1. 11/19/08
Lecture 31 - Molecular Genetics 6
 Differential Gene Expression
 Development
 In cells at any point in time.
 mRNA
 Unstable.
 A number of copies of the polypeptide are made.
Then, mRNA breaks down.
 Operons
 Control units in the DNA (mostly within prokaryotes – ie. E. Coli).
 Anabolic Process (Eg. Trp operon)
 A synthetic (building up) process.
 Tryptophan (amino acid).
 This process requires 5 enzymes.
 A gene which codes for each enzyme.
 If tryptophan is present, RNA polymerase cannot bind.
 If tryptophan is absent, there will be transcription.
 There is 1 promoter for all 5 of the enzymes.
 All 5 enzymes are transcribed as 1 unit.
 Upstream is a repressor gene which makes a repressor protein that may or maybe not bind to the
operator.
 Enzymes are 'said” to be repressible.
 The presence of tryptophan blocks transcription.
 Catabolic Process (Eg. Lac operon)
 A process of breaking down molecules.
 Lac is short for lactose.
 Lactose is a sugar.
 Sugar is used as an energy source.
 In the lac operon, only 3 enzymes are needed.
 Enzymes are 'said' to be inducible.
 The presence of lactose induces transcription.
 Eukaryotic cells (control of transcription)
 The chromatin structure of a cell in interphase (G1) is important.
 DNA is highly condense (folded).
 This is called heterochromatin.
 While DNA is in this form, transcription cannot occur.
 Small proteins called histones are involved in the folding.
 Histones wrap the DNA into bead-like structures called nucleosomes.
 By changing the chemical structure of the Histone, unraveling the regions of DNA for transcription is
possible.

2.  Once opened, there are activators and enhancers which are environmentally controlled through signal.
 When the signal is received, transcription occurs.
 Proto-oncogenes
 Proteins which control the cell cycle.
 When the mutation of these proto-oncogenes occurs, cancer production can happen.
 5-6 different oncogenes are needed before this can happen.
 The factors are from:
 the environment
 smoke
 radiation
 chemicals
 inherited tendency for genes to mutate