The Lac Operon

The Lac Operon is an example of Enzyme Induction – when bacteria change the synthesis rates of specific enzymes in response to environmental changes. Here, the Lac Operon was theorised after experiments where glucose and lactose were given to E. Coli, which then used glucose before using lactose in two growth phases.

The enzyme E. Coli uses to process lactase is called (Beta) B-Galactosidase and Lactose Permease. B-Galactosidase catalyses the hydrolysis of lactose to glucose and galactose, whereas Lactose Permease transports lactose into cells.

The actual structure of the Lac Operon can be read as I-POZY, where:

I is a regulatory gene, and IS NOT part of the operon and is some distance away. I is used in the Lac Operon description to show the start of the process.

P – The Promoter region. This is DNA that RNA polymerase binds to, to transcribe the genes Z and Y.

O – The Operator region. A length of DNA sitting next to the structural genes (Z and Y). It can switch them on or off. The operator region is seen next to the promoter region in diagrams.

Z and Y – Structural Genes. Z codes for the enzyme B-Galactosidase, and Y codes for Lactose Permease.

TO TURN THE LAC OPERON OFF:

Step 1:

I, the regulatory gene, is transcribed and translated to produce a repressor protein. The protein can bind to lactose and to the operator region.

Step 2:

The repressor protein binds to the O-region, and in doing so covers the promoter region as well. This not only stops the operator region from switching Z and Y `on`, but also prevents RNA Polymerase from binding to P (which in turn stops production of the mRNA that codes for Z and Y).

Step 3:

Without the mRNA the B-Galactosidase and Lactose Permease cannot be created.

TO TURN THE LAC OPERON ON:

Step 1:

As above, the repressor protein is produced and binds to O, covering P also.

Step 2:

Lactose is now present. When lactose binds to the repressor protein they are both removed from O.

This frees O and P to do their own processes – that is, for RNA polymerase to bind to P to begin production of Z and Y, and O switches Z and Y on.

Step 3:

B-Galactosidase and Lactose Permease are now produced, and each act with the Lactose.

 This diagram shows both `on` and `off`.

 This diagram doesn't show the process, but is a good labelled picture of the seperate parts of the Operon. The CAP is the Regulatory Gene, I.

Protein Synthesis

A protein is synthesised during two processes. The first is transcription.

Transcription begins with the specified DNA strand coding for the gene dipping into the nucleolus. This `unzips` the DNA (by breaking hydrogen bonds between complementary bases). The half-DNA acquired is the template strand . Free floating nucleotides line up and bind to the template strand (via Hydrogen bonds). In the mRNA strand about to be created, the base pairs are complementary to one another, but here U-nucleotides bind to A-Template strand nucleotides. The bindings are catalysed by RNA polymerase.

The mRNA produced is complementary to the nucleotide base sequence on the template DNA strand and is therefore a copy of the base sequence on the coding strand of DNA.  The mRNA is the released and goes through a nuclear pore.

Translation is the second stage of protein synthesis, when amino acids are lined up to create the protein.

The mRNA strand is able to fit into a groove in a ribosome. When this happens one can move along the other. There is space for two amino acid-tRNA complexes on the ribosome, where there are six bases (two codons) to bind to. The first base sequence is always AUG.

An amino acid-tRNA complex attaches to the ribosome. The tRNA has an anti-codon, which can bind to a complementary codon on the mRNA. Another complex binds to the adjacent base sequence. The amino acids bound to the tRNA molecules are placed together using peptide bond. The first tRNA molecule is now free to move (it has given up its Amino Acid), and the Ribosome or mRNA shifts along. Another complex binds to the next codon that’s just been revealed. The new amino acid bind to the other amino acids with a peptide bond. This is repeated till the end of the mRNA strand. The chain of amino acids is the coded protein.

A few pictures showing the full process.

 A picture showing Translation across a Ribosome.