6.1.4 Gene Control: Transcription Factors

Question 1

What are transcription factors and what are they used for?

Answer 1

• Eukaryotes can use transcription factors to control gene expression
• Transcription factors are proteins that bind to specific regions of DNA to control the transcription of genes
• Transcription factors allow organisms to respond to their environment

Question 2

What percentage of human genes code for transcription factors?

Answer 2

It is estimated that ~10% of human genes code for transcription factors

Question 3

How do transcription factors work?

Answer 3

• Some transcription factors bind to the promoter region of a gene (i.e. the region of DNA ‘upstream’ of the gene that controls the expression of the gene)
  This binding can either allow or prevent the transcription of the gene from taking place
  The presence of a transcription factor will either increase or decrease the rate of transcription of a gene

Question 4

Describe the role of oestrogen in gene control.

Answer 4

• In mammals, the hormone oestrogen is involved in controlling the oestrus cycle and also in sperm production
• Oestrogen is a lipid-soluble molecule and can therefore diffuse through the plasma membrane of cells
• It then moves to the nucleus and binds to an oestrogen receptor
• These receptors are actually transcription factors that are able to initiate transcription for many different genes by binding to their promoter regions
• Once bound, oestrogen causes a change in the shape of the receptor
• As a result, the receptor moves away from the protein complex it is normally attached to and binds to the promoter region of one of its target genes
• This allows RNA polymerase to bind and to begin transcribing that gene

Question 5

Describe the role of Gibberellin in gene control.

Answer 5

• Plant cells use transcription factors in a similar way to animal cells
• Gibberellin is a hormone found in plants (e.g. wheat and barley) that controls seed germination by stimulating the synthesis of the enzyme amylase
• It does this by influencing transcription of the amylase gene
• When gibberellin is applied to a germinating seed there is an increased amount of the mRNA for amylase present

Question 6

Describe the mechanism of the breakdown of DELLA protein by gibberellin.

Answer 6

• The breakdown of DELLA protein by gibberellin is necessary for the synthesis of amylase
  The following components are involved:
• Repressor protein DELLA
• Transcription factor (the one involved is called PIF)
• Promoter of amylase gene
• Amylase gene
• Gibberellin
• Gibberellin receptor and enzyme
  The process occurs as follows:
• DELLA protein is bound to the transcription factor, preventing it from binding to the promoter of the amylase gene so no transcription can occur
• Gibberellin binds to a gibberellin receptor and enzyme which starts the breakdown of DELLA
• The transcription factor is no longer bound to DELLA protein and so it binds to the promoter of the amylase gene
• Transcription of amylase gene begins
• Amylase is produced