Regulation of transcription and translation Flashcards
Describe how transcription is initiated
- Molecules called transcription factors move from the cytoplasm, into the nucleus to initiate
transcription. - Each has a binding site, specific to a base sequence on the DNA called the promoter region.
- Once bound, transcription is stimulated.
Describe how genes are not expressed
- When the gene is switched off, the binding site on the transcription factor is inactivated (either by using an inhibitor molecule that binds to
the DNA binding site, or by using a repressor
molecule to block the promoter) - this means that transcription is inhibited
How are genes not expressed using inhibitor molecule?
- Binding site on transcription factor is inactivate by the inhibitor molecule binding to the DNA binding site
- RNA polymerase cannot bind
- Transcription is inhibited
How are genes not expressed using a repressor molecule?
- Binding site on transcription factor is inactivated by repressor molecule blocking the promoter
- RNA polymerase cannot bind
- Transcription is inhibited
How are genes expressed again?
- Oestrogen (which is non-polar and lipid soluble) diffuses directly across the phospholipid bilayer. Once inside the cytoplasm, it binds to complementary shaped sites on special receptor molecules on the transcription factor
- This causes the DNA binding site on the transcription factor to change shape. The transcription factor is now complementary to the promoter region of the target gene. If an inhibitor molecule were present on the transcription factor, this gets released
- The transcription factor is now able to enter the nucleus, and bind to promoter region at start of gene. This activates RNA polymerase to transcribe the gene.
Oestrogen is a hormone that affects transcription. It forms a complex with a receptor in the cytoplasm of target cells. Explain how an activated oestrogen receptor affects the target cell.
- (Receptor/transcription factor) binds to promoter which stimulates RNA polymerase
/enzyme X ; transcribes gene / increases transcription
Oestrogen only affects target cells. Explain why oestrogen doesn’t affect other cells in the body.
- Other cells do not have the oestrogen receptors
Some breast tumours are stimulated to grow by oestrogen. Tamoxifen is used to treat these breast tumours. In the liver, tamoxifen is converted into an active substance called endoxifen. Figure 2 shows a molecule of oestrogen and a molecule of endoxifen. Use
figure 2 to suggest how endoxifen reduces the growth rate of these breast tumours.
- Similar shape to oestrogen ;
- binds receptor / prevents oestrogen from binding ;
- receptor not activated / will not attach to promoter / no transcription
Suggest how transcription factors can reprogramme cells to form iPS cells
- Attach to gene / DNA / promoter region
- Stimulate / inhibit transcription / RNA polymerase
Using all the information provided, evaluate the use of iPS cells to treat type I diabetes in humans
- (Effective as) group A / with iPS / treated lower than group B / with diabetes
- Effective as) group A similar to group C / without diabetes
- Investigation done on mice, not humans
- Only shows results for 12 weeks / short time-period / long term effects not know
Describe the Post-transcriptional regulation of gene expression in RNAi (RNA interference)
- DICER enzyme breaks double-stranded mRNA into smaller siRNA.
- A molecule of siRNA binds to RISC enzyme, and then separates, leaving a single siRNA strand attached
- This strand of siRNA guides the RISC enzyme towards the mRNA, as the siRNA is able to bind to complementary bases.
- RISC enzyme hydrolyses (and destroys) the mRNA, cutting it into small sections. The mRNA cannot be translated into a polypeptide – the gene is not expressed
What is RISC?
- RNA induced silencing complex enzyme
What is siRNA?
- short interfering RNA
Why may Post-transcriptional regulation of gene expression in RNAi (RNA interference) be used?
- This process allows us to identify the role of genes by observing the effect when we block them
- This process allows us to prevent certain
genetic diseases by using siRNA to block genes and production of harmful protein
Give two differences between siRNA and ai) DNA and aii) mRNA
ai) DNA contains thymine but siRNA contains uracil / DNA made of deoxyribose sugar as opposed to ribose sugar
aii) mRNA is a bigger molecule than siRNA / mRNA is single stranded whereas siRNA is double stranded
Huntingtons disease is a disorder in which the protein produced by a mutant gene causes progressive death of cells in the brain. The cells of sufferers from this condition frequently contain one mutant gene and one normal gene. Figure 1 shows how siRNA could be used in the treatment of such conditions. Use the diagram, and your knowledge of siRNA, to explain how siRNA might one day be used to treat Huntingtons disease.
