Chapter 18: Regulation of Gene Expression

3 Main Questions:

- What is an operator?

In bacterial DNA, a sequence of nucleotides near the start of an operon to which an actice repressor can attach. The binding of the repressor prevents RNA polymerase form attaching toe hte promoter and transcribing the genes of the operon.

- What is operon?

A unit of genetic function found in bacteria and phages, consisting of a promoter, an operator and a coordinately regulated cluster of genes whose products function in a pathways.

- What is repressor?

A protein that inhibits gene transcription. In prokaryotes, repressors bind to the DNA in or near the promoter. In eukaryotes, repressors may bind to control elements within enhancers, to activators, or to other proteins in a way that blocks activators form binding to DNA.

5 Main Facts:

- When glucose and lactose are both present in its environment, E. coli prefertentially uses glucose.

- If the amount of glucose in the cell increases, the cAMP concentration falls, and without cAMP, CAP detaches from the operon.

- For the lac operon, the inducer is allolactose, an isomer of lactose formed in small amounts from lactose that enters the cell.

- A typical human cell probably expresses about 20% of its genes at any given time.

- Almost all the cells in an organism contain an identical genome.

Diagram:

In the pathway for trytophan synthesis, an abundance of trytophan can both (a) inhibit the activity of the first enzyme in the pathway (feedback inhibition), a rapid response, and (b) repress expression of the genes encoding all subunits of the enzymes in the pathway, a long term response. Genes trpE and trpD encode the two subunits of enzyme 1, and gene trpB and trpA encode the two subunits of enzyme 3.

Summary:

Cells intricately and precisely regulate their gene expression. Both prokaryotes and eukaryotes must alters their patterns of gene expression in reponse to changes in environmental conditions. Multicellular eukaryotes must also develop and maintain multiple cell types.

Each cell type contains the same genome but expresses a different subset of genes, a significant challenge in gene regulation. At every stage, gene expression is carefully regulated, ensuring that the right genes are expressed only at the corrected time and place.

Video:

http://www.youtube.com/watch?v=OEWOZS_JTgk