CIE A Level Biology (9700) 2019-2021

Revision Notes

16.3.1 Gene Control

Gene Control

  • The nucleus of every cell in the human body contains the same genes
    • However not every gene is expressed in every cell
    • Not all of these genes are expressed all the time
  • There are several mechanisms that exist within cells to make sure the correct genes are expressed in the correct cell at the right time
    • They involve regulatory genes

Structural & regulatory genes

  • A structural gene codes for a protein that has a function within a cell
    • For example, the F8 gene codes for the protein Factor VIII involved in blood clotting
  • A regulatory gene codes for a protein that helps to control the expression of another gene
  • Structural and regulatory genes that work together are usually found close together

Inducible & repressible enzymes

  • Some genes code for proteins that form enzymes
  • Some enzymes are required all the time and some are required only at specific times
  • The expression of enzyme-producing genes can be controlled
  • Inducible enzymes are only synthesized when their substrate is present
    • The presence of the substrate induces the synthesis of of the enzyme by causing the transcription of the gene for the enzyme to start
  • Repressible enzymes are synthesized as normal until a repressor protein binds to an operator
    • The presence of the repressor protein represses the synthesis of the enzyme by causing the transcription of the gene for the enzyme to stop
  • Controlling when enzymes are synthesized can be beneficial for cells as it stops materials and energy being wasted
    • For example, using materials and energy to synthesize an enzyme when its substrate is not present and it can’t carry out its function would be highly wasteful

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Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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