- DNA coding for siRNA is introduced and siRNA is produced is specific to the Huntington mRNA
- siRNA forms silencing complex
- Complex joins to Huntington mRNA and destroys it. mRNA cannot be translated into the Huntington protein
- reduces symptoms of disease
The scientists began by lysing (breaking open) cells and organelles using a detergent that
dissolves lipids in water. Suggest how detergent releases CENP-W from cells.
- Cell membranes made from phospholipid; (Detergent) dissolves membranes / phospholipid (bilayer) (Reject hydrolysis)
Explain how ultracentrifugation separates CENP-W from other molecules.
- Spin (liquid / supernatant) at (very) high speed (not just faster);
- Molecules / CENP-W separates depending on (molecular) mass / size / density / weight
The diagram shows the results of the electrophoresis. The size of each band is proportional to the amount of CENP-W or tubulin present.
c) Suggest how the siRNA produced these results.
- siRNA binds to / destroys mRNA for CENP-W; (Reject if siRNA binds to gene / DNA / tubulin)
- Prevents translation of CENP-W;
- After / as) CENP-W reduces so does tubulin production
When siRNA binds to mRNA, name the complementary base pairs holding the siRNA and mRNA together
- Cytosine with Guanine and (Adenine) with Uracil
This siRNA would only affect gene expression in cells infected with HIV. Suggest two reasons why.
- Only infected cells have HIV protein on surface; - carrier only attaches to / specific to these cells /
siRNA can only enter these cells; - OR siRNA complementary / specific to one mRNA;
- Only infected cells contain mRNA of HIV / stops translation of this gene / destroys this mRNA
The carrier molecule on its own may be able to prevent the infection of cells by HIV. Explain how.
- Carrier binds to (protein on) HIV;
- Prevents HIV / it binding to (receptor on human) cell
Epigenetics
- study of heritable changes to gene function and expression that are a result of environmental factors, and NOT the result of altering the base sequence of DNA itself.
What does the epigenome determine?
- determines and changes the shape of the DNA histone complex
Why can tightly packed DNA genes not be expressed? (Heterochromatin)
- gene cannot be accessed by RNA polymerase or transcription factors
Why can loosely packed DNA genes be expressed? (Euchromatin)
- gene can be accessed by RNA polymerase or transcription factors
Acetylation of histones
- addition of acetyl groups to the histones (taken from acetyl coenzyme A).
- This causes the nucleosomes to form less
tightly and transcription to occur
Deactylation of histones
- removal of acetyl groups from histones (by histone deacetylase, HDAC).
- This causes stronger positive charges on histones, increasing their attraction to phosphate groups in DNA, therefore leading to a more tightly packed structure which is inaccessible to gene transcription.
- This is known as epigenetic gene silencing
Methylation of DNA
- addition of a methyl group (CH3) to the cytosine bases of DNA.
- Methylated DNA is usually silenced, so genes are not transcribed to mRNA.
How does methylation of cytosine
turns off gene expression by changing the state
of the chromatin?
- Blocking binding of transcriptional factors
- Attracting proteins that condense the DNA –
histone complex (forming heterochromatin)
Huntington’s disease is a disorder caused when a protein called Huntingtin damages the brain.
Huntingtin is produced because of a dominant, mutant allele. Suggest and explain one way epigenetics may affect the age when symptoms of Huntington’s disease start.
- Increased methylation of DNA/gene which inhibits / prevents transcription
- OR Decreased methylation of DNA/gene which stimulates / allows transcription
- OR Decreased acetylation of histones which inhibits transcription
- OR Increased acetylation of histones which stimulates / allows transcription
Factors when genes are active
- Increased acetylation of histones
- Decreased methylation of DNA
- Loose DNA histones complex
- Euchromatin
- Access to transcriptional factors
Factors when genes are inactive
- Decreased acetlyation of histones
- Increased methylation of DNA
- Tight DNA histones complex
- Heterochromatin
- No access for transcriptional factors
