Gene Regulation and Expression

3/29/2011

Prokaryotes

• Genes are regulated to conserve energy and resources• DNA binding proteins (called

regulatory proteins – because they regulate gene expression) in prokaryotes regulate gene by controlling transcription–Some turn genes “on” – transcribe the gene–Some turn genes “off” – do not transcribe the gene

Prokaryotes cont…

• Genes are organized into operons, a group of genes with related functions that are regulated together• An example of a prokaryotic

operon is the E. coli’s Lac operon – 3 genes that must be turned on before E. coli can break down lactose for energy

Lac Operon

• E. coli must transport lactose across its cell membrane to break lactose (disaccharide) down into its monomers galactose and glucose to get the energy from it

• These tasks are performed by proteins coded for by genes of the Lac operon

• The Lac operon genes are only expressed (transcribed and translated into protein) when lactose is present in the environment

Promoters and Operators

• On one side of the operon’s 3 genes is a promoter (where transcription begins)• Then there is an operator region–This is where DNA binding protein, called the lac repressor protein, binds to DNA

• The lac repressor blocks transcription by blocking RNA polymerase from reaching the genes to be transcribed

When Lactose is Present…

• The lac repressor protein has a site where lactose can bind to it• When lactose binds the lac repressor

protein, the repressor falls off the operator• Now that the lac repressor is no

longer bound to the operator region, RNA polymerase can bind to the promoter and begin transcription of the lac operon genes which are needed to break down lactose

Eukaryotic Gene Regulation

• Similar but more complex than prokaryotic gene regulation• Eukaryotic genes have what are called TATA boxes, short regions of DNA right before a gene that helps position RNA polymerase for transcription

Transcription Factors

• DNA binding proteins called transcription factors control expression of genes• Some:–Enhance transcription by making DNA more accessible–Attract RNA polymerase–Block access to certain genes

Cell Specialization

• Cell specialization requires genetic specialization• Complex gene regulation in eukaryotes is what makes specialization of cells possible

RNA Interference

• miRNA are RNA molecules only a few dozen bases long, produced by transcription

• These miRNA molecules attach to protein clusters and form a silencing complex–Binds to destroys mRNA which is

complementary to it–“Sticks” to certain mRNA molecules

and therefore blocks the translation of the mRNA

• Blocking gene expression in this way is called RNA interference

Genetic Control of Development

• During early stages of development, different sets of genes are regulated by transcription factors and repressors• The expression of different genes

is what makes cells become specialized for specific functions (nerve, heart, muscle, etc)

Homeotic Genes

• Master control genes which control and regulates the identifies of body parts, organs during development• Master control genes are like

switches that trigger particular patterns of development and differentiation in cells and tissues

Homeobox Genes

• Homeotic genes all share a similar a DNA sequence called the homeobox• These genes code for transcription factors that activate other genes important in cell development and differentiation

Hox Genes

• A group of homeobox genes• Determines of each segment of

an organism's body–All organisms share the same basic tools for building parts of the body

• They are arranged in the exact order they are expressed “head to tail”

Environmental Influences

• Factors such as temperature, salinity, population size, and nutrient availability can influence gene expression• Metamorphosis, such as in the

bull frog, is regulated by internal (gene/hormonal) and external (environmental) factors