Gene Regulation • Remember! All of your cells contain the exact same DNA. • Your nerve cells do not look like or act like skin cells, so do they use the same genes? NO!
Gene Regulation • Genes are to be turned on/off based on whether they will be used or not! • When a gene is on, it is said to be expressed.
Basic Structure of a Gene • When looking at a gene sequence, there are several important regions that enzymes & other proteins recognize.
Basic Structure of a Gene • Promoter: region of gene where RNA Polymerase binds • Start: transcription begins • Stop: transcription ends • Regulatory Sites: near the promoter where regulatory proteins bind (turn genes on/off)
Prokaryotic Gene Regulation • Remember! Prokaryotes have a single chromosome & are unicellular. • Small amount of DNA (simple) • Turn genes on/off to be efficient • Operon: group of genes that operate together for a function (turned on/off together) • Inducible • Repressible
The Promoter Sequence • DNA sequence where RNA Polymerase binds • Remember! RNA Polymerase will make mRNA during transcription.
The Operator Sequence • DNA sequence where a repressor protein binds
Repressor Proteins • Bind to the operator sequence & block transcription • When the repressor is bound, NO GENES ARE EXPRESSED!
2 Types of Operons • Inducible Operon: normally “off” but can be switched “on” • In the presence of a substance, the substance causes the repressor to let go. • Repressible Operon: normally “on” but can be switched “off” • In the presence of a substance, the substance causes the repressor to bind to the operator.
The lac Operon • E. coli uses lactose (sugar) as a food source. • The lactose needs to be metabolized (broken down). • There are 3 genes in the lac operon that work together to metabolize the lactose. • The presence or absence of lactose determines whether the genes are turned on/off. • Why would the bacteria express genes to break down lactose if there is no lactose?
The lac operon will be turned on when: • Lactose is present • Lactose is absent When lactose is present! There would be no point in expressing genes to break down lactose if there is no lactose.
When lactose is absent… • The repressor is bound & blocks transcription of the lac operon.
The lac operon: • Normally “on” • Normally “off” Normally “off” (the repressor is bound)!
The lac operon: • Inducible • Repressible Inducible! The operon is turned “on” when lactose is present!
Eukaryotic Gene Regulation • No operons, genes are regulated individually • Similar process, but is much more complex than prokaryotic gene regulation
Eukaryotic Gene Regulation • There is a promoter, several enhancers, & a “TATA Box” • TATA: helps position RNA Polymerase • Enhancers can act like operators & block transcription, & others can signal to unpack chromatin
Eukaryotic Gene Regulation • Exons are EXPRESSED. • Introns are cut out of the mRNA copy & not expressed.
Fun Fact! • Over 98% of the human genome is considered “junk” DNA! • This DNA is not expressed & the function is unknown.
Cell Differentiation • After fertilization & mitosis occurs, cells specialize into their life-long functions. • Differentiation is controlled by hox genes. • Some genes get turned off permanently (your liver cells do not express genes that make proteins in the skin). • Manipulation of these genes can alter what parts grow where (a leg out of your head?!).