Transcript Gene Expression and Regulation

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 Promoter
sites
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 βgalactosidase) 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
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?!)