OCR A Level Biology

Revision Notes

6.1.3 Gene Control: Lac Operon

Gene Control: Lac Operon

  • Regulatory genes control structural genes and their levels of protein production
  • Regulatory genes sometimes have control over several structural genes at once
    • If the structural genes being controlled are in any way involved in the process of transcription, then gene control is occurring at the transcriptional level
  • The lac operon provides an example of a regulatory mechanism at the transcriptional level (i.e. a regulatory mechanism that occurs during transcription)

The lac operon

  • Structural genes in prokaryotes can form an operon: a group or a cluster of genes that are controlled by the same promoter
  • The lac operon found in some bacteria is one of the most well-known of these
  • The lac operon controls the production of the enzyme lactase (also called β-galactosidase) and two other structural proteins
  • Lactase breaks down the substrate lactose so that it can be used as an energy source in the bacterial cell
  • It is known as an inducible enzyme (this means it is only synthesized when lactose is present)
  • This helps prevent the bacteria from wasting energy and materials

Structure of the lac operon

  • The components of the lac operon are found in the following order:
    • Promoter for structural genes
    • Operator
    • Structural gene lacZ that codes for lactase
    • Structural gene lacY that codes for permease (allows lactose into the cell)
    • Structural gene lacA that codes for transacetylase
  • Located to the left (upstream) of the lac operon on the bacterium’s DNA there is also the:
    • Promoter for regulatory gene
    • Regulatory gene lacI that codes for the lac repressor protein
  • The lac repressor protein has two binding sites that allow it to bind to the operator in the lac operon and also to lactose (the effector molecule)
    • When it binds to the operator it prevents the transcription of the structural genes as RNA polymerase cannot attach to the promoter
    • When it binds to lactase the shape of the repressor protein distorts and it can no longer bind to the operator

Lac Operon Structure, downloadable AS & A Level Biology revision notes

The components of the lac operon along with the upstream regulatory gene and its associated promoter

When lactose is absent

  • The following processes take place when lactose is absent in the medium that the bacterium is growing in:
    • The regulatory gene is transcribed and translated to produce lac repressor protein
    • The lac repressor protein binds to the operator region upstream of lacZ
    • Due to the presence of the repressor protein RNA polymerase is unable to bind to the promoter region
    • Transcription of the structural genes does not take place
    • No lactase enzyme is synthesized

Lac Operon when Lactose is Absent, downloadable AS & A Level Biology revision notes

The repressor protein binding to the operator region of the lac operon and preventing transcription of the structural gene

When lactose is present

  • The following processes take place when lactose is present in the medium that the bacterium is growing in:
    • There is an uptake of lactose by the bacterium
    • The lactose binds to the second binding site on the repressor protein, distorting its shape so that it cannot bind to the operator site
    • RNA polymerase is then able to bind to the promoter region and transcription takes place
    • The mRNA from all three structural genes is translated
    • Enzyme lactase is produced and lactose can be broken down and used for energy by the bacterium

Lac Operon when Lactose is Present, downloadable AS & A Level Biology revision notes

The binding of lactose to the repressor protein frees up the operator region of the lac operon so RNA polymerase can bind and begin transcription of the structural genes

Exam Tip

The example above explains how the genetic control of an inducible enzyme works.

However, you could get some questions on the genetic control of repressible enzymes.

In this mechanism, an effector molecule also binds to a repressor protein produced by a regulatory gene. However this binding actually helps the repressor bind to the operator region and prevent transcription of the structural genes. So it’s the opposite of the lac operon: when there is less of the effector molecule, the repressor protein cannot bind to the operator region and transcription of the structural genes goes ahead, meaning the enzyme is produced.

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