Protein Synthesis

Control Mechanisms

Control Mechansisms

• the human genome contains about 20000-25000 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/animations/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/content/chp13/1302002.html