CIE A Level Biology复习笔记16.3.1 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