BSC-2010 Lecture Notes - Lecture 4: Catabolism, Trp Operon, Lac Operon

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11 Dec 2016
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Viruses and Virology
minimal virus structure: nucleic acid + protein capsid
virus is either a DNA or an RNA virus
depends on the kind of nucleic acid that makes up its genome
capsid — protein chill that encloses the viral genome
rod-shaped, polyhedral, or more complex in shape (depends on type of virus)
built from capsomeres — protein subunits
viral envelope — membranous envelope that surrounds the capsids of the virus
contains host cell phospholipids and membrane proteins, & proteins and glycoproteins of
viral origin
glycoprotein — protein w carbohydrates covalently attached
bacteriophages — viruses that infect bacteria
aka phages
viruses cannot make proteins
can only replicate inside the host cell
viruses have limited host range — can infect cells of only a limited number of host species
viral reproductive cycle:
1. Virus enters the cell and is uncoated, releasing viral DNA and capsid proteins
2. Host enzymes replicate the viral genome
3. Meanwhile, host enzymes transcribe the viral genome into the viral mRNA, which host
ribosomes use to make more capsid proteins
4. Viral genomes and capsid proteins sled-assemble into new virus particles, which exit the
cell
lytic cycle — a phage replicative cycle that culminates in death of the host cell
virulent phage — a phage that replicates only by a lytic cycle
ex: lytic cycle of phage T4
1. Attachment — phage uses tail fibers to bind to bacterium surface proteins
2. Entry of phage DNA & degradation of host DNA — host cell’s DNA is hydrolyzed
3. Synthesis of viral genomes & proteins
4. Self-assembly
5. Release — bacterium lyses (breaks open) & releases the phages produced within the
cell
lysogenic cycle — allows replication of the phage genome without destroying the host
prophage — phage DNA integrates into the bacterial chromosome
temperate phages — capable of using both models of replicating within a bacterium
Regulation of Gene Expression in Bacteria
repressible operon —
operon transcription turned off (inhibited) when a specific molecule is present
generally associated with anabolic (synthetic) pathways
ex: trp operon
transcription is inhibited by tryptophan
inducible operon —
operon transcription turned on (stimulated) when a specific molecule is present
generally associated with catabolic (degradative) pathways
ex: lac operon
transcription stimulated by allolactose, an isomer of lactose
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