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VIRUSES. Viral replication  genetic variation Viral Infection  can introduce genetic variation in hosts. Structure. Composition: n ucleic a cid (viral DNA = prophage ) surrounded by a p rotein coat Very small! (~ 20nm ) Capsid – Protein shell that surrounds genetic material

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Viral replication genetic variation

Viral Infection can introduce genetic variation in hosts


  • Composition: nucleic acid (viral DNA = prophage) surrounded by a protein coat

  • Very small! (~20nm)

  • Capsid – Protein shell that surrounds genetic material

  • Viral Envelope – surrounds the capsid of some viruses, helping to infect hosts


  • Viruses that infect bacteria cells

  • Aka: Phages


  • Host Range – Range of hosts a virus can infect

     can only infect a limited variety of hosts

    (ex. Human cold virus can only infect upper respiratory tract)

A lytic cycle
a. Lytic Cycle

  • Bacteriophage injects its DNA into host cell and takes over the cell’s machinery to make new copies of the viral DNA and protein coats

  • These self assemble (component assembly model) and host bursts open (lysis), releasing new viruses

B lysogenic cycle
b. Lysogenic Cycle

  • Bacteriophage DNA is incorporated into hosts DNA, replicated by host along with its own genome, can enter lytic cycle (but not always)

  • Host is not killed

  • Can sometimes

    result in new properties

    for the host

Transduction in bacteria
Transduction (in Bacteria)

  • Bacterial DNA is ‘accidentally’ picked up into phages’ (viruses) DNA during lytic cycle and integrated into a new bacteria (host) cells DNA


  • Highly effective replicative capabilities - allows for rapid evolution and acquisition of new phenotypes

    • Allows for mutations

    • Lack error-checking mechanisms (higher rates of mutations)

    • Related viruses can combine/recombine info if same host is infected


  • RNA viruses – use the enzyme reverse transcriptase to transcribe DNA from RNA template

  • New DNA permanently integrates into DNA of animal cell

  • Host (animal) transcribes viral DNA into RNA used to make viral proteins


  • Human Immunodeficiency Virus  causes Acquired Immunodeficiency Syndrome (AIDS)

  • Retrovirus

  • HIV infects cells in the

    human immune system

    (white blood cells)

  • Rapid evolution of a virus within host contributes to pathogenicity (deadliness) of viral infection

Similar pathogens in plants and animals
Similar pathogens in plants and animals:

  • Viroid – Circular RNA molecules that infect plants causing errors in regulatory systems that control plant growth

  • Prions – misfolded, infectious proteins that cause misfolding in normal proteins they come in contact with in various animal species

    (ex: Mad Cow Disease)