Chapter 12.5 Gene Regulation

Genes are able to turn on and off Genes are able to turn on and off. Only a few genes are expressed at any time. An expressed gene is a gene this is transcribed into RNA. In front of genes there is a promoter region, but there are also regulatory sites – these turn on and off the gene.

Gene Regulation: An Example We will use E. coli to give an example of turning a gene on and off. The 4288 protein region is made of 3 genes. These 3 genes get turned off and on together. These genes are responsible for breaking down lactose for the bacteria. A group of genes that that turn off and on is called an operon. In E. coli this group of genes is called the lac operon.

The E. coli only needs these genes turned on when it uses lactose for food. If there is no lactose it does not need these genes turned on. The lac genes are turned off by repressors and turned on by the presence of lactose.

How Does it Work? On one side of the operon’s genes are 2 regulatory proteins. The promoter region is where RNA polymerase can attach and start transcription. There is another region called the operator. This contains a proteins called a lac repressor. If the lac repressor is attached to the DNA the gene is turned off. The repressor will be on DNA unless lactose is present. When lactose is present it changes the shape of the repressor and the repressor is removed.

Eukaryotic Gene Regulation There are differences between prokaryotes and Eukaryotes. Eukaryotes usually do not have operons. Most eukaryotic genes are controlled individually and are more complex. Usually eukaryotic genes will have a TATA box (a sequence of TATATA) before the gene. This area helps RNA polymerase bind to the DNA.

Development and Differentiation Regulation of gene expression is very important in cell specialization. When an organism is developing it’s cells undergo differentiation – they become specialized in structure and function. Hox Genes control cell differentiation and help make different body parts.