Download Chapter 4 - HCC Learning Web

Authored by Peter J. Russell
CHAPTER 4
Gene Function
Gene Control of Enzyme Structure
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Genes encode proteins including
enzymes, which catalyze reactions
Genes work in sets to accomplish
biochemical pathways
Genes often work in cooperation with
other genes
Garrod and Bateson’s Hypothesis of Inborn
Errors of Metabolism – work in Alkaptonuria
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Alkaptonuria is symptomized by blackened,
oxidized urine and late onset arthritis
The etiology of the disease (genetic) was
elucidated by examining familial inheritance
Pathologically, patients with alkaptonuria lack the
necessary enzyme to metabolize homogenistic
acid (HA) due to a recessive mutation on
chromosome 3 (found in studies performed later
on)
Garrod’s work provided the first evidence of a
specific relationship between genes and enzymes.
Notice the
relationship of
pathways within
the metabolome
One-Gene One-Enzyme Hypothesis
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Next, George Beadle and Edward Tatum exposed a
bread mold (haploid fungus Neurospora crassa) to
X-rays, creating mutants unable to survive on
minimal medium due to an inability to synthesize
certain molecules
Using crosses, they identified three classes of
arginine-deficient mutants, each lacking a different
enzyme necessary for synthesizing arginine
They developed a one-gene one-enzyme
hypothesis, stating that each gene dictates the
production of a specific enzyme
Animation: Meselson-Stahl Experiment
Life Cycle of Neurospora
 Asexual propogation
occurs through the
duplication of condidia
or mycelium (haploid
genome – mitotic)1
 Sexual reproduction
occurs through the
fusion of two mating
types producing
ascospores (diploid
genome – meiotic)2
 Prototrophs (wild-type)
grow on minimal
medium (precursors
only)
 Auxotrophs (mutants)
need nutritional
medium supplements
(products)
2
1
1
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Experimental Protocol
One mating type of
conidia were
mutagenized with X-rays
and crossed with wildtype conidia of the
opposite mating type
(Why?)
Fruiting bodies were
produced; microscopic
ascospores were
dissected and
transferred to culture
tubes containing
complete or minimal
medium
No growth on minimal
medium identified a
nutritional mutant that
was further investigated
and specifically identified
Mutants affirm that methionine biosynthesis proceeds
through a series of reactions catalyzed by enymes
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But some proteins aren’t enzymes. So what
then? Thus, the researchers revised the
hypothesis to: one-gene one-protein
But, But! Many proteins are composed of
several polypeptides, each of which has its own
gene
Therefore, Beadle and Tatum’s hypothesis is
now restated as the one-gene one- polypeptide
hypothesis
Note that even though it is not accurate (or
sometimes correct) it is common to refer to gene
products as proteins rather than polypeptides
Genetically based enzyme deficiencies
Presentation Projects
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Each of you will be required to present a 15 minute
slide presentation (overheads in powerpoint) of a
genetically based enzyme deficiency
The format of the presentation will be like my
presentation of Fibrodysplasia Ossificans Progressiva
(a genetically based developmental defect)
Your information resources are unlimited and some
are included in my presentation for FOP
For the NCBI database, go to http://omim.org
Put in the OMIM entry number (on previous slide) for
the genetic disorder you drew
Pull up the information and construct your
presentation from what is given
Gene Control of Protein Structure
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Genes also encode proteins that are not
enzymes
Structural proteins, such as hemoglobin, are
often abundant, making them easier to isolate
and purify (enzymes are generally produced in
much smaller amounts and thus, are more
difficult to purify to homogeneity)
Animation: Gene control of Protein Structure and Function
Sickle Cell Anemia and Hemoglobin
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Hemoglobin is formed by
four polypeptide chains
and is responsible for O2
and CO2 distribution
Two subunits of the
protein contain the a
polypeptide
Two subunits of the
protein contain the b
polypeptide
Each subunit associates
with a heme group,
which contains an iron
center (reactive site)
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Sickle Cell Hemoglobin
Phenotypically, the mutant red
blood cells change shape
(sickle) under low O2 tension
Sickled cells are fragile, causing
anemia. They are also less
flexible, blocking up capillaries
Effects are pleiotropic, including
damage to the extremities,
resulting in heart failure,
pneumonia, paralysis, kidney
failure, abdominal pain, and
rheumatism
Heterozygous individuals have
the sickle-cell trait, a much
milder form of the disease
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Hemoglobin Form
Electrophoresis of the protein
showed the sickle cell form of Hb
(Hb-S) has altered mobility
compared with normal
hemoglobin (Hb-A)
Hemoglobin from individuals with
the sickle-cell trait shows equal
amounts of Hb-A and Hb-S,
indicating that heterozygotes
make both forms of Hb
Thus, the sickle-cell mutation
changes the form of its
corresponding protein. Since
protein structure is controlled by
genes…
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Hemoglobin Genetics
The 6th amino acid of the
b chain in sickle-cell
hemoglobin is valine (no
electrical charge) rather
than the negatively
charged glutamic acid in
the b chain of normal
hemoglobin
Over 200 types of
hemoglobin mutants
have been genetically
characterized (most are
point mutants)
The mutant form is
codominantly expressed
Cystic Fibrosis
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The affected gene is on the
long arm of chromosome 7
and encodes a protein
called Cystic Fibrosis
Transmembrane
Conductance Regulator
Mutation results in an
abnormal CFTR protein,
preventing chlorine ion
transport, resulting