Transcript Chapter 18, 19, 20 Summaries

Chapter 18, 19, 20 Summaries
Gene Expression
Viruses
Biotechnology
Gene Expression (Chapter 18)
• Regulated by altering gene expression in
response to a changing environment
• Regulates both development and
differentiation of cells
• RNA molecules play many roles in gene
expression in eukaryotes
Prokaryotes (Bacteria)
• Often respond to their environment by
regulating transcription
• This regulation can be done by feedback
inhibition or gene regulation
• Gene expression in bacteria is controlled by
the operon model
Operons
• A cluster of functionally related genes can be
under the control of a single on-off switch
• This switch is a segment of DNA called an
operator (usually positioned within the promoter
sequence)
• So an operon is the entire stretch of DNA
including the promoter the operator and the genes
they control
How it works
• The operon can be switched off by a
repressor (protein)
• The repressor prevents gene transcription by
binding to the operator and blocking the
action of RNA polymerase
• The repressor is the product of a separate
regulatory gene
How it Works 2
• The repressor may have an active or
inactive form, depending on the presence of
a particular molecule
• A corepressor is a molecule that cooperates
with a repressor protein to switch an operon
off
• Example: E. coli can make the amino acid
tryptophan
How it works (3)
• The “default “ setting allows the genes for
tryptophan synthesis to be tanscribed
• If tryptophan is present, especially in large
amounts, it binds to a tryptophan repressor protein
and turns the operon off, no longer transcribing
genes that make tryptophan
• This repressor is only active when the corepressor
tryptophan is present
• This prevents the bacteria from making too much
tryptophan (form of feedback inhibition)
Repressible and Inducible
Operons
• The tryptophan operon
is a repressible operon
and is repressed in the
presence of tryptophan
• Inducible operons are
ones that are usually
off
• A molecule called an
inducer inactivates the
repressor and turns on
transcription
The lac Operon
• An inducible operon; 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 second molecule called an inducer then
inactivates the repressor to turn the lac
operon on….
• Inducible operons usually function in catabolic
pathways and their synthesis is induced by a
chemical signal
• Repressible enzymes usually function in anabolic
pathways; their synthesis is repressed by high
levels of the end product
• This type of regulation is referred to as negative
gene regulation because operons are turned off by
the active form of the repressor
Positive Gene Regulation
• Some operons can be stimulated by a protein
(ex:CAP or catabolite activator protein) to activate
transcription
• When glucose is scarce, the CAP binds with
Cyclic AMP
• Activated CAP attaches to the promoter of the lac
operon and increases the chemical affinity of RNA
polymerase, thus accelerating transcription
• When glucose levels increase, CAP
detaches from the lac operon and
transcription continues at its normal rate
• CAP helps regulate other operons that
encode enzymes used in catabolic pathways
Differential Gene Expression
• All multicellular organisms gene regulation is
essential for cell specialization which makes it
important in development of embryos
• Almost all the genes in a cell are genetically
identical, so how do we get our many types of
cells?
• Differential gene expression is the expression of
different genes by cells with the same genome
• Errors in gene expression can lead to cancer and
other diseases
• Gene expression is regulated at many stages
Control Elements &
Transcription Factors
• Control elements are
segments of
noncoding DNA that
help regulate
transcription by
binding certain
proteins
• Transcription factors
are proteins that act
along with RNA
polymerase to start
transcription
• There are both general
and specific
transcription factors
Post-Transcriptional Regulation
• Transcription alone cannot account for gene
expression
• Regulatory mechanisms can operate after
transcription
• These allow the cell to fine-tune its
response to changes in the environment
• There are several things that can be
involved in this type of regulation
Alternative RNA Splicing
• Different mRNA molecules are produced
from the same primary mRNA transcript,
depending on which RNA segments are
treated as introns and which are treated as
exons
Differential Gene Expression and
Embryonic Development
• Development of multicellular organisms is
controlled by gene expression
• Materials in the egg can set up gene regulation that
is carried out as cells divide
• Cytoplasmic determinants are maternal substances
in the egg that influence early development
• Early mitotic divisions still contain these and lead
to different gene expression
Induction
• Process by which signal molecules from cells in
the environment cause transcriptional changes in
nearby target cells
• So interactions between cells cause differentiation
into particular cell types
• Cell differentiation is marked by production of
tissue specific proteins (ex. Muscle-specific
proteins for muscle cells and tissue)
Setting up the Body Plan
• Pattern formation is the development of a
spatial (3D) formation of tissues and organs
• It begins with the formation of axes and
body areas such as ventral and dorsal
• Positional Information-molecular clues that
control pattern formation by telling a cell
where it is in relation to other cells or
tissues or axes
Viruses (Chapter 19)
• Viruses consist of either DNA or RNA
surrounded by a protein coat
• They were detected before they were able to
be seen
• They are not cells
• 1935 Wendell Stanley discovered the
Tobacco Mosaic Virus while researching
the disease that stunted tobacco plants
• Viruses are sometimes referred to as
obligate intracellular parasites because they
can reproduce only within a host cell
• Each virus has a Host Range or limited
number of host cells that it can infect
Viral Envelopes
• Some viruses have membranous envelopes
that help them infect host cells
• These surround the capsids of influenza
viruses
• These can be derived from the host’s cell
membrane and contain a combination of
host cell and viral molecules
Reproductive Cycles
• Once the viral genome has entered the cell,
it begins to manufacture viral proteins
• It makes use of host cell enzymes,
ribosomes, tRNA’s, amino acids, ATP and
other molecules
• Viral parts spontaneously self-assemble into
new viruses
Bacteriophages
• Viruses that infect bacteria are called
bacteriophages or phages
• They are the most studied of all viruses and
have directly and indirectly provided us
with many tools we now use in
biotechnology
• They have two reproductive cycles: Lytic
and Lysogenic
The Lytic Cycle of A
Bacteriophage
• The lytic cycle of a virus destroys the
bacterial cell
• The viral DNA or RNA enters the
bacterium and takes over the host cell DNA
• It begins producing viral parts
• The viral parts self-assemble and can cause
the cell membrane to rupture in several
places
Lysogenic Cycle
• This is a reproductive cycle where the virus
enters the cell, becomes a part of the host
cell genome and remains dormant for a time
• If something stimulates it to become
virulent it will enter the lytic cycle and
destroy the cell
• If not, its genome is reproduced along with
the host cell and it “hitches a ride”