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Chapter 15
Regulation of Gene Activity
Consider that....
Every cell of the body contains ALL the DNA for the organism.
Not all genes are necessary all the time
Cells must then have the ability to turn a gene on and off
ENTER: THE OPERON MODEL
OF GENE EXPRESSION
Francois Jacob and Jacques Monod
Experiments with E. Coli showed that it is capable of
regulating the expression of its genes
An operon consists of the following elements
1. Promoter - where RNA polymerase attaches, signalling
the start of the gene
2. Operator - where a repressor
binds, stopping the transcription of
that gene
3. Structural Genes - genes
coding for the enzyme, they are
transcribed as a unit
The trp Operon
It exists in the "on" state and controls the production of
tryptophan.
If tryptophan is already present, it binds to the repressor and
prevents more tryptophan from being made
The lac Operon
This region is normally in the "off" position, it turns on when
lactose is present
Repressors versus Inducers
The tryp operon is a repressible operon, it is normally on but
can be turned off when tyrptophan is present
The lac operon is an inducible operon because it is normally
off but can be turned on when lactose is present
Grammar Time
A woman is pregnant and the baby is
later. The doctor says they will "INDUCE"
labor tomorrow. What does he mean?
What does this shirt mean?
Tying it All Together: Lac Operon
In eukaryotes, a
variety of
mechanisms regulate
gene expression
1. chromatin structure
2. transcriptional control
3. post transcriptional control
4. translational control
5. post translational control
Chromatin Structure
DNA is wound around a
core of eight protein
molecules, the result
resembles beads on a
string. The protein
molecules are histones
and each individual bead
is called a nucleosome
BARR BODIES
In females, chromatin of one of the X chromosomes
inactivates, this inactive chromosome is called a BARR BODY
Evidence for barr bodies
1. Women who are heterozygous for duchenne muscular
dystrophy have patches of muscle tissue that are degenerative
2. Women who are heterozygous for a condition that causes
no sweat glands have patches of skin that are normal, and
patches that lack sweat glands
Barr Body in Cats
Genotype:
XB XO
The black patches are XB and the
orange patches are XO, the cat is
multicolored because not all X's are
activated
Gene Mutations
Point Mutations - single base changes, causing a
change in the amino acid structure (protein)
Frameshift mutation
A base is added or deleted and changes the reading frame
GAT
CAT
AAA
GAT
A CA TAA A
What happens when you have a
nonfunctional protein?
Hemophilia
PKU
Cystic Fibrosis
Androgen Insensitivity caused by a faulty receptor
for androgens, individual is a
chromosomal male but
appears
female (hermaphrodite)
Cancer and Genetics
Growth of cancer often begins with the loss of the TUMOR
SUPPRESSOR GENE
ONCOGENES activate, cell division occurs uncontrollably
Breast Cancer Gene
BRCA1 (brak-uh)
Transposons
http://www.thenakedscientists.com
/HTML/articles/article/jamilcolumn
1.htm/
Barbara McClintock studying maize corn found that controlling
elements could move from one location to another on a
chromosome
-- Transposons or Jumping Genes
In maize corn a colorless kernel results from the
inability to create purple pigment.
If the transposon jumps to
another location, the cells
regain the ability to make the
pigment, this creates a
speckled pattern on the kernel
Introns or Exons
Exons = parts of DNA that are
transcribed and code for a
protein
Introns = sections of DNA that
are not transcribed, function
unknown
sometimes once called this
"JUNK DNA" - scientists now
believe this is not accurate
What is Epigenetics?
See this article from Learn.Genetics
Video at Nova narrated by Neil DeGrasse Tyspon