Gene Control (Cambridge International A-Level Biology)

Spec point — 16.3.1 · describe the differences between structural genes and regulatory genes and the differences between repressible enzymes and inducible enzymes

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 and 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

• In prokaryotes, structural genes that work together are often grouped in an operon with a single promoter and operator, e.g. the lac operon; the regulatory gene for the lac operon (lacI) is found close to it

Inducible and 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 synthesised 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 synthesised as normal until a repressor protein binds to an operator

  • When the product of the pathway (for example tryptophan) builds up, it binds to and activates the repressor protein, which then binds to the operator and stops transcription of the gene for the enzyme (end-product inhibition)

• 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