Operon Model
1. Bacteria do not require same
enzymes all the time; they produce enzymes as needed.
2. In 1961, French microbiologists
Francis Jacob and Jacques Monod proposed operon model to explain regulation
of gene expression in prokaryotes (they won the Nobel prize for this model).
a. Operon model: several genes code for an enzyme in same metabolic pathway
and are located in a sequence on chromosome; expression of structural genes
is controlled by same regulatory genes.
b. An operon consists of four types of genes that function as a single
unit:
1) Regulator gene---gene that codes for a repressor protein molecule.
2) Promotor---sequence of DNA where RNA polymerase attaches when a gene
is transcribed.
3) Operator---sequence of DNA where repressor can bind, preventing RNA
polymerase from attaching to the promotor.
4) Structural genes---genes coding enzymes of a metabolic pathway; transcribed
as a unit.
A. lac Operon
1. If E. coli cannot
get glucose, but lactose is present, it makes 3 enzymes to metabolize lactose.
2. These three enzymes are encoded
by three genes.
a. One gene codes for -galactosidase that breaks lactose to glucose and
galactose.
b. A second gene codes for a permease that facilitates entry of lactose
into the cell.
c. A third gene codes for an enzyme called transacetylase, which is an
accessory in lactose metabolism.
3. The three genes are adjacent
on chromosome and under control of one promoter and one operator. |
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4. The regulator gene some distance
ahead of the promoter codes for a lac operon repressor protein that binds
to the operator and prevents transcription of the three genes.
5. When E. coli is confronted
with lactose, lactose binds to the repressor, the repressor undergoes a
change in shape that prevents it from binding to the operator.
6. 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.
7. 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.
8. E. coli can tell when
there is no glucose available because:
a. When glucose is absent, cyclic AMP (cAMP) accumulates.
b. Cytosol contains catabolite activator protein (CAP).
c. When cAMP binds to CAP, the complex attaches to the lac promoter.
d. Only then does RNA polymerase bind to the promoter.
B. trp Operon
1. Jacob and Monod found other
operons in E. coli that are normally on (instead of normally off).
2. E. coli produces five
enzymes to synthesize the amino acid tryptophan.
3. If tryptophan is already
present in medium, these enzymes are not needed.
4. 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---if tryptophan is present,
it binds to the repressor.
5. This changes the shape of
the repressor that now binds to the operator.
6. The entire unit is called
a repressible operon and tryptophan is called the corepressor.
7. Repressible operons are involved
in anabolic pathways that synthesize substances needed by cells.
Eukaryotic Gene Expression - short verison
When genes are expressed, the genetic
information (base sequence) on DNA is first transcribed (copied) to a molecule
of messenger RNA in a process similar to DNA replication.
The mRNA molecules then leave
the cell nucleus and enter the cytoplasm, where triplets of DNA bases (codons)
forming the genetic code specify the particular amino acids that make up
an individual protein. This process, called translation, is accomplished
by ribosomes (cellular components composed of proteins and another class
of RNA) that read the genetic code from the mRNA, and transfer RNAs (tRNAs)
that transport amino acids to the ribosomes for attachment to the growing
protein.
Eukaryotic Gene Expression - long version
A. Expression of Genes
1. Eukaryotes lack a universal
regulatory mechanism to control expression of genes coding proteins.
2. There are four primary levels
of control of gene activity.
a. Transcriptional control determines which structural genes are transcribed
and rate of transcription; this includes organization of chromatin and
transcription factors that initiate transcription.
b. Posttranscriptional control occurs in nucleus after DNA is transcribed
and preliminary mRNA forms.
1) This may involve differential processing of preliminary mRNA before
it leaves the nucleus.
2) Speed with which mature mRNA leaves nucleus affects ultimate amount
of gene product.
c. Translational control occurs in cytoplasm after mRNA leaves nucleus
but before protein product.
1) Life expectancy of mRNA molecules can vary, as well as their ability
to bind ribosomes.
2) Some mRNAs may need additional changes before they are translated at
all.
d. Posttranslational control takes place in the cytoplasm after protein
synthesis.
1) Polypeptide products may have to undergo additional changes before they
are biologically functional.
2) A functional enzyme is also subject to feedback control; the binding
of an end product can change the shape of an enzyme so it no longer carries
out its reaction.
