Chapter 13 Notes

Gene Regulation and Expression

 

Every cell has the same DNA.  Differentiation occurs because some of those genes are expressed and others are not. 

Examples of Different Cells

  Sperm Cell

  Red Blood Cell

 

 

How are prokaryotic genes regulated?

DNA binding proteins regulate genes by controlling transcription.

 

Operon: A group of genes that are regulated together.

·     These genes must be turned on and off together.

 

Example: The bacterium E. coli has a group of genes that must be in order so that the bacterium is able to use the sugar lactose as food.  They are referred to as the lac operon.

 

 

 

       The lac genes are turned off by the lac repressor protein being bound to a region called the operator.

       The repressor protein blocks RNA polymerase and transcription does not occur. The genes are turned off.

       The lac genes are turned on by the presence of lactose which binds to the repressor and releases the repressor protein from the operator.

       Once the repressor is released, RNA polymerase can bind to the promoter and transcription occurs. The genes are turned on.

 

 

 

Eukaryotic genes are more complex than prokaryotic genes because they are controlled individually and have regulatory sequences that are more complex.

 

Most eukaryotic genes have a short region of DNA (about 30 base pairs long) containing a sequence of TATA.

 

The “TATA box” marks the place for RNA polymerase to bind at a point just before transcription begins.  It is the primary promoter found in eukaryotes.

 

 

       The “TATA box” seems to help position RNA polymerase to start transcription.

       The TATA box is necessary for transcription because RNA polymerase cannot recognize the promotor on its own.

       There are several different enhancer sequences located before the “TATA box” that will attract different proteins.

 

 

 

These proteins, called transcription factors decide which genes are turned on and which are turned off. 

·     some serve as promoters (binding sites for RNA polymerase)

·      some code for enhancer proteins that turn the process of transcription “on”!

·     some code for repressor proteins that stop transcription, turn it “off”!

·     multiple transcription factors are usually needed to start transcription.

 

Gene regulation is important for cell differentiation and specialization

 

Example:  Genes that code for liver enzymes are not the same genes expressed in nerve cells even though they have the same DNA.

 

Homeotic Genes: are like switches that trigger particular patterns of development and differentiation in cells and tissues in embryos.

·     Often called the “master control genes”.

·     Control where organs will develop in specific parts of the body.

 

Homeobox Genes: genes that control basic body structure.

·     Are expressed in certain regions of the body

·     Can determine factors like the presence of wings or legs.

 

Example: A group of homeobox genes called Hox genes exist in a single cluster in fruit flies.  Similar clusters exist in the DNA of other animals, including humans.

 

 

In fruit flies, mutations affecting the Hox genes can replace the fly’s antennae with legs growing on its head.