Gene Expression and Regulation

Chapter 12-5

• All of your cells contain the same DNA.

• Your nerve cells do not look like or act like skin cells, so how do they know what to do and what to produce?– Genes are to be turned on or turned off based

on whether they will be used or not– When a gene is actually transcribed and

translated, it is said to be expressed.

I. Basic Structure of a gene

• When looking at a gene sequence, there are several important regions that enzymes and other proteins recognize.

Regulatory sites

Promoter

Start transcription

DNA strand

Stop transcription

1. Promoter- region of gene where RNA Polymerase binds

2. Start- transcription begins (TAC)

3. Stop- Transcription ends (stop codon)

4. Regulatory sites- near the promoter where regulatory proteins can bind (turn genes on and off)

II. Prokaryotic Gene Regulation

• Prokaryotes have a single chromosome and are unicellular

• Turn genes on and off when needed

• Operon- group of genes that operate together for a function, can be– Inducible– Repressible

• A repressor molecule can block, or repress, transcription by binding to a region called the operator.– Inducible- in presence of a substance, the

substance causes the repressor to let go– Repressible- in presence of a substance, the

substance causes the repressor to grab onto the operator

Example: lac operon

• The lac operon is a series of genes in E. coli that operates together to metabolize (use as food) lactose, the sugar found in milk– Lactose is a disaccharide made of glucose

and galactose– In the presence of lactose, certain enzymes

must be produced to break them down

Example: lac operon

• P (promoter)- region that RNAP binds• O (operator)- region that a repressor can

bind, blocking RNAP from transcribing• Gene Z- codes for β-galactosidase

– which breaks down lactose into glucose and galactose

• Gene Y- codes for Permease– Which causes the cell membrane to be more

permeable to lactose (let it in)

• Gene A- codes for a protein whose function is unknown

Steps for lac operon

• In absence of lactose, a repressor binds to the operator, blocking RNAP

• 1. In the presence of lactose, lactose binds to the repressor causing it to let go of the operator– 2. RNAP transcribes gene Z (makes β-

galactosidase) and gene Y (makes permease)– 3. Ribosomes translates β-galactosidase and

permease

– 4. β-galactosidase breaks down lactose into glucose and galactose

– 5. Permease allows lactose to flood into the bacterial cell

– 6. Once lactose is all broken down, lactose lets go of the repressor so it can rebind to the operator

– 7. This stops transcription and translation of β-galactosidase and permease

http://vcell.ndsu.nodak.edu/animations/lacOperon/index.htm

III. Eukaryotic Gene Regulation

• No operons, genes are regulated individually

• Similar process, but is much more complex than prokaryotic gene regulation– Prokaryotes have no cell specialization

• There is a promoter region, several enhancer sequences and a “TATA box”.– TATA- helps position RNA Polymerase– enhancers can act like operators

and block transcription, and others can signal to unpack chromatin

• Exons actually get “EXPRESSED”

• Introns are cut out of the mRNA copy and not transcribed and translated by ribosomes.

• After fertilization and mitosis occurs, cells specialize into their life-long functions through a process called differentiation

• 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)

– Like master controls of what cells become what part of the body

– Manipulation of these genes can alter what parts grows where (a leg out of your head?!)