Transcription Factors
Biology: Cambridge International A Level, topic 16.3.2
Transcription Factors
Genes are expressed by transcription and translation. In eukaryotic cells, not all of the genes are expressed in a specialised cell. Gene expression is controlled by transcription factors.
Function
- Transcription factors are proteins that control gene expression by stimulating or inhibiting the transcription of target genes.
- Transcription factors are produced in the cytoplasm and move to the nucleus.
- In the nucleus, transcription factors bind to a specific region of DNA to stimulate or inhibit the gene.
Activators
- Transcription factors that stimulate gene expression are called activators.
- Activators promote the transcription of the genes by interacting with an enzyme called RNA polymerase and allowing it to bind to DNA.
Repressors
- Transcription factors that inhibit gene expression are called repressors.
- Repressors prevent the transcription of genes by stopping RNA polymerase from binding to DNA.
Hormones
Transcription can also be regulated by hormones. There are two types of hormone involved:
Peptide hormones
- Peptide hormones bind to the cell surface membrane and trigger a secondary messenger response.
- The secondary messenger will lead to the activation or inhibition of transcription of some genes.
Lipid-soluble steroid hormones
- Lipid-soluble steroid hormones can pass through the phospholipid membrane.
- Steroid hormones interact directly with DNA to promote or inhibit gene expression.
- E.g. Oestrogen.
Oestrogen
Oestrogen is an example of a lipid-soluble steroid hormone that can enter the cell and directly interact with DNA to initiate gene transcription. The steps involved are:
1) Enter the cell
- Oestrogen enters the cytoplasm of the cell through the cell surface membrane.
- Oestrogen is lipid-soluble so it can pass through the phospholipid bilayer.
2) Bind to transcription factors
- Oestrogen binds to receptors on transcription factors in the cytoplasm.
- Binding of oestrogen causes the transcription factors to change shape.
- The transcription factors form a receptor-hormone complex that can now enter the nucleus.
3) Bind to DNA
- The receptor-hormone complex binds to the promoter region of the DNA.
- Binding to DNA activates transcription.
- This stimulates protein synthesis.
Test yourself on Transcription Factors
Which of these statements about the transcription factors (TFs) are true?
Which enzyme do repressors bind to in order to inhibit gene expression?
What are the two types of hormone involved in regulating transcription?
Which of the following statements about regulatory hormones are true?
Which of these are NOT a feature of lipid-soluble steroid hormones?