Regulation of Transcription (AQA A Level Biology): Revision Note

Transcription factors

• Eukaryotes use transcription factors to control gene expression

• A transcription factor is a protein that helps control the process of transcription

• Transcription factors either activate or repress the transcription of a particular gene

How transcription factors work

• Transcription factors enter the nucleus from the cytoplasm through nuclear pores

• They bind to the promoter region at the start of a gene

  • The promoter is a section of DNA upstream of the coding region that is the binding site for proteins that control the expression of the gene

• This binding can either allow or prevent the transcription of the gene

  • Transcription factors either assist in the binding of RNA polymerase (to stimulate expression of the gene) or prevent it from binding (to inhibit gene expression)

• The presence of a transcription factor will either increase or decrease the rate of transcription of a gene

Example: oestrogen

• Oestrogen is a steroid hormone, found in mammals, that functions as a transcription factor

  • Steroid hormones are small, hydrophobic, lipid-based hormones that can diffuse through the cell membrane and can pass directly into the nucleus through nuclear pores

• Oestrogen is involved in controlling the female fertility cycle and is also responsible for stimulating sperm production in males

  • Up to 100 different genes are controlled by oestrogen

The oestrogen stimulation pathway

1. Oestrogen diffuses through the cell surface membrane into the cytoplasm

2. Oestrogen diffuses through a nuclear pore into the nucleus

3. Within the nucleus, oestrogen attaches to an ERα oestrogen receptor that is held within a protein complex, which causes the ERα oestrogen receptor to undergo a conformational change

4. The new shape of the ERα oestrogen receptor allows it to detach from the protein complex and diffuse towards the gene to be expressed

5. The ERα oestrogen receptor binds to a cofactor which enables it to bind to the promoter region of the gene; this stimulates RNA polymerase binding and gene transcription