Nov 25 Chapter 11

 

In this chapter, you will learn mechanisms of gene regulation. Here are the headings:

1. Gene regulation mechanisms in prokaryotes
2. Gene regulation mechanisms in eukaryotes
3. Therapeutic and reproductive cloning
4. Genetic basis of cancer

 

1. Gene regulation mechanisms in prokaryotes

A. Gene expression in prokaryotes

- Prokaryotes turn ON or OFF sets of genes according to the environment.

Example: Escherichia coli (E. coli) is bacterium reside in the intestine.  E. coli cells turn on expression of the lactose digesting enzyme genes only when they are bathed in solution containing lactose
(i.e. when you eat or drink milk products).

- Operon: a cluster of genes in prokaryotes, which are controlled by a common ‘ON/OFF’ switch.

 

B. The lac operon

- The lac operon is a cluster of genes involved in lactose digestion in E. coli.

- Two control genes, promoter and operator, followed by a group of lactose-utilizing genes (Fig 11.1 top, p211).

- Promoter: the binding site of RNA polymerase.

- Operator: regulatory sequence that act as switch.

 

i) When lactose is absent:

- A regulatory gene produces repressor all the time.

- The repressor protein binds to the operator sequence.

- RNA polymerase cannot attach to the promoter. ® The lac operon is turned off.

ii) When lactose is present:

- Lactose molecule binds to repressor protein, inactivating the repressor.

- Repressor cannot bind to the operator.

- RNA polymerase can now bind to the promoter. ® The lac operon is turned on.

 

B. The Trp Operon

- A cluster of genes involved in the synthesis of the amino acid tryptophan (Trp).

 

i) When Trp is absent:

- A regulatory gene produces repressor proteins all the time but the repressor cannot bind to the operon by itself. (i.e. Repressor is inactive.)  Fig 11.1C, p211

- The trp operon is turned on.

ii) When Trp is present:               

- Binding of Trp to the repressor activates the repressor.

- The active repressor binds to the operator.

- The trp operon is turned off.

 

C. The activator system

- Operons may be controlled by an activator instead of a repressor.

- Binding of the activator to the operator facilitates binding of RNA polymerase to the promoter sequence.

- An environmental stimulus may activate or inactivate the function of the activator.

 

 

2. Gene regulation mechanisms in eukaryotes

A. Cell differentiation and gene expression

- Multicellular organisms consist of specialized cells with specialized functions.

Examples:         muscle cells                  - muscle contraction

                        alpha cells in pancreas   - production of glucagon (The hormone that increases the blood sugar level.)

                        beta cells in pancreas    - production of insulin (The hormone that decreases the blood sugar level.)

                        white blood cells            - defense (removal of pathogen)

                        red blood cells               - carries oxygen            

 

- Cell differentiation: the process of cell specialization.

- Different sets of genes are expressed in different types of cells (cell-specific gene expression). (Table in p212) 

- Eukaryotic genes are controlled by both developmental and environmental signals.

Example:          The insulin production is specific to the alpha cells in pancreas (developmental control).

                        The insulin production is turned on only when the blood sugar level is too high (environmental control).

 

 

B. Gene regulation in eukaryotes

i) Overview

- The strength of eukaryotic gene expression is controlled at different levels (Fig 11.11, p219).

- The flow of gene expression is similar to water flowing through pipes.

- There are at least nine ‘valves’ (control mechanisms) to control gene expression.

 

ii) Chromatin structure

iii) Transcriptional regulation

iv) Posttranscriptional regulation