Class notes

Trp and Lac Operon Explained

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Course
Biology

Institution
12th Grade

Here is an in-depth explanation detailing the mechanisms of the Trp and Lac Operon in terms of gene expression in prokaryotic organisms. The following document summarized the general process, enzymes and molecules involved and the key difference between both operons.

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Uploaded on January 18, 2025
Number of pages 2
Written in 2023/2024
Type Class notes
Professor(s) N/a
Contains All classes

Institution Secondary school
Education 12th Grade
Course Biology
School year 4

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Let’s talk about the lac and trp operon. Both, very important gene control mechanisms
used by prokaryotic organisms in bacteria such as E. Coli to regulate gene expression in
terms of a cell’s environment. An operon, in general, consists of a band of genes,
alongside something called a promoter and an operator and are all transcribed together
as one mRNA molecule . Let’s demonstrate this by talking about the Lac Operon. The
Lac Operon specifically contains 3 genes, called lacZ, lacY, and lacA that code for the
proteins that help metabolize lactose, for it to be converted into usable energy for a cell.
The LacZ gene specifically encodes for an enzyme called β-galactosidase, which breaks
down lactose, a disaccharide into glucose and galactose, two monosaccharides. The
lacY gene encodes for a transmembrane pump protein called permease that lets the cell
accept lactose and the lacA gene encodes for transacetylase, an enzyme which is
believed to attaches a acetyl group to molecules like β-galactosidase. The promoter I
mentioned earlier is where RNA polymerase is binded to perform transcription, and the
operator, which is home to a sequence of bases that control the function of RNA
polymerase, thus controlling transcription. The transcription of the genes in the lac
operon and a protein called the lacI are dependent on the amount of lactose in the cell.
LacI is an important regulatory molecule in the operon, specifically a repressor protein
that prevents transcription in the lac operon by binding itself to the operator by blocking
RNA polymerase. This LacI gene is encoded upstream from the operon. When lactose is
available to the cell, some lactose is converted into allolactose (which lactose in its
isomer form) and it will bind to the lac repressor. This action will alter the shape of the
repressor protein, inactivating it to not bind to the operator. Lactose, or specifically
allolactose is a type of signal molecule, known as an inducer which will set off the
expression of the lac genes that produce the mRNA needed to produce the needed
protein. In turn, RNA polymerase is successfully able to bind to the promoter, and those
3 lac genes, LacZ, LacA, and LazY are translated into proteins, which help metabolize
lactose. When lactose is not available to the cell, the lac repressor binds to the operator,
preventing transcription, barring RNA Polymerase from functioning. The Lac Operon
contains a terminator segment as well, which terminates transcription, essentially the
end point of the lac operon segment. Another essential regulatory compound in the lac

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