Gene control in prokaryotes (lac operon)

In both prokaryotes and eukaryotes, transcription of a gene is controlled by transcription factors.









Transcription factors are:

• proteins that bind to a specific DNA sequence
• control the formation of mRNA --> control the flow of information from DNA to RNA 

Structural genes code for proteins required by a cell.

Regulatory genes code for proteins that regulate the expression of other genes.

The synthesis of repressible enzymes can be prevented by binding a repressor protein to the operator (a specific site) on the bacterium's DNA.

The synthesis of inducible enzymes occur when its substrate is present.

Prokaryotic lac operon

operon: a length of DNA making up a unit of gene expression in a bacterium.

The lac operon consists of a length of DNA with operator and promoter regions and a cluster of 3 structural genes:

• lacZ - coding for β-galactosidase (hydrolyses lactose to glucose + galactose)
• lacY - coding for permease (allows lactose to enter cell)
• lacA - coding for transacetylase

Close to the promoter, although not part of the operon, is the regulatory gene for the lac operon.

Lactose absent

• regulatory gene codes for repressor protein*
• repressor binds to operator region, close to lacZ
• RNA polymerase can't bind to DNA at promoter region
• no transcription of the 3 structural genes

*repressor protein is allosteric (has 2 binding sites). The repressor binds to DNA with one site and binds to lactose with another site. 

Lactose present:

• lactose taken up by bacterium
• lactose binds to repressor protein, distorts shape and prevents it from binding to the operator region on the DNA (closes DNA-binding site)
• transcription no longer inhibited
• mRNA produced from 3 structural genes

This makes sure that the bacterium can produce β-galactosidase, permease and transacetylase only when lactose is available in the surrounding medium. This avoids waste of energy and materials.

Question 10a from 2016 specimen paper

http://papers.gceguide.com/A%20Levels/Biology%20(9700)/9700_y16_sp_4.pdf

http://papers.gceguide.com/A%20Levels/Biology%20(9700)/9700_y16_sm_4.pdf

16.3 Gene control  Some genes are transcribed all the time to produce constitutive proteins; others are only ‘switched on’ when their protein products are required.  a) distinguish between structural and regulatory genes and between repressible and inducible enzymes  b) explain genetic control of protein production in a prokaryote using the lac operon  c) explain the function of transcription factors in gene expression in eukaryotes  d) explain how gibberellin activates genes by causing the breakdown of DELLA protein repressors, which normally inhibit factors that promote transcription