Transcript Lecture #14 - Suraj @ LUMS

Control of Expression
Bacteria do not require the same enzymes
all the time; they produce enzymes
needed at the moment.
Bacterial Genome
Bacterial Genetics
• A segment of the DNA that codes for a specific
polypeptide is known as a structural gene.
• These often occur together on a bacterial chromosome.
• The location of the polypeptides, which may be enzymes
involved in a biochemical pathway, for example, allows for
quick, efficient transcription of the mRNAs.
• E. coli can synthesize 1700 enzymes. Therefore, this small
bacterium has the genes for 1700 different mRNAs.
• Regulation (control) of expression is explained by the
Operon Model.
Operons
• Operons are either inducible or repressible
according to the control mechanism.
• Seventy-five different operons controlling
250 structural genes have been identified
for E. coli.
The Operon Model
• Proposed by Fancois Jacob and Jacques Monod.
• Groups of genes coding for related proteins are arranged in units
known as operons.
• An operon consists of an:– 1.operator
– 2.promoter
– 3.regulator
– 4.structural genes
• The regulator gene codes for a repressor protein that binds to the
operator, obstructing the promoter (thus, transcription) of the
structural genes.
• The regulator does not have to be adjacent to other genes in the
operon.
• If the repressor protein is removed, transcription may occur.
The lac Operon
1. If E. coli is denied glucose and given lactose instead, it
makes three enzymes to metabolize lactose.
2. These three enzymes are encoded by three genes. One
gene codes for b-galactosidase that breaks lactose to
glucose and galactose. A second gene codes for a
permease that facilitates entry of lactose into the cell. A
third gene codes for enzyme transacetylase, which is an
accessory in lactose metabolism.
3. The regulator gene codes for a lac operon repressor
protein that binds to the operator and prevents
transcription of the three genes.
Regulation of the lac Operon
1.
2.
3.
When lactose is present it binds to the repressor,
the repressor undergoes a change in shape that
prevents it from binding to the operator.
Because the repressor is unable to bind to the
operator, the promoter is able to bind to RNA
polymerase, which carries out transcription and
produces the three enzymes.
An inducer is any substance, lactose in the case of
the lac operon, that can bind to a particular
repressor protein, preventing the repressor from
binding to a particular operator, consequently
permitting RNA polymerase to bind to the promoter,
causing transcription of structural genes.
Lactose absent
Lactose present
The trp Operon
1. Some operons in E. Coli exist in the on rather than the off
condition.
2. This cell produces five enzymes to synthesize the amino
acid tryptophan. If tryptophan is already present in
medium, these enzymes are not needed.
3. In the trp operon, the regulator codes for a repressor that
usually is unable to attach to the operator; the repressor
has a binding site for tryptophan.
4. When tryptophan binds this changes the shape of the
repressor that can now bind to the operator.
5. The entire unit is called a repressible operon;
tryptophan is the corepressor.
6. Repressible operons are involved in anabolic pathways
that synthesize substances needed by cells.
Plasmids
• Are small DNA fragments,
• They carry between 2 and 30 genes.
• Some seem to have the ability to move in and out of the
bacterial chromosome they are called episomes.
• Plasmids are self-replicating in a manner like the bacterial
chromosome.
• Many plasmids have been recognized for E. coli, including
the F plasmids ("sex factors") and R plasmids
(drug/antibiotic resistance).
• The F plasmid contains 25 genes, some of which control
the production of F pili (proteins which extend from the
surface of F+, or male, cells to the surface of F-, or female,
cells).
R Plasmids
• As many as 10 resistance genes can be contained on a
single R plasmid.
• The R plasmids can be transferred to other bacteria of the
same species, to viruses, and even to bacteria of different
species.
• Drug (antibiotic) resistance has been found among
pathogens causing the diseases typhoid fever,
gastroenteritus, plague, undulant fever, meningitis, and
gonorrhea.
• In addition to the more common modes of transfer, R
plasmids may be passed through the cell membrane.
Regulation of Eukaryotic Gene
Expression
• Complicated by the process of development unique to
multicellular organisms.
• Each multicellular organism begins as a single-celled zygote
which divides by mitosis.
