NOTES FOR BIOLOGY 1201
Section 001
Spring 2005
DR. STEVEN POMARICO
>>>>>>Most viruses consist of a genome enclosed in a protein shell
Viruses and Prions: Living or Nonliving? - 11.14.1
Viruses range in size between 20 - 100 nm in diameter
Depending on the virus four types of genomes are seen:
1. double stranded DNA
2. single stranded DNA
3. double stranded RNA
4. single stranded RNA
Nucleic acid may be circular or linear
>>>Capsids and viral envelopes
The viral genome is protected by a capsid
---Capsid
Some viruses have viral envelopes that make them more infective
---Viral envelopes
>>>>>>Viruses can only replicate (reproduce) with a host cell
Viruses are obligate intracellular parasites. The types of cells a virus can invade determine the host range of the virus.
---Host range
The host range may be broad (rabies) or narrow (AIDS)
The host specificity is determined by the interaction between virus proteins and receptor sites on host cell surface.
>>>There are a variety of viral life cycles but all include:
-Infecting the host cell
-Using the host resources to
Replicate the viral genome.
Make the capsid protein.
-Assembly of new virus particles
The replication of the viral genome may use the host cells enzymes or enzymes coded for by the virus.
Genome:
DNA => DNA (Normal host cell enzymes; DNA polymerase)
RNA => RNA (Viral coded enzyme; RNA replicase)
RNA => DNA => RNA (Viral coded enzyme; reverse transcriptase, then host cell
enzymes; RNA polymerase)
Host materials used to synthesize viral components
The assembly of new viral particles is spontaneous and weak bond hold the particle together (i.e., hydrogen bonds, van der Waals)
>>>>>>Bacteriophages exhibit two reproductive cycles, the lytic and lysogenic cycles.
Bacteriophage lambda is a double stranded DNA virus that was first shown to reproduce by both the lytic cycle and the lysogenic cycle.
>>>The lytic cycle
Virulent phages reproduce by a lytic replication cycle.
---Virulent phages
---Lytic cycle
The lytic cycle proceeds rapidly through the following steps:
1. Phage attaches to the cell surface.
2. Viral DNA is injected into the cell.
3. Host DNA is destroyed.
4. Viral genes transcribed and translated; viral genome replicated
5. New viruses assembled and cell lyses
Typically this cycle can take 20-30 minutes and produce 100's of virus particles per bacterial host cell infected.
>>>The lysogenic cycle
Some viruses are temperate viruses and can coexist with their bacterial hosts.
---Temperate viruses
These viruses can reproduce by lytic or lysogenic cycle
---Lysogenic cycle
In the lysogenic cycle the viral genome (DNA form) is incorporated into the bacterial host cell’s genome.
A repressor protein inactivates the incorporated prophage.
---Prophage
-The prophage DNA reproduces along with the bacterial DNA
-If prophage DNA leaves bacterial chromosome, then the lytic cycle begins
A lysogenic bacterial cell may express some viral genes. The expression of the viral genes is a lysogenic conversion and changes the host cell’s phenotype
Often this lysogenic conversion results in the production of a toxin
(e.g., botulism, diphtheria, scarlet fever)
Bacterial cells have a defense against bacterial viruses
-Change the receptor site.
-Restriction nucleases that cut up foreign DNA
>>>Enveloped viruses
A membranous viral envelope surrounds some animal viruses. In these viruses the viral envelope is derived from the host cell (either plasma membrane or nuclear envelope) and aid the virus in the inflection of new cells.
The reproductive cycle of an envelope virus is different than those seen in bacteriophage.
1. The virus fuses with the cell membrane.
2. The capsid and viral genome enter the cell.
3. The cell removes the viral capsid.
4. Copies of the viral genome and proteins are reproduced
5. New virus particles assemble and bud off from the cell surface.
The host cell doesn’t lyse so there is no “lytic cycle”
>>>Viral “hide and seek”
HIV: An Attack on the Immune System - 12.9.1
Some animal viruses are able to form a provirus and remain inactive (latent) after infection. After a productive cycle the virus may return to it inactive state.
---Provirus
Two human viruses that form proviruses are Herpes viruses and the retrovirus HIV.
---Retrovirus
Retroviruses carry an enzyme, reverse transcriptase, which makes DNA from the virus RNA genome.
HIV is a retrovirus
>>>>>>Bacteria gene expression
In glycolysis we saw that a pathway could be shut off at a regulatory enzyme. In addition to this type of control a pathway can be shut off by not producing the enzymes at all. This is done by regulating gene expression
- regulation of an enzyme at the gene level (i.e., stop RNA synthesis) is slower, but
more economical
>>>Gene regulation and the operon.
Control Mechanisms: Lactose Metabolism in E. coli - 6.8.1
In bacteria (and some viruses) structural genes with related functions (i.e., the genes of a pathway) are often organized into an operon.
---Operon
The entire cluster of genes is transcribed as one large mRNA
This mRNA contains start and stop codon to produce all the proteins coded for in the operon.
Jacob and Monod’s Model: The lac Operon - 6.8.2
The entire operon is under the control of a single operator.
---Operator
The operator is downstream from the promoter and before the structural genes.
The operator acts as an off/on switch for the operon.
>>>>>>Inducible operons (catabolic pathways) - the lac operon
In this type of operon the operator is off because the repressor is active unless an inducer switches it off
---Inducer
>>>>>>Repressible operons (anabolic pathways) - The trp operon
In this type of operon the operator is on unless a protein repressor switches it off.
---Repressor
The repressor is coded for by a regulatory gene that is located away from the operon it controls.
---Regulatory genes
The repressor is “activated” when a corepressor binds to it.
---Corepressor