Lecture 13: Prokaryotic Gene Regulation
How do cells produce the right gene product (protein or RNA)
in the right amount, in the right place, at the right time?
All genes are in the cell’s DNA but only certain genes are expressed as RNA depending on what is needed
for a specific part of the organism.
Prok cells undergo rapid and reversible alterations in biochemical pathways that allow them to adapt quickly
to changes in their environment. Versatile/responsive control system allows bacterium to make most efficient
use of particular array of nutrients and energy sources available at any given time.
Many operons are controlled by more than one regulatory mechanism and many mechanisms control more
than one operon. Everything in an operon results in a complex network of superimposed controls that
provides regulation of transcription, allowing instantaneous responses to changes in environmental
1. identify the main features of bacterial operons
2. identify the function of repressor proteins
- When bound to DNA, reduces the likelihood of genes being transcribed.
- When the repressor is bound to the operator it blocks the RNA polymerase from binding to the
promoter so no DNA can be transcribed – stops transcription.
- Repressor binding acts as equilibrium - in short moments the repressor detaches from operator and
transcription proceeds and all three enzymes are synthesized rapidly – therefore, there is always a
small amount in lac operon in cell.
How repressor is turned off.
- Lactose enters cell – beta-galactosidase converts lactose to allolactose (isomer of lactose).
Allolactose is an inducer for lac operon.
- Inducer (allolactose) binds to lac repressor changing it’s shape so the repressor can no longer bind
to the operator DNA – then RNA polymerase is able to bind freely to promoter and begin rapid
(inducer molecule increases expression) transcription.
- When lactose is used up, regulatory system switched lac operon off – no more allolactose
molecules to stop repressor binding.
3. identify location of various components of the lac operon
DNA signals in RNA-coding genes
DNA sequence of anticodon in tRNA gene, given the codon
- *On test
- mRNA 5’-3’ UGG
- anticodon: tRNA – antiparallel and complementary – 3’-5’ ACC as read off template strand of
- DNA – 3’-5’ GGT
- Anticodon coded in DNA that after transcription appears in tRNA that during translation
antiparallel and complimentary with an mRNA codon
likely effect of base sequence substitutions in various DNA signals
change in amino acid coded, given a change in the DNA sequence (and Genetic Code table)
- Amino acids can have many codons
- Tyr UGG
- Codons are coded 3’-5’ in DNA but only understood as 5’-3’ mRNA
- Mutations in DNA:
o If you destroy one codon but create another one that codes for same amino acid – protein
is not affected at all – silent mutation – 0 effect on phenotype
o Destroy one codon and create a new one that codes for a new amino acid
Effect: substitute negative amino acid for positive one – effect on hydrophobic
Effect: similar function would have little effect - Substitution mutations may create “nonsense” codons
o Single base pair substitution creates stop codons
o Translation stops early – protein will be too short – probably be nonfu