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
Viruses
Viruses consist of
the following
structures:
1. Nucleic acid either DNA or
RNA contains the
heredity
information of the
virus.
2. Protein coat which encloses the
nucleic acid.
3. An envelope
made of
phospholipds and
proteins from the
host cell surround
the protein coat
(capsid) of some
viruses.
Viral Replication
Lytic Cycle
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
In the lytic cycle, a virus
penetrates the cell membrane of
the host and uses the enzymes of
the host to produce viral nucleic
acids and viral proteins. The viral
components are assembled into
new viruses which subsequently
burst from the host cell,
destroying the host cell in the
process.
Retroviruses are special case of
the lytic cycle. Retroviruses use an
enzyme called reverse
transcriptase to make a DNA
complement of their RNA. The
DNA complement can then begin
the lytic cycle or the lysogenic
cycle. HIV is a retrovirus.
Lysogenic Cycle


In the lysogenic cycle, a virus
penetrates the cell membrane
of the host but rather than
take over the cell the nucleic
acid becomes incorporated
into the host cell DNA.
A virus in this dormant state is
called a provirus (prophage).
The virus remains inactive
(being copied every time the
cell divides), until some trigger
causes the virus to begin the
lytic cycle.
Bacterial DNA

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).
Operon Assignment
Due 1/16/13

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.