Regulation of Gene ExpressionProkaryotes

I. Genetics of Bacteria• A. Bacteria are prokaryotes

– meaning they do NOT have a nucleus

• B. Instead they have 1 tightly condensed double-stranded DNA circular chromosome called a nucleoid.

• C. During replication DNA is copied in both directions at a single point of origin

II. Bacteria Reproduction

• A. Bacteria can reproduce several ways although the most common is asexual binary fission. Where all genes are identical with a few mutations every 1,000 replications which can result in significant variations.

II. Bacteria Reproduction• B. Bacteria can also reproduce through conjugation

a primitive method of sexual reproduction. – 1. With the use of a plasmid which allows the bacteria to

create a pillis, or bridge between two different bacteria that allows for DNA exchange. (F plasmid)

• C. Plasmids are foreign, small circular, self-replication DNA molecules that are found within bacteria.– 1. other plasmids include the R plasmid which

allows for bacteria to be resistant to antibiotics

• D. Other variation due to– Transformation– Transduction

III. Gene Expression• A. In eukaryotes to go from DNA to a protein the

steps are compartmentalized due to the nuclear envelope; in prokaryotes gene expression happens all together.

IV. Gene Regulation

• A. In prokaryotes gene regulation is controlled by operons. An operon is a set of genes and the switches that control the expression of those genes.

• B. There are 2 main types of operons:– 1. Repressible (tryptophan operon)– 2. Inducible (lac operon)

1. Trp Operon – Repressible always turned on unless repressor is activated

• a. Consists of a promoter and 5 adjacent structural genes that code for the 5 separate enzymes necessary to synthesize the amino acid tryptophan.

• b. RNA polymerase binds to the operator along the promoter creating one long strand of mRNA.

• c. Trp operon is only turned off when the repressor, activated by the corepressor trp, binds to the operator does transcription stop. (Recall what this is called from previous chapters, enzyme changes shape)

2. Lac operon – Inducible always turned off but can be stimulated or induced

• a. In order for E. coli to utilize lactose as an energy source they need 3 enzymes to break the disaccharide which are coded in the lac operon.

• b. In order for this to happen the repressor must be removed from the operator and RNA must bind to the promoter region.

• c. Allolactose, isomer of lactose, is the inducer that binds to the active repressor inactivating it.

• Repressible – anabolic pathways

• Only when sufficient end product is there

• Inducible – catabolic pathways

• Only active when product is available

V. Positive Gene Regulation

• A. When both glucose and lactose are both present E. coli prefers using glucose; but when glucose is in low supply and lactose is high enzymes are created to allow for this breakdown.

• B. When glucose is in low supply cyclic AMP (cAMP) accumulates. This molecule can bind to a regulatory protein called catabolite activator protein (CAP).

• C. When cAMP binds with CAP it can attach to the lac operon at the promotor (increase affinity of RNA polymerase)to stimulate transcription.