B. Looking at Transcriptional Control
1. Chromatin Plays a Role
a. In eukaryotes, DNA is associated with histone proteins.
b. Nucleosome is a unit made of a segment of DNA wound around complex of
histone proteins.
c. At the beginning of cell division, during condensation, the strings
of nucleosomes coil, loop, and wind to form microscopically visible chromosomes.
d. During interphase, some chromatin is highly compact heterochromatin;
rest is diffuse euchromatin.
e. Histones differ little between species; histones can both repress and
activate genes.
f. Heterochromatin
is a form of chromatin that is genetically inactive.
g. Barr body
1) In mammalian females, one of the X chromosomes is in the Barr body.
2) Which X chromosome is condensed is determined by chance.
3) Body of heterozygous females is mosaic; half her cells express alleles
on one X chromosome and half of her cells express the alleles on the other
X chromosome.
4) Female gonads do not show Barr bodies; X chromosomes are both needed
in development.
h. Euchromatin is genetically active.
i. Lampbrush chromosomes in egg cells of vertebrates present many loops
for mRNA synthesis.
j. Polytene chromosome puffs in larval insects made of many duplicated
sister chromatids.
k. Gene amplification is replication of a gene so there are many copies;
Xenopus frog germ cells increase nucleoli (rRNA genes) 1000-fold.
2. Transcription Factors are
Regulatory Proteins
a. Transcription is controlled by DNA-binding proteins called transcription
factors.
b. Each cell contains different transcription factors; different combinations
regulate activity of gene.
c. A group of transcription factors binds to a promotor adjacent to a gene;
then the complex attracts and binds RNA polymerase. Transcription may still
not begin.
d. As well as DNA sequences, enhancers are involved in controlling transcription
in eukaryotes.
1) Enhancers are regions where factors that help regulate transcription
of the gene can bind.
2) Enhancers can be quite a distance from the promoter.
3) Hairpin loop in DNA brings factor attached to enhancer into contact
with transcription factors and RNA polymerase at promotor; enables transcription
to begin.
e. Transcription factors are always present in cell and most likely they
have to be activated in some way (e.g., regulatory pathways involving kinases
or phosphatases) before they bind to DNA.
C. Looking at Posttranscriptional Control
1. Posttranscriptional control
involves differential processing of preliminary mRNA before it leaves the
nucleus and regulation of transport of mature mRNA.
2. Differential excision of
introns and splicing of mRNA can vary type of mRNA that leaves nucleus;
both the hypothalamus and thyroid glands produce calcitonin but the mRNA
that leaves the nucleus is not same in both types of cells; radioactive
labelling shows variation in mRNA splicing.
3. Speed of transport of mRNA
from nucleus into cytoplasm affects amount of gene product realized.
4. There is difference in length
of time it takes various mRNA molecules to pass through nuclear pores.
D. Looking at Translational Control
1. Frog eggs contain mRNA that
is not translated at all until fertilization occurs; these mRNAs are masked
messengers; when fertilization occurs, they unmask and there is a rapid
gene product synthesis.
2. The longer an active mRNA
molecule remains in the cytoplasm, the more product is produced.
3. Ribonucleases are enzymes
associated with ribosomes that degrade mRNA.
4. Mature mRNA has noncoding
segments at 3' and 5' ends; differences in these segments influence how
long the mRNA avoids being degraded.
5. Prolactin promotes milk production
by affecting the length of time mRNA persists and is translated.
6. Estrogen interferes with
action of ribonuclease; prolongs vitellin production in amphibian cells.
E. Looking at Posttranslational Control
1. Some proteins are not active
after translation; polypeptide product has to undergo additional changes
before it is biologically functional.
a. Bovine proinsulin is inactive when first produced.
b. Single long polypeptide folds into a three-dimensional structure, a
sequence is removed from the middle, and the two polypeptide chains are
bonded together resulting in an active protein.
2. The metabolic activity of
proteins is often under feedback control.
Cancer
A. Characteristics of Cancer Cells
1. Cancer cells have a failure
in regulation of genes coding for products that determine cell division.
2. Cancer Cells Lack Differentiation
a. Cancer cells lack structural and functional specialization of normal
epithelial cells, muscle cells, etc.
b. Instead, cancer cells have a shape and form that is distinctly abnormal.
c. Cancer cells exhibit uncontrolled growth.
d. Normal cells enter cell cycle about 50 times and die.
e. Cancer cells cycle indefinitely and are "immortal"; in cell tissue culture,
they die only because they run out of nutrients or are killed by their
own toxic waste products.
3. Cancer Cells Have Abnormal
Nuclei
a. The nuclei of cancer cells are enlarged, and there may be an abnormal
number of chromosomes.
b. The chromosomes have mutated; some parts may be duplicated and some
may be deleted.
c. In addition, gene amplification is seen much more frequently than in
normal cells.
