Principles of Biology – Bio 101
Fall Quarter
Lake Tahoe Community College
Instructor: Sue Kloss
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Chapter 19 – Viral Genetics
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I.
Viruses have genome, but can only reproduce in a host cell
A. Structure of viruses (Fig. 19.1)
1. Bacteria are prokaryotes
2. viruses are smaller and simpler = genes packaged in protein coats, sometimes a membrane
3. tiniest viruses are 20 nm in diameter, smaller than a ribosome; millions fit on pinhead (Fig. 18.2)
4. Viral genomes
a. many viruses defy convention of double helix dna
b. their genomes may be double or single stranded dna, double or single stranded rna
c. they are named by the type of nucleic acid in genome- eg. DNA or RNA virus
d. nucleic acid normally is linear or circular molecule
e. smallest viruses have 4 genes, largest have several hundred
5. Capsids and envelopes
a. protein shell of a virus is called a capsid
b. capsids are made of many proteins, but a small number of kinds of proteins called
capsomeres (Fig. 19.3)
1. helical capsid (tobacco mosaic virus)
2. adenoviruses – infect respiratory tracts of mammals= 252 identical proteins arranged
in a polyhedral capsid with 20 triangular facets – icosahedron
3. some viruses have accessory structures – eg. Envelope of membrane surrounds
capsids of flu viruses in animals
4. these envelopes are derived from membranes of host cells
5. contain host cell phospholipids and membrane proteins
6. they also contain proteins and glycoproteins of viral origin
7. most complex capsids are found in bacteriophages – heads and tails w/fibers for
attaching
B. Reproduction of Viruses
1. obligate intracellular parasites= can only reproduce inside a host cell
2. unable to do anything by itself
3. viruses lack metabolic enzymes, ribosomes and other equipment for making proteins
4. they are merely packaged genes traveling from one host cell to another
5. each type of virus can only infect a limited number of host cells, called its host range
6. this results from the evolution of host recognition systems by the virus
7. viruses depend on lock and key fit btn their proteins and host cell receptors
8. some viruses have broad range- W. Nile- mosquitoses, birds, and humans
a. other viruses have narrow ranges- measles and polio can only infect humans
b. infection often is limited to particular host tissues
c. cold viruses infect only upper respiratory tract
d. AIDs infects only white blood cells
9. Viral infection begins when virus inserts DNA into host cell – eg. Phages inject e.coli
a. viral genome conmmandeers host, causing host to reproduce viral genome and manufacture
viral proteins (fig. 19.4)
b. host provides nucleotides for viral nucleic acids, as well as enzymes, ribosomes, tRNAs,
aminos, ATP, etc.
c. after viral components are produced, self assembly occurs
d. hundreds or thousands of viruses produced from infected host cell, which damages or kills it
e. progeny infect other cells
f. many variations of this overview
C. Reproductive cycles of phages
1. lytic cycle
a. bacteria lyses open and releases phages that were produced in cell
b. death of host cell
c. an entire population of bacteria can be killed in just a few hours
d. lytic cycle phages are called virulent phages
e. T4 is a typical virulent phage (Fig. 19.5)
2. Lysogenic cycle
a. temperate phages can use lytic or/and lysogenic cycles
b. lambda, a temperate phage, resembles T4 but has only 1 tail fiber
c. forms a prophage but doesn’t enter lysogenic cycle
3. why haven’t all bacteria been killed by viruses?
a. mutation causes recognition proteins to be altered
b. bacteria may recognized foreign DNA and cut it up w/ restriction endonucleases, aka
restriction enzymes
1. bacterial cells own dna is chemically modified to prevent attack by restriction enzymes
2. host-parasite relationship – constantly evolving
3. many phages coexist w. bacterial hosts in the lysogenic cycle (fig. 19.6)
D. reproductive cycle of animal viruses table 19.1
Lesson Objectives
1. List and describe the structural components of viruses.
2. Explain why viruses are obligate intracellular parasites.
3. Explain how a virus identifies its host cell.
4. Describe bacterial defenses against phages.
5. Distinguish between the lytic and lysogenic reproductive cycles, using phage lambda as an example.
6. Describe the reproductive cycle of an enveloped virus. Explain the reproductive cycle of the herpesvirus.
7. Describe the reproductive cycle of retroviruses.
8. List some characteristics that viruses share with living organisms and explain why viruses do not fit our usual definition of
life.
9. Explain how viral infections in animals cause disease.
10. Describe viroids and prions.
11. Explain how a non-replicating protein can act as a transmissible pathogen.