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Chapter 19
I. Viruses
1. bacteriophages (phages)
a) capsid (capsomeres)
b) genome (usually DNA)
c) cycles (lytic / lysogenic)
Lytic Cycle
Virulent phages use just this cycle ( )
1) viral DNA enters bacterium
2) host’s DNA hydrolyzed
3) phage DNA directs formation of viral
DNA and proteins
4) self assembly
5) new viruses “burst” out
Lysogenic Cycle
1. Viral DNA enters bacterium
2. Viral DNA integrates into host chromosome
3. viral DNA now called a prophage
4. bacteria reproduces, daughter cells all
receive prophage
5. prophage randomly exits to initiate lytic
stage
Final notes
A temperate phage can use both lytic and
lyosogenic cycles
• Bacteria contain “restriction enzymes” to
cleave foreign DNA
Animal viruses
a)Usually contain an “envelope” surrounding the
capsid -from?
b) Can be RNA or DNA viruses
c) Involves formation of “proviruses” which are
permanent in host genome
d) Have cycles “sort of” like bacteriophages
HIV retrovirus (
)
• Carries RNA and reverse trancriptase in capsid
Viruses and cancer
Tumor viruses disrupt normal cell cycle:
a) some may carry oncogenes- genes to send
cell cycle out of control
b) some viruses may turn on protooncogenes at inappropriate times
Plant viruses
I. Horizontal transmission – plant is infected
from external source of virus( insect vector)
II. Vertical transmission – plant is infected from
a parent (cuttings, or infected seeds)
Bacteria chapter 27
• Bacterial genomes are larger than viral
genomes, but much smaller than a typical
eukaryotic genome
• Most DNA in a bacterium is found in a single
circular chromosome that is composed of
double-stranded DNA found in the nucleiod
region.
• Many bacteria also contain extra DNA in
plasmids- beneficial but not normally
essential
• Bacterial reproduction
reproduce by binary fission, which is preceeded
by DNA replication.
• rapid reproductive rate,useful for genetic
studies.
• Binary fission is asexual reproduction that
produces daughter cells that are genetically
identical to the parent
Genetic recombinations
1. Transformation – the process during which a
bacterial cell assimilates foreign DNA from
the surroundings.
•  Specialized proteins on the cell membrane
of some bacteria facilitate this kind of DNA
uptake.
2. Transduction – the process of gene transfer
from one bacterium to another by a
bacteriophage (a virus)
•  General transduction occurs when random
pieces of host cell DNA are packaged within a
phage capsid during the lytic cycle.
•  specialized transduction involves a
temperate phage
• Conjugation – the direct transfer of genes
between two cells that are temporarily joined
•  A donor bacterium (F+ or Hfr) produces a
tube, or pilus, that connects to the recipient
bacterium.
• Transposons – DNA sequences that can move
from one chromosomal site to another
•  When the transposon’s genes are not
replicated before the move it is called
conservative transposition
•  When the transposon’s genes are replicated
before the move it is called replicative
transposition
Gene regulation and Operons
I. Operon
a) promoter
b) operator
c) genes to be transcribed(transcription unit)
Repressible operons
ex) trp operon (tryptophan)
1. operon is usually turned “on”
2. repressor protein (made at a distant
regulatory gene) is manufactured in
an “off” formation
3. tryptophan acts as a corepressor to
activate the repressor to shut off own
production
Inducible operons ex) lac operon – for lactose
hydrolysis
1. operon is usually off because repressor
protein (from distant regulatory gene) codes
for repressor in “on” formation
2. allactose (lactose isomer) acts as inducer
to turn “off” repressor – thus induces the
turning on of a gene for its own breakdown.
In the presence of lactose
Cyclic AMP (cAMP) accumulates in a cell
without glucose
cAMP activates CRP (cAMP receptor protein)
which binds to promoter area to increase rate
of transcription of lac gene.
When glucose is present, cAMP levels fall
Transcription of lac gene slows due to inactive
CRP