4. Cancer Cells Form Tumors
a. When normal cells come into contact with a neighbor, they stop dividing;
cancer cells do not.
b. In culture, cancer cells form disorganized, multilayered aggregations;
c. Cancer cells have reduced need for growth factors: signaling proteins
received by membrane receptors and needed by normal cells to grow.
d. Cancerous tumor is a neoplasia, an abnormal mass of cells that invades
neighboring tissue.
e. An anaplasia is reversion of cells to primitive form; results in growth
into a disorganized mass.
f. A benign tumor is a disorganized mass of cells, but they do not invade
adjacent tissues.
5. Cancer Cells Undergo Angiogenesis
and Metastasis
a. Angiogenesis is formation of new blood vessels; required to bring nutrients
and oxygen to tumor.
b. Cancer cells release growth factor causing neighboring blood vessels
to branch into cancer tissue.
c. Some modes of cancer treatment are aimed at preventing angiogenesis
from occurring.
d. Cancer in situ has not yet invaded other tissues.
e. Metastasis is the spread of cancer away from the place of origin.
1) Cancer cells must cross the extracellular matrix into a blood or lymphatic
vessel; they have receptors that allow them to adhere to the extracellular
matrix.
2) Cancer cells produce proteinases to degrade matrix and invade underlying
tissue.
3) Cancer cells are motile, have disorganized cytoskeleton, and lack actin
filament bundles.
f. A patient's prognosis is dependent on the degree to which the cancer
has progressed: whether the tumor has invaded surrounding tissues; if so,
whether there is any lymph node involvement; and, whether there are metastatic
tumors in distant parts of the body.
B. Oncogenes and Tumor---Suppressor Genes
1. A growth control network
controls cell division in cells.
2. Two genes regulate growth
control network: proto-oncogenes and tumor-suppressor genes.
3. Proto-oncogenes and Oncogenes.
a. Proto-oncogenes are normal genes that code for proteins in the growth
control network.
b. Mutation of a proto-oncogenes may produce a cancer-causing oncogene.
c. The ras family of genes are implicated in several types of cancers.
1) Alteration of one nucleotide pair converts normal functioning ras proto-oncogene
to oncogene.
2) rasK oncogene found in 25% of lung cancers, 50% of colon cancers, 90%
of pancreatic cancer.
3) The rasN oncogene is associated with leukemias and lymphomas.
4) Both rasK and rasN oncogenes are frequently found in thyroid cancers.
4. Tumor-suppressor genes are
genes that code for a protein that normally suppresses cell division.
a. Mutated tumor-suppressor genes have lost ability to prevent reactions
that lead to cell division.
b. The RB tumor-suppressor gene was discovered in inherited retinoblastoma---a
mutated gene causes eye tumors in young children---RB has been implicated
in breast, prostate and bladder cancers.
c. Signaling proteins act as negative growth factors: TGF can attach to
a receptor and activate RB protein, which turns off expression of proto-oncogene
c-myc.
C. Causes of Cancer
1. Carcinogens are environmental
agents; mutagens increase chances of mutations that lead to cancer.
2. Mutagenic carcinogens include
radiation, organic chemicals, and viruses.
3. Ultraviolet radiation from
sunlight and tanning has resulted in increased skin cancers, esp. melanomas.
4. Diagnostic X rays provide
great medical benefit and minimal risk.
5. Tobacco smoke contains many
organic chemical carcinogens; likely causes many lung cancer deaths and
is associated with cancers of mouth, larynx, bladder, kidney, and pancreas.
6. A diet rich in saturated
fats and low in fiber may cause colon, rectum, and prostate cancer.
7. Industrial chemical carcinogens
include benzene, carbon tetrachloride, asbestos fibers, etc.
8. DNA viruses have been linked
to human cancers: hepatitis B virus with liver cancer, human papillomavirus
with cancer of cervix, and Epstein-Barr virus with Burkitt's cancer.
9. An RNA virus is cause of
adult T-cell leukemia.
10. Breast cancer is prone to
run in families, indicating a genetic risk factor.
11. Immunodeficiencies allow
development of cervical cancer and Kaposi's sarcoma in AIDS patients.
D. Prevention of Cancer
1. Avoid smoking, sunbathing,
alcohol, and unnecessary radiation.
2. Have regular examinations
for cancer.
3. Follow proper dietary guidelines:
avoid obesity; lower total fat intake; eat plenty of high-fiber foods;
increase consumption of foods that are rich in vitamins A and C; cut down
on consumption of salt-cured, smoked, or nitrite-cured foods; and include
vegetables from the cabbage family (e.g., cabbage, broccoli, brussel sprouts,
kohlrabi, and cauliflower) in the diet.
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