Chapter 24

Viruses

I Viruses: a very small particle made of nucleic acid and protein; coat or shells.

A) A lifeless particle – non-cellular, no organelles.

B) Can only reproduce inside a living cell; obligate intracellular parasite.

C) Structure: different shapes and sizes

1) Central core of nucleic acid – DNA or RNA

2) Outer protein shell = CAPSID

(a) Some viruses have a 2nd layer or envelope of protein and lipids in addition to the capsid (outermost)

3) Capsomere: single protein units making up the capsid.

(a) Arrangement of capsomeres determines the kind of virus.

D) Viruses alter and change host cells

1) Virulent – cause a disease immediately.

2) Temperate – viruses that do not cause a disease immediately.

II Classification

A) Viruses are classified as DNA or RNA

B) DNA viruses – 2 strands

1) May produce new RNA and new viruses or DNA virus may join DNA of host = new viruses.

C) RNA viruses – single strand

1) Make new proteins directly in host cell by using the hosts ribosomes.

2) Retroviruses – use an enzyme called reverse transcriptase to make DNA from RNA

(a) RNA makes DNA

(b) (HIV – AIDS)

D) Also classified if they infect plants or animals or bacterial cells.

E) Presence or absence of an envelope.

III Viral Replication – most viruses replicate alike

A) Lytic cycle:

1) Virus: attaches to host at a receptor site.

2) Virus puts a hole in the in the host

3) Viral nucleic acid enters host

(a) Empty capsid stays inside

(b) Entire virus enters host

4) Viral nucleic acid destroys host

5) Viral nucleic acid duplicates copies of itself.

6) Host cell bursts releasing new ciruses to infect other cells.

B) Virulent: viruses which (and/or) are deadly and cause Lysis of host cell – diseases

C) Temperate: virus which is not deadly but DNA of Virus joins with DNA of host; can lead to new viral strains.

D) Lysogenic Cycle: some viruses are not virulent – called temperate viruses.

1) Attachment to the host is the same

2) DNA of virus when injected into the host becomes part of DNA of host chromosomes – called provirus or prophage.

3) When host cell reproduces it catties DNA of provirus or prophage; causes no harm

4) At any time – external stimuli can cause provirus to become virulent

(a) Herpes Simplex I: cold sores or fever blisters

(b) Herpes Simplex II: genital

(c) Hepatitus B: chicken pox – later in life shingles and affects nerves.

IV Other pathogens smaller than viruses

A) Viroids

1) A short single strand of RNA – no capsid

2) Causes dieseases in plants

(a) Color break

(b) Tobacco mosaic

B) Prions

1) A glycoprotein (chains of attached sugars and proteins) particle containing about 250 amino acids.

2) Reproduce in mammal cells

3) Long incubation period (about 40 years)

(a) EX: madcow

C) Bacteriophages: viruses that infect only bacteria.

1) Cause host cells to reproduce quickly

2) Complex shape

3) Used in lab research – used in genetic engineering of insulin and interferon.