Regulation of Transcription and Translation

Question 1

what are transcription factors?

Answer 1

• proteins which control the expression or inhibition of certain genes by promoting or inhibiting the binding of RNA polymerase

Question 2

how are specialised cells made via transcription factors?

Answer 2

• they move into the nucleus and attach to a promoter region close to the target genes that it affects
• part of each transcription factor is complementary in shape to a particular sequence of nucleotides in a promotor sequence = specificity
• the binding of the transcription factor to the promotor region either promotes or blocks the recruitment of RNA polymerase resulting in either gene expression or inhibition
• the expression of different genes results in different proteins being coded for resulting in specialised cells being produced

Question 3

what is a promotor region

Answer 3

• a region of DNA where transcription of a gene is initiated

Question 4

describe how oestrogen acts as an activator

Answer 4

• oestrogen = lipid soluble therefore can easily pass through cell membrane
• it binds specifically to a receptor protein that is part of a transcription factor
• this changes the shape of the transcription factor and allows it to bind specifically to the promoter sequence of a particular gene
• this allows RNA polymerase to attach to the gene and catalyse the transcription of the gene
• mRNA is then transcribed from the gene and translated into protein at the ribosome

Question 5

what is the rate of cell division controlled by?

Answer 5

• proto-oncogenes - code for proteins which stimulate cell division
• tumour suppressor genes- code for proteins that slow cell division

Question 6

what causes rapid, uncontrolled cell division?

Answer 6

• a mutated proto-oncogene called an oncogene stimulates cells to divide too quickly = rapid cell division
• a mutation in a tumour suppressor gene. The tumour suppressor protein is not made or is non functional = rapid cell division

Question 7

what are the differences between a malignant and benign tumour?

Answer 7

• benign tumours grow slower than malignant tumours
• benign tumours don’t break off and spread to other tissues (non cancerous) whereas malignant tumours do (cancerous)
• the cells in benign tumours often remain differentiated whereas cells in malignant tumours often become undifferentiated
• in benign tumours , cell nucleus is relatively normal in comparison to malignant tumours where the nucleus is larger and darker

Question 8

define epigenetics?

Answer 8

changes in gene function without changes in the DNA base sequence

Question 9

what may cause epigenetic changes to gene function

Answer 9

aspects of environment - e.g. stress, diet, exposure to toxins

Question 10

what is increased methylation of DNA and its effect

Answer 10

• methyl group attaches to DNA sequence of a gene (CpG site)
• increased methylation inhibits transcription by preventing the binding of transcription factors to the promotor sequence so that the gene is not expressed

Question 10

what is decreased acetylation of Histones and its effect

Answer 10

• histones are less acetylated so the chromatin is more condensensed.
• this results in transcription being inhibited as the genes are not accessible to transcription factors

Question 11

how can hypermethylation of tumour-suppressor genes cause cancer

Answer 11

• hypermethylation of tumour suppressor genes = non-transcription of the genes therefore proteins that slow cell division are not produced = rapid uncontrollable cell division

Question 12

How does Endoxifen reduce the growth rate of breast tumours

Answer 12

• it has a similar shape to oestrogen
• Endoxifen binds to receptor on transcription factor preventing Oestrogen from binding
• therefore receptor not activated and so cannot attach to promotor region
• therefore transcription is not initiated

Question 13

explain how siRNA prevents translation

Answer 13

• siRNA are short single stranded sequences of RNA
• siRNA binds to an enzyme that hydrolyses mRNA
• the siRNA binds to a specific molecule of mRNA by complementary base pairing. Thus siRNA guides the hydrolytic enzyme to a target molecule of mRNA
• the enzyme hydrolyses the mRNA molecule preventing the translation of mRNA into protein

Question 14

how can hypomethylation of proto-oncogenes cause cancer

Answer 14

• hypomethylation of proto-oncogenes so they are transcribed more than usual = increase in cell-division stimulating proteins = rapid uncontrollable cell division