Gen expression

Chap. 19: Regulation of Gene Expression in Prokaryotes and Their Viruses
I. Constitutive, Inducible, and Repressible Gene Expression
a. Constitutive genes-is transcribed at a relativelyconstant level regardless of the cell environmental conditions. Constitutive genes are expressed continuously. It is because constitutive genes such as housekeeping genes code for products withhousekeeping functions in cells, maintaining the basic cell processes or structure.
b. Inducible genes- need an "induction", For example there are defense related genes in your body, and their expressionis highly increased when you are infected with bacteria. Similarly there are heat shock induced, stress induced genes. Some catabolic enzymes are characteristically as inducible ones.
c.Repressible genes- are on except in the presence of some molecule that suppresses gene expression. The molecule is said to "repress expression". The manner in which this happens is dependent on the controlmechanisms as well as differences between prokaryotic and eukaryotic cells. Some anabolic enzymes are characteristically as inducible ones.

II. Positive and Negative control of Gene Expressiona. Positive control mechanism- requires the presence of an activator protein before RNA polymerase will attach. The activator protein itself must be bound to an inducer molecule before it attachesto mRNA.
b. Negative control- a repressor blocks transcription. In one case the repressor is active and prevents transcription. In the other case (trp) the repressor is inactive and must beactivated to prevent transcription.

III. Operons: Regulated units of Gene Expression
Operons are groups of genes that function to produce proteins needed by the cell. There are two different kindsof genes in Operons:
a. Structural genes code for proteins needed for the normal operation of the cell. For example, they may be proteins needed for the breakdown of sugars. The structural...