Download Control of Gene Expression

Control of Gene Expression
1. Recognize example of the operon model in prokaryotes
2. Describe the method of gene control in eukaryotes
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.
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.