Transcript DNA, and in some cases RNA, is the primary source of heritable

DNA, AND IN SOME CASES
RNA, IS THE PRIMARY SOURCE
OF HERITABLE INFORMATION
Noneukaryotic Genetic Information
Bacterial DNA
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Two types of DNA in bacteria.
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The main form of genetic material in
bacteria is a single circular
chromosome made of DNA. The
chromosome replicates via binary
fission. In binary fission, the
chromosomes replicates and the cell
divides into two cells, with each cell
gets an identical copy of the
chromosome.
Bacteria also contain plasmids, small,
circular DNA molecules outside the
chromosome. Plasmids replicate
independently of the chromosome.
Plasmids are not always necessary to
the survival of the bacteria but can
be beneficial to the survival of the
bacteria.
Binary Fission
Genetic Variation in Bacteria
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Conjugation is a process of DNA exchange
between bacteria.
Transduction occurs when DNA is introduced into
the genome of a bacterium by a virus.
Transformation occurs when bacteria absorb DNA
from their surroundings and incorporate it into their
genome,
Mutation occurs when there is a random change in
the DNA
Conjugation
Transduction
Transformation
Summary of Sources of Genetic Variation
Mutations also results in genetic variation
Regulation of Gene Expression
Gene expression in
bacteria is controlled by
the operon model.
An operon is the entire
stretch of DNA that
includes the operator,
the promoter, and the
genes that they control.
Natural selection has
favored bacteria that
produce only the
products needed by
that cell.
A bacteria cell can
regulate the production
of enzymes by
feedback inhibition or
gene regulation via an
operon.
Parts of an Operon
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promoter - region is a sequence of DNA
to which the RNA polymerase attaches to
begin transcription
operator - region can block the action of
the RNA polymerase if the region is
occupied by a repressor protein
structural genes - contain DNA sequences
that code for several related enzymes
that direct the production of some
particular end product
regulatory genes - produces proteins
that either (1) bind to the operator and
block transcription(repressor proteins) or
(2) bind to the repressor which causes it
to release the operator and allow
transcription to take place (activator
proteins).
Repressible Operons – always ON
Repressible operons are
always turned on - meaning
they produce their protein
product until they are
turned off.
The trp operon is an
example. By default the trp
operon is on and the genes
for tryptophan synthesis are
transcribed. When
tryptophan is present, it
binds to the trp repressor
protein, which turns the
operon off. The repressor is
active only in the presence
of its corepressor tryptophan; thus the trp
operon is turned off
(repressed) if tryptophan
levels are high. Repressible
enzymes usually function
anabolic pathways; their
synthesis is repressed by
high levels of the end
product
Inducible Operons – always OFF
Inducible operons are
usually turned off - meaning
they don't produce the
protein until a molecule called
an inducer inactivates the
repressor and turns on
transcription.
The lac operon is an inducible
operon and contains genes
that code for enzymes used in
the hydrolysis and metabolism
of lactose. By itself, the lac
repressor is active and
switches the lac operon off. A
molecule called an inducer
inactivates the repressor to
turn the lac operon on. When
there is no lactose present there is not need for the
enzymes that break it down.
Inducible enzymes usually
function in catabolic
pathways; like digestion; their
syntesis is induced by a
chemical signal (the item that
is meant to be digested).
Positive Gene Regulation
Promoter
The compounds present
determine which operons
are turned on. For
example: positive control
of the lac operon by
catabolite activator
protein (CAP). RNA
polymerase has high
affinity for the lac
promoter only when CAP
is bound to a DNA site at
the upstream end of the
promoter. CAP attaches
to its DNA site only when
associated with cyclic
AMP (cAMP), whose
concentration in the cell
rises when glucose
concentration falls.
DNA
lacI
CAP-binding site
cAMP
lacZ
Operator
RNA
polymerase
Active binds and
CAP
transcribes
Inactive lac
Inactive
repressor
CAP
Allolactose
(a) Lactose present, glucose scarce (cAMP level high):
abundant lac mRNA synthesized
Promoter
DNA
lacI
CAP-binding site
Inactive
CAP
lacZ
Operator
RNA
polymerase less
likely to bind
Inactive lac
repressor
(b) Lactose present, glucose present (cAMP level low):
little lac mRNA synthesized
Operon Assignment
Due 1/20/15
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Explain the concept of an operon and
the function of the operator, repressor,
and corepressor. Be sure to state the
adaptive advantage of grouping
bacterial genes into an operon.
Discuss how repressible and inducible
operons differ and how those
differences reflect differences in the
pathways they control.