AP Bio Chapter 18

 

Virus

• Infectious particle consisting of a nucleic acid enclosed in a protein coat

• Small in size

• Cannot reproduce without a host

• Alcohol Resistant

• Examples- measles, small pox, HPV, HIV

(Erin Cockrum, Elena Wilcoxson)

 

Viroids

• Infectious particles of naked RNA (has no capsid to protect it)

• Found only in plants

• Examples- Coconut Blight, Chrysanthemum Wilh

(Erin Cockrum)

 

Prions (Awan)

• infectious proteins

• cause many degenerative brain diseases in animals.

• The incubation period of prions until symptoms appear is about 10 years

• Prions are indestructible: they are not destroyed or deactivated by cooking to the suggested temperatures.

Campbell Nail a., and Jane B. Reece. AP Edition: Biology. San Francisco: Pearson: Benjamin: Cummins.2005 pg. 345

 Viral Genomes

• consist of double DNA, single-stranded DNA, double-stranded RNA or single-stranded RNA-depending on the kind of virus

• called a DNA virus or RNA virus according to the kind of nucleic acid that makes up its genome

• the genome is usually organized as a single linear or circular molecule of nucleic acid. 

• smallest viruses have only four genes, largest can have several hundred

•  

(Fernandez)

http://www.goalfinder.com/images/SBMVIR9/L-RNA-virus-genomes.jpg

Capsid:

• the protein shell enclosing the viral genome
• may be rod-shaped, polyhedral, or more complex in shape
• built from a large number of protein subunits called capsomeres

(Fernandez)

 

The number of different kinds of proteins is typically small. To help infect their hosts, some viruses have accesory structures. A viral envelope is a membrane that cloaks the capsid that in turn encloses a viral genome. These structures contain host cell phospholipids and membrane proteins.  They also contain proteins and glyocoproteins of viral organs. A bacteriophage, or phage, is a virus that infects bacteria. The most complex capsids are found among these viruses. The first phages studied were those the infect E. coli. These were named type 1 (T1), T2, T3, T4, T5, T6, and T7. The three T-even phages were very similar in structure. Their capsids have elongated icosahedral heads enclosing DNA. The head has a protein tail piece with fibers that the phages use to attach to a bacterium. In other words, the T-even phages have a complex capsid consisting of a polyhedral head and a tail apparatus. (Mroz)

 

Campbell, Neil A., and Jane B. Reece.  AP Edition: Biology.  7th ed.  San Francisco: Pearson: Benjamin Cummings, 2005.

 

 

Prions are infectious proteins that cause a number of degenerative brain diseases such as mad cow and Creutzfeldt-Jakob.  They are slow acting and have incubation periods up to 10 years; they are also virtually indestructible and are unaffected by cooking.  Other details about prions are unkown, and they are quite lethal.  (Evan Johnson)

 

Campbell, Neil A., and Jane B. Reece.  AP Edition: Biology.  7th ed.  San Francisco: Pearson: Benjamin Cummings, 2005.

 

The Lytic Cycle

•     is a phage reproductive cycle that ends in with the deah of the host cell.
•     refers to the last stage of infection, during which the bacterium lyses and releases phages that were producted within the cell.
•     each of these phages can then infect a healthy cell, and a few successive lytic cycles can destroy an entire bacterial population.
•     a virulent phage is a phage that only reproduces by this cycle.

 

 http://www.youtube.com/watch?v=wVkCyU5aeeU&feature=related

 (Fernandez)

 

The Lysogenic Cycle

By definition, the lysogenic cycle is a phage replication cycle in which the viral genome becomes incorporated into the bacterial host chromosome as a prophage and does not kill the host. It replicated the phage genome without destroying the host. A temperate phage is a phage that is capable of reproducing by either the lytic or lysogenic cycle. Infection of an E. coli cell by phage lambda begins with the phage binds to the surface of the cell and injects its DNA. Either the lytic or lysogenic cycle occurs next. During a lysogenic cycle, the lambda DNA molecule is incorporated by genetic recombination into a specific site on the host cell's chromosome. The viral DNA is known as a prophage when integrated into the bacterial chromosome in this way. (Mroz)

 

Campbell, Neil A., and Jane B. Reece. AP Edition: Biology. 7th ed. San Francisco: Pearson: Benjamin Cummings, 2005.

 

http://diverge.hunter.cuny.edu/~weigang/Lecture-syllabus.html 

This is a diagram of the lytic and lysogenic cycles of phage lambda, a temperate phage. After entering the bacterial cell and circularizing, either the lytic cycle or lysogenic cycle will be initiated by the lambda DNA. The lytic cycle occurs when the lambda DNA initiates the production of a large number of progeny shapes. The lysogenic cycle occurs when the lambda DNA integrates into the bacterial chromosome. Most of the time, phage lambda follows the lytic pathway. However, once the lysogenic cycle begins, the prophage may be carried in the host cell's chromosome for many generations. Phage lambda has one main short tail fiber. (Mroz)

 

Campbell, Neil A., and Jane B. Reece. AP Edition: Biology. 7th ed. San Francisco: Pearson: Benjamin Cummings, 2005.

 

 

http://www.youtube.com/watch?v=_J9-xKitsd0 (Medellin)

Review Questions!

1. What would occur if the repressor of an inducible operon were mutated so it could not bind the operator?

A. continuous transcription of the operon's genes

B. reduced transcription of the operon's genes

C. buildup of a substrate for the pathway controlled by the operon

D. irreversible binding of the repressor to the promorter

E. overproduction of catabolite activator protein

 

2. Emerging viruses arise by

A. mutation of existing viruses

B. the spread of existing viruses to new host species

C. the spread of existing viruses more widely within their host species

D. all of the above

E. none of the above

 

3. Transposition differs from other mechanisms of genetic recombination because it

a. occurs only in bacteria

b. moves genes between homologous regions of the DNA

c. plays little or no role in evolution

d. occurs only in eukaryotes

e. scatters genes to new loce in the genome

 

4. Which of the following characteristics or processes is common to both bacteria and viruses?

a. binary fission

b. ribosomes

c. genetic marerial of nucleic acid

d. mitosis

e. conjugation

 

 

answers: a, d, e, c (Ramey)

 

MORE Review Questions:

 

1.  A bacterium is infected with an experimentally constructed bacteriophage composed of the T2 phage protein coat and T4 phage DNA. The new phages produced would have

     a) a mixture of the DNA and proteins of both phages

     b) T4 protein and T4 DNA

     c) T2 protein and T2 DNA

     d) T2 protein and T4 DNA     

     e) T4 protein and T2 DNA

 

2. RNA virus require their own supply of certain enzymes because

     a) host cells lack enzymes that can replicate the viral genome

     b) these enzymes cannot be made in host cells

     c) host cells rapidly destroy the viruses

     d) these enzymes translate viral mRNA into proteins

     e) these enzymes penetrate host cell membranes

 

3.  Genetic Variation in bacterial populations never results from

     a) conjugation

     b) meiosis     

     c) mutation

     d) transformation     

     e) transduction

 

4.  If a particular operon encodes enzymes for making an essential amino acid and is regulated like the trp operon, then

     a) the amino acid acts as a corepressor     

     b) the amino acid inactivates the repressor     

     c) the enzymes produced are called inducible enzymes

     d) the amino acid turns on transcription of the operon

     e) the repressor is active in the absence of the amino acid

 

5. Which of the following is descriptive of an R plasmid?

     a) it is a good example of a composite transposon

     b) its transfer converts and F- cell into an F+ cell

     c) it has genes for antibiotic resistance and maybe for sex pili

     d) it makes bacteria resistant to phage

     e) it is transferred between bacteria by transduction

 

ANSWERS: 1)B  2)A  3)B  4)A  5)C

 

(Langguth)

 

 

 

 

 Even More Review Questions

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Works Cited

Campbell, Neil A.. AP Edition Biology. 7th. San Francisco, CA: Pearson Benjamin Cummings, 2005.