OCR A Level Biology

Revision Notes

6.1.4 Gene Control: Transcription Factors

Gene Control: Transcription Factors

  • Prokaryotes use operons to control the expression of genes in cells
  • Eukaryotes also use transcription factors to control gene expression
    • Transcription factors are proteins that bind to specific regions of DNA to control the transcription of genes
  • 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

How transcription factors work

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

Transcription Factor binding to Promoter, downloadable AS & A Level Biology revision notes

A transcription factor binding to the promoter region of a gene which allows RNA polymerase to bind and for transcription to occur

Gene control: oestrogen

  • 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

Gene control: gibberellin

  • 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

Mechanism

  • 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

Gibberellin-Mechanism, downloadable AS & A Level Biology revision notes

The breakdown of DELLA protein by gibberellin allows the transcription factor PIF to bind to the promoter for the amylase gene and for transcription to initiate

Exam Tip

In your exam you may be asked to explain why RNA analysis is important with regards to gene expression. From the outside most cells look almost identical with the same DNA in their nucleus. However we know that they are most likely expressing different genes.

When a cell expresses a gene, RNA is produced by transcription. This RNA present in a cell can be analysed. Scientists can match the RNA present in a cell to specific genes and work out which genes are being expressed in that specific cell.

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