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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)