Gene Control: Transcription Factors (Cambridge (CIE) A Level Biology): Revision Note

Prokaryotes use operons to control the expression of genes in cells

Eukaryotes also use transcription factors to control gene expression

A transcription factor is a protein that controls the transcription of genes by binding to a specific region of DNA

They ensure that genes are being expressed in the correct cells, at the correct time and to the right level

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

• There are several types of transcription factors that have varying effects on gene expression

• This is still a relatively young area of research and scientists are working hard to understand how all the different transcription factors function

• Transcription factors allow organisms to respond to their environment

• Some hormones achieve their effect via transcription factors

Some transcription factors bind to the promoter region of a 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

For example, PIF is a transcription factor found in plants that activates the transcription of the amylase gene

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

The breakdown of DELLA protein by gibberellin is necessary for the synthesis of amylase

The following components are involved:

• Repressor protein DELLA

• Transcription factor PIF

• Promoter of amylase gene

• Amylase gene

• Gibberellin

• Gibberellin receptor and enzyme

The process occurs as follows:

• DELLA protein is bound to PIF, 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

• PIF 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