8.2.2 Regulation of transcription and translation

What are transcription factors?

• Proteins which regulate (stimulate or inhibit) transcription of specific target genes in eukaryotes

• By binding to a specific DNA base sequence on the promoter region of a target gene

Describe the first step in how transcription factors regulate gene expression.

1. Transcription factors diffuse from the cytoplasm to the nucleus

What is the specific action of a transcription factor once it enters the nucleus?

Bind to DNA at a specific DNA base sequence on the promoter region of a target gene (before / upstream of target gene)

What is the final effect of a transcription factor binding to the promoter?

This stimulates or inhibits transcription (production of mRNA) of target gene(s) by activating or preventing RNA polymerase binding

Explain step 1 of how oestrogen affects transcription.

1. Oestrogen is a lipid-soluble steroid hormone so diffuses into cell across the phospholipid bilayer

What happens to oestrogen once inside the cytoplasm?

2. In cytoplasm, oestrogen binds to its receptor, an inactive transcription factor, forming an oestrogen-receptor complex

What is the key conformational change that occurs after binding?

3. This changes the shape of the inactive transcription factor, forming an active transcription factor

Where does the active complex go next?

4. The complex diffuses from cytoplasm into the nucleus

What is the specific action of the complex inside the nucleus?

5. Then binds to DNA at a specific DNA base sequence on the promoter region of a target gene

What is the final effect of this binding?

6. This stimulates or inhibits transcription (production of mRNA) of target gene(s) by activating or preventing RNA polymerase binding

Explain why oestrogen only affects target cells.

• Other cells do not have oestrogen receptors.

What property allows oestrogen to diffuse directly into a cell?

• It is a lipid-soluble steroid hormone.

What are interfering RNA (RNAi)?

• Small lengths of non-coding RNA that regulate translation of specific target genes.

What is RNA interference (RNAi)?

• Inhibition of translation of mRNA produced from target genes, by RNA molecules eg. siRNA, miRNA

• This inhibits or silences the expression of a target gene

• This happens in eukaryotes and some prokaryotes.

What process does RNAi directly inhibit?

• The translation of mRNA.

What is the overall effect of RNAi on a gene?

• It silences or inhibits the expression of the target gene.

What is the first step in the regulation of translation by RNA interference?

1. Small interfering RNA (siRNA) or micro-RNA (miRNA) is incorporated into and binds to a protein, forming an RNA-induced silencing complex (RISC)

   • siRNA synthesised as double-stranded RNA → 1 strand incorporated

   • miRNA synthesised as a double-stranded hairpin bend of RNA → both strands incorporated

What does the single-stranded RNA within the RISC do?

2. Single-stranded miRNA / siRNA within RISC binds to target mRNA with a complementary base sequence

What are the two possible outcomes of this binding?

3. This leads to hydrolysis of mRNA into fragments which are then degraded OR prevents ribosomes binding

What is the final result of this process?

4. Reducing / preventing translation of target mRNA into protein