• Cells differentiate into functional types by using some genes
but ignoring others.
• The timing of certain gene expressions seems to follow a
sequence, such as the production of different types of fetal
hemoglobins by mammalian red blood cells, which switch to
adult hemoglobin sometime after birth.
The Eukaryotic Genome
• Eukaryotes have only 10% of their DNA coding for
proteins.
• The rest has no function or has a function not yet known.
• Humans may have a little as 1% coding for proteins.
Viruses and prokaryotes use a great deal more of their
DNA.
• Almost half the DNA in eukaryotic cells is repeated
nucleotide sequences. Protein-coding sequences are
interrupted by non-coding regions. Non-coding
interruptions are known as intervening sequences or
introns. Coding sequences that are expressed are exons.
Introns and exons
• Most, but not all structural eukaryote genes contain
introns.
• These introns are excised (cut out) before translation (a
seemingly energy inefficient process).
• The number of introns varies with the particular gene, even
occurring in tRNAs, rRNAs and viral genes!
• Generally the more complex and recently evolved the
organism, the more numerous and larger the introns.
Which came first: continuous genes lacking introns or
interrupted genes containing introns?
• Introns have been hypothesized to promote genetic
recombination (via crossing-over), thus speeding up the
evolution of new proteins.
• Exons code for different functional regions of proteins
There are four primary levels of
control of gene activity.
a.
b.
Transcriptional control – in nucleus
Posttranscriptional control - in nucleus after DNA
is transcribed and preliminary mRNA forms.
1.
2.
c.
d.
This may involve differential processing of preliminary
mRNA before it leaves the nucleus.
Speed with which mature mRNA leaves nucleus affects
ultimate amount of gene product.
Translational control occurs in cytoplasm after
mRNA leaves nucleus but before protein product.
Posttranslational control – in the cytoplasm after
protein synthesis.
1.
2.
Polypeptide products may undergo additional changes
before they are biologically functional.
A functional enzyme is subject to feedback control; binding
of an end product can change the shape of an enzyme so
it no longer carries out its reaction.
Transcription
• The process of transcription in eukaryotes is similar to that
in prokaryotes, although there are some differences.
• Eukaryote genes are not grouped in operons as are
prokaryote genes.
• Each eukaryote gene is transcribed separately, with
separate transcriptional controls on each gene.
• Prokaryote translation begins even before transcription has
finished, while eukaryotes have the two processes
separated in time and location.
Processing of mRNA
• After eukaryotes transcribe an RNA, the RNA transcript is
extensively modified before export to the cytoplasm.
• A cap of 7-methylguanine is added to the 5' end of the
mRNA; this cap is essential for binding the mRNA to the
ribosome.
• A string of adenines (as many as 200 nucleotides known as
poly-A) is added to the 3' end of the mRNA after
transcription. The function of a poly-A tail is not known,
but it can be used to capture mRNAs for study.
• Introns are cut out of the message and the exons are
spliced together before the mRNA leaves the nucleus.
• Protein molecules are attached to mRNAs that are
exported, forming ribonucleoprotein particles (mRNPs)
which may help in transport through the nuclear pores and
also in attaching to ribosomes.
Processing of Eukaryotic mRNA
Splicing
Many Viruses Cause Disease in
Animals
Reproductive Cycle of an Animal Virus
1. Entry: Virus gets inside cell.Attachment: Virus
attaches to a specific receptor on cell
surface.Penetration: Virus fuses to cell membrane and
enters cell.
2. Uncoating: Viral capsid releases genetic material.
3. Synthesis: Genetic material is copied, viral proteins
are made.
4. Assembly: Genetic material is packaged into
capsids.Release: New viruses (50-200) leave the cell
through: Lysis: Cells burst and die.Budding: Cell
does not necessarily die.
The AIDS Virus Makes DNA from
RNA
• Human Immunodeficiency Virus (HIV):
Causes AIDS.
• HIV is a retrovirus, which contains the enzyme
reverse transciptase.
• Flow of genetic information is reversed:
Transcription
Translation
RNA
DNA
Protein
Reverse Transcription
• Viral DNA is inserted into host chromosome as
a provirus.
Infection of a Cell by HIV