Lecture AP Biology Chapter 19 Viruses

Ch. 19 Warm-up
1.

Why do many scientists classify viruses
as non-living?

2.

Draw the basic structure of a virus.
Label and define capsid, viral envelope
and nucleic acid.


Ch. 19 Warm-up
1.

Draw the lytic/lysogenic cycle.

2.

What stage of the lytic-lysogenic cycle
is a virus virulent? Temperate?

3.

What determines a host range?


Viruses
Chapter 19

What you must know:
The components of a virus.
 The differences between lytic and
lysogenic cycles.
 How viruses can introduce genetic
variation into host organisms.
 Mechanisms that introduce genetic
variation into viral populations.



Bacteria vs. Viruses
Bacteria





Prokaryotic cell
Most are free-living (some
parasitic)
Relatively large size
Antibiotics used to kill
bacteria

Virus







Not a living cell (genes
packaged in protein shell)
Intracellular parasite
1/1000 size of bacteria
Vaccines used to prevent
viral infection
Antiviral treatment


Viruses



Very small (Components = nucleic acid + capsid
◦ Nucleic acid: DNA or RNA (double or
single-stranded)
◦ Capsid: protein shell
◦ Some viruses also have viral envelopes
that surround capsid


Viruses



Limited host range

◦ Entry = attach to host cell membrane receptors
through capsid proteins or glycoproteins on viral
envelope (animal)
◦ Eg. human cold virus (rhinovirus)  upper
respiratory tract (mouth & nose)




Reproduce quickly within host cells
Can mutate easily
◦ RNA viruses: no error-checking mechanisms





Simplified viral replicative cycle


Viral Reproduction
Lytic Cycle:
◦ Use host machinery to replicate, assemble, and
release copies of virus
◦ Virulent phages: Cells die through lysis or apoptosis
 Lysogenic (Latent) Cycle:
◦ DNA incorporated into host DNA and replicated
along with it
◦ Bacteriophage DNA = prophage
◦ Animal virus DNA = provirus

◦ UV radiation, chemicals: lysogenic  lytic cycle
◦ Temperate Phage: uses both methods of replication



Bacteriophage


Virus that infects bacterial cells


Lytic Cycle of T4
Phage


Lytic Cycle vs. Lysogenic Cycle


Animal viruses have a membranous envelope




Host membrane
forms around exiting
virus
Difficult for host
immune system to
detect virus

VIDEO: HIV LIFE CYCLE


Retrovirus





RNA virus that uses reverse transcriptase
(RNA  DNA)
Newly made viral DNA inserted into
chromosome of host (provirus)
provirus
Host transcribes provirus to make new virus
parts
Example: HIV (Human Immunodeficiency Virus)


HIV =
Retrovirus


HIV
◦ Origin: Chimpanzee virus
◦ Infects white blood cells
(helper T)
◦ HIV+: provirus (DNA
inserted), latent

◦ AIDS: <200 WBC count,
opportunistic infections


Other Human Viruses


Herpes virus



Smallpox

Herpes Simplex Virus 1 (HSV-1)

Herpes Simplex Virus 2 (HSV-2)

Eradicated in 1979 due to
worldwide vaccination campaigns


Emerging Viruses = mutation of existing
viruses
Pandemic: global epidemic


Current Outbreaks


Zika Virus

◦ Spread by Aedes mosquitoes (Aedes aegypti)
◦ Major outbreak in Brazil and Latin America
◦ Linked to birth defects (microcephaly)




Dengue Fever
Chikungunya


Zika Virus (as of Dec. 2015)


Drugs for Prevention/Treatment


Vaccine: weakened virus or part of pathogen that
triggers immune system response to prevent
infection
◦ Ex. HPV, MMR, HepA, Flu shot



Antiviral Drugs: block viral replication after
infection
◦ Ex. Tamiflu (influenza), AZT (HIV)