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Micorbiology: Chapter 17


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What is occuring during the eclipse period in a one-step growth experiment with a lytic phage?
During the eclipse period, the initial segment of the latent period, the virions that infected the host cell are concealed or eclipsed.
What is occuring during the latent period in a one-step growth experiment with a lytic phage?
During the latent period no phages/virions are released.
What is occuring during the rise period in a one-step growth experiment with a lytic phage?
During the rise period or burst the host cells rapidly lyse and release infective phages.
What is the burst size?
The burst size is the number of viruses produced per infectd cell.
How do T-even phage effect adsorption and penetration?
In adsorption and penetration, the phage attaches to the the host cell by a tail fiber coming in contact with the appropriate receptor. As more tail fibers make contact, base plate binding takes place. Conformational changes occur in the baseplate and sheath , and the tail sheath becomes shorter and wider; The central tube or core is pushed through the bacterial wall and DNA is extruded from the head, through the tail tube, and into the host cell.
What is meant by "early gene expression"?
"Early gene expression" refers to genes that are expressed in the first 5 minutes (for T4) that direct take over of the host cell: nucleases degrade host DNA to nucleotides, alter host RNA polymerase to recognize phage protomers (early mRNA=mRNA transcribed before phage DNA is made)
What kind of genes are expressed in the late period?
In the late period, virion proteins, assembly proteins, and the proteins required for cell lysis and release are transcribed. Mainly the genes required for packaging the genome are expressed.
How does T-even DNA differ from the host DNA?
T-even DNA differs from host DNA in that it contains HMC (hydroxymethylcytosine) instead of cytosine which is modified further by glycosylation.
How do the genomic concatemers arise?
T4 DNA shows terminal redundancy (a base sequence is repeated at both ends). When many DNA copies have been made, about 6 to 10 copies are joined by their terminally redundant ends with the aid of several enzymes, and form concatemers.
What genes are encoded by late mRNA?
Genes that encode for (1) phage structural proteins, (2) proteins that help with phage assembly without becoming part of the virion structure, and (3) proteins involved in cell lysis and phage release are encoded by late mRNA.
How is the virion assembled?
The baseplate is constructed of 15 gene products, when that's finished the tail tube is built on it and the sheath is assembled around the tube. The phage prohead or procapsid is constructed separately of more than 10 proteins and then spontaneously combines witht he tail assmebly. The procaspid is assmebled with the aid of scaffolding proteins that are degraded or removed after construction is completed. A special portal protein is located at the base of the procapsid where it connects to the tail. The portal protein is part of the DNA translocating vetex, a structure that helps initiate head assmebly and aids in DNA movement into and out of the head. Tail fibers attach to the baseplate after the head and tail have come together.
What is encapsidation?
Encapsidation is the process by which a virus' nucleic acid is enclosed in a capsid. The capsid is stuffed with 102% of the genome in a circular permutation.
How does the circular permutation of the genome arise?
The genome that enters the host cell is terminally redundant, there are direct repeats on the ends; when the genome is packaged into the capsid ligase seal these direct repeats and brings about the circular permutation.
How are the virions released from the cell?
At the end of the intracellular phase, genes that encode for lysosyme like proteins that digest peptidoglycan are expressed and the virions are released. (no swelling/bursting)
What is a lysogen?
A lysogen consists of teh host cell and the prophage
What are cI and cro?
CI is a regulator that acts as a lambda repressor - promotes lysogeny. Cro is a regulator protein that promotes lytice growth.
Describe the regulation of the divergent promoters PRM and PR.
When cro is present it prefers to bind to OR3 as a dimer; as the concentration of cro increases it binds to OR2 and OR3; if cro binds then cI can't bind, there is no cI produced, and there is lytic growth. When cI is present it prefers to bind to OR1 as dimers and can form cooperative complexes; when cI binds to OR1 or OR2 it represses PR so no cro is prodcue, therefore lysogeny occurs; if cI binds to OR3 then PRM is repressed and no more cI is produce resulting in lytic growth.
What is the role of the repressor in conferring immunity to the lysogen?
The repressor inhibits mRNA synthesis of aby other lambda phage that tries to infect the lysogen.
How is the prophage induced by RecA?
DNA damage triggers the SOS response/RecA to cleave LexA; cI is cleaved due to resemblance to LexA; when cI (the lambda repressor) is cleaved lysogeny is ended, cro is produced, and it enters a lytic growth cycle.

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