracellular parasites)

• Infect all life forms, including bacteria
• Lack metabolic enzymes and equipment for

making proteins (i.e. ribosomes)

• Infect cells of only a limited variety of hosts

(host range of the virus)

Cell Specificity

• Viruses identify host cells by a “lock and key”

fit between viral surface proteins and specific receptor molecules on the outside of cells

Structure of a Virus

Structure of a Virus

• Composed of a nucleic acid core (DNA or RNA)

and a protein coat (capsid)

• Some viruses are also enveloped with a

material composed of lipids and proteins

RNA or DNA?

• The majority of viruses have RNA genomes
• Plant viruses frequently have single-stranded

RNA genomes

• Bacteriophages (viruses that infect bacteria)

tend to have double-stranded DNA genomes

Virus Groups

DNA Viruses RNA Viruses Retroviruses Chickenpox, herpes, hepatitis B Cold, influenza, rabies, ebola, polio HIV, human T- lymphotropic virus (HTLV-1)

How do viruses reproduce?

Lytic Cycle

• Virus replicates immediately via a component

assembly model allowing one virus to produce many viral progeny simultaneously Five steps:

• Attachment
• Penetration
• Replication
• Assembly
• Release (lysis of host cell)

Lytic Cycle of Reproduction

Lysogenic Cycle

• Virus integrates its genome into the host cell

genome (DNA)

• Results in a latent (lysogenic) infection
• May result in new properties for the host such

as increased pathogenicity in bacteria

• Environmental conditions then trigger virus to

initiate the lytic cycle

Lysogenic Cycle of Reproduction

Viruses and Genetic Variation

• Viruses transmit DNA or RNA when they infect

a host cell and introduce genetic variation

• Examples:
• 1. Transduction in bacteria – type of horizontal

gene transfer

• 2. Transposons in DNA – mobile genetic

elements, viruses may move DNA within genomes and between individuals

Viruses and Evolution Fast Track

• Replication is highly

efficient

• RNA viruses lack

replication error-checking mechanisms, higher rates

• f mutations
• Related viruses can

combine/recombine information if they infect the same host cell

Evolution of Drug Resistant HIV

• 3TC is an HIV drug that interferes with viral

genome replication

HIV Virus

HIV (Retrovirus)

• Retrovirus – alternate flow of information

from RNA to DNA using reverse transcriptase

• Demonstrates rapid evolution of a virus

–Accumulates many mutations during replication –Reproduces quickly – one virus to billions of viral progeny in one day