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Gene Action - Operon Hypothesis
- Control of Growth & Development

As mentioned in the genetic control page of this tutorial, some genes are switched on and off depending on the function of the cell. Genes that code for vital metabolites, i.e. essential proteins always remain switched on.

Operon Hypothesis - Enzyme Induction

There is a model hypothesised by two famous scientists, Jacob and Monod that illustrates how certain genes can be active or inactive.

The following diagrams the basics of their model of an operon (where it is possible for their to be more than one structural gene and operator gene). The goal of these genes is to breakdown lactose (a sugar) when it is present in the cell.

Basic Jacob-Monod Operon Model

Lactose Absent

In the absence of lactose, the repressor molecule is freely available to disable the gene

When lactose is absent, the repressor molecule attaches itself to the operator gene, effectively switching it off.

Lactose Present

In the presence of lactose, the repressor molecule is inhibited and the gene remains switched on, and codes for beta galactosidase

  • When lactose is present, the repressor molecule is unable to attach itself to the operator gene, meaning the gene is left 'switched on'
  • The structural gene is now operational, and begins to code for beta-galactosidase, an enzyme which is capable of breaking down lactose into glucose and galactose.
  • The enzyme is produced and breaks down the lactose.
  • In this case, lactose is deemed the inducer, as the presence of it activates enzyme induction.

Inborn Error of Metabolism

Sometimes for hereditary reasons, or mutations, the biochemical pathway cannot be fully executed, due to a dysfunctional or missing gene.

Hereditary defects are known as inborn errors of metabolism, and are present since birth. Mutations can occur at the initial meiotic stages in the formation of gametes, and are possible throughout an organisms lifetime preventing the induction of various enzymes.

Gene Control of Hormones

The next page investigates how genes control the production of hormones within the endocrine system of animals.

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