[20.3] regulation of transcription and translation

Question 1

what is a transcription factor?

Answer 1

a protein that controls the transcription of genes by binding to a specific region (promoter) of DNA in eukaryotes

Question 2

what is the promoter region?

Answer 2

section of DNA before the start of the coding section of the coding region that is the binding site for proteins that control the extression of the gene

Question 3

which proteins control the expression of the gene? (2)

Answer 3

• RNA polymerase
• transcription factors

Question 4

how do transcription factors work? (4)

Answer 4

1. enter nucleus from cytoplasm via nuclear pores
2. activated through signalling pathways that start from outside of cell
3. TF binds to promoter region of gene
4. binding allows or prevents transcription of the gene taking place

Question 5

how do transcription factors interact with RNA polymerase? (2)

Answer 5

• assist RNA polymerase binding to the gene to stimulate expression of the gene
• prevent RNA polymerase binding to the gene to inhibit gene expression

Question 6

what is the importance of oestrogen?

Answer 6

hormone which can switch on a gene and thus start transcription by combining with a receptor site on the transcriptional factor

Question 7

describe the oestrogen stimulation pathway [7]

Answer 7

1. oestrogen diffuses through CSM into cytoplasm
2. oestrogen diffiuses through a nuclear pore into the nucleus
3. in nucleus, oestrogen attaches to an oestrogen receptor that is held in a protein complex
4. oestrogen receptor undergoes a conformational change
5. new shape of oestrogen receptor allows it to detach from the protein complex and diffuse towards the gene to be expressed
6. oestrogen receptor binds to a cofactor which enables it to bind to the promoter region of gene
7. stimulates RNA polymerase binding and gene transcription

Question 8

how does RNA interference work?

Answer 8

• in eukaryotes and some prokaryotes, siRNA binds to mRNA that has been transcribed from target genes to be silenced as their base sequence is complementary
• each siRNA is attached to a protein complex which is able to break down the mRNA that has been transcribed from target genes
• therefore, mRNA is unable to be translated into proteins

Question 9

describe the RNA interference pathway (8)

Answer 9

1. double-stranded RNA (dsRNA) is produced by RNA-dependent RNA polymerases (RDRs)
2. dsRNA is hydrolysed into smaller fragments, roughly 23 nucleotides long (siRNAs)
3. in the cytoplasm, siRNAs bind to protein complexes which use energy from ATP to separate the two single strands of the siRNA
4. this exposes the nucleotide bases so they are able to pair with bases from an mRNA molecule
5. once the target mRNA leaves the nucleus and enters the cytoplasm, single-stranded siRNA (SSsiRNA) binds to the target mRNA through complementary base pairing
6. mRNA molecule is cut into fragments by the enzyme / protein complex associated with the siRNA
7. cut mRNA cannot be translated and therefore will not produce proteins
8. after the target mRNA has been cut up into fragments, the fragments are broken down into RNA nucleotides by enzymes

Question 10

what are 2 uses of siRNA in medicine?

Answer 10

• to identify the role of genes in a biological pathway - by using siRNA to block certain genes, you can observe what effects occur which tells you what the role of the blocked gene is
• genetic diseases caused by the expression of certain genes - if these genes could be blocked by siRNA, you could prevent the disease caused by them