Transcript Gene Expression and Regulation

Expression and Regulation
Introduction to an advanced topic in
gene regulation
The Lac gene example
• E. coli needs 3 enzymes to digest
lactose……….
• …….But only when lactose is present in the
environment (where the bacteria are living)
The lac-operon
• On one side (upstream) to the DNA sequences for these
three proteins is a region of DNA called the promoter.
This is where RNA polymerase binds
• There is another region of DNA before the sequence that
codes for the three lactose proteins called the lac operator
• When there is no lactose in the environment a protein
called the lac repressor binds to a region of DNA called the
lac operator
• The lac repressor blocks the RNA polymerase from getting
to the sequence for the three lactose proteins
The lac operon
• When lactose is present in the environment the bacteria
need the three proteins (enzymes)
• Lactose binds to the lac repressor which makes it change
shape and fall off the lac operator
• This frees RNA polymerase to move down the DNA and
transcribe the sequence into mRNA to make the three
enzymes to digest lactose
• When all the lactose is used up (digested) the lac repressor
retains its original shape and binds to the lac operator to
shut the gene off
Eukaryotes do it too
• Operons work for eukaryotes too, but there’s
more…….
• Transcription factors
– Open tightly packed chromosomes
– Attract RNA polymerase
– Block access to genes
• RNA interference (mRNA is interfered with)
– The dicer enzyme
– The silencer complex
Development starts here
• Differentiation = becoming specialized
(remember cells…tissues…organs…)
• Homeotic gene
– Homeobox gene
• The BIG PICTURE: all animals use the
same basic tools for building the different
parts of the body
• Oh, and don’t forget the environment….