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Protein Synthesis
Control Mechanisms
Control Mechansisms
• the human genome contains about 2000025000 genes that code for proteins
• housekeeping genes code for proteins that
are always needed in a cell; they are
constantly being transcribed and translated
• not all proteins are needed by all cells at all
times, so gene regulation is important to an
organism's survival
Control Mechansisms
• in eukaryotic cells, there are 4 levels of
gene regulation:
– transcriptional
– post-transcriptional
– translational
– post-translational
*see Table 1 on page 255*
Control Mechansisms
• transcription factors are proteins that bind
to DNA and assist RNA polymerase in binding
to DNA to "turn on" genes when they are
needed
• prokaryotic cells use operons to control gene
expression:
-an operon is a cluster of genes with one
promoter region, and a sequence of
bases called an operator in between
The lac operon
• lactose is a disaccharide found in milk
or milk sugars
• intestinal bacteria cells split lactose (into
glucose and galactose) to generate
energy for growth using the enzyme
beta-galactosidase
The lac operon
• the lac operon contains a promoter, an
operator, and 3 genes:
• lacZ codes for beta-galactosidase
• lacY codes for beta-galactosidase permease
(makes cell membrane permeable to lactose)
• lacA codes for transacetylase (unknown
function)
The lac operon
The lac operon
• when there is no lactose present, the LacI
protein binds to the operator, which partially
blocks the promoter region, preventing RNA
polymerase from binding to it
• LacI is a repressor protein
• when lactose is present, it binds to the LacI
protein, changing its shape and causing it to
fall off of the DNA
• lactose is a signal molecule or inducer
lac operon animation
• Narrated animation:
http://www.sumanasinc.com/webcontent/a
nimations/content/lacoperon.html
The trp operon
• tryptophan is an amino acid needed for
protein synthesis
• intestinal bacteria cells can obtain
tryptophan from a mammalian diet, or
they can synthesize it themselves
The trp operon
• the trp operon contains a promoter, an
operator, and 5 genes:
– the 5 genes code for 3 enzymes needed to
synthesize tryptophan
The trp operon
• when there is no tryptophan present, the trp
repressor protein is not able to bind to the
operator, and RNA polymerase is free to bind
to the promoter and initiate transcription
• when there is tryptophan present, it binds to
the trp repressor protein, altering its shape so
that it can bind to the operator; tryptophan is
a corepressor
trp operon animation
• Narrated animation:
http://bcs.whfreeman.com/thelifewire/cont
ent/chp13/1302002.html