CONTROL MECHANISMS




Cells control when, how, and to what extent their genes are expressed.
housekeeping genes are genes that are always needed (for life functions)
transcription factors are proteins that bind to the DNA and helps RNA polymerase to bind.
gene regulation is the turning on or off of a gene
Four Levels of Control of Gene Expression in Eukaryotic Cells
1. Transcriptional Control: controls the rate at which transcription occurs
2. Posttranscriptional: pre-mRNA transcripts undergo modifications (ie. introns are
removed and exons are spliced together)
3. Translational: controls when, how rapidly and how often an mRNA transcript can be
translated
4. Posttranslation: most polypeptide chains must past through the cytomembrane system
(a series of organelles) before they become functional. To do this they can undergo
modifications such as having a specific oligsaccharide or phosphate group attached to
them
Enzyme Induction: The lac Operon
 lactose is a disaccharide of glucose and galactose; if the monosaccharides are separated,
bacteria cells like E. coli use the energy from them for growth
 β-galactosidase is the enzyme used to split the molecule apart
 It is a waste of energy to produce the enzyme when it is not needed, so bacteria will only
produce this enzyme when lactose is present
 The gene for β-galactosidase is part of an operon
 An operon is located on the bacterial chromosome and is made up of structural genes, a
promoter and a short sequence of bases known as an operator
 Only prokaryotes use operons to regulate genes and their products
 a promoter ( a base sequence that signals the start of a gene) is found before the three
genes
 an operator ( the binding site for the repressor) is found between the promoter and the three
genes
 see Fig. 2a on pg. 256
 the repressor protein (LacI protein) prevents transcription from occurring by binding to the
lactose operator.
 when [lactose] is low the repressor is free to block transcription (it overlaps the promoter
and prevents binding by RNA polymerase so the enzymes are not produced)
 when [lactose] is high, the lactose binds to the repressor and changes its shape so it can not
bind to the operator; the promoter is exposed and the genes can be transcribed
 see Fig. 2 on pg. 256
Enzyme Repression: The trp Operon
 when E. coli lives in the intestines of a mammal, it gains the amino acid, tryptophan, from the
mammal’s diet; if it lives elsewhere, the bacteria has to produce its own tryptophan
 the trp operon is repressed when high concentrations of tryptophan are present
 it consist of 5 genes, that code for 5 polypeptides that make 3 enzymes needed to make
tryptophan.
 when there is high levels of this amino acid, the aa binds to the trp repressor protein changing
its shape co it can now bind to the operator, preventing the genes from being made, so the
production of tryptophan ceases.
 when there is a low level of this aa, the repressor protein changes it's shape; unbinding it
from the operator, allowing the genes to be read and the enzymes made.
 tryptophan is called a corepressor because it is needed to inactivate the trp operon.
Seatwork
Pg. 258 # 1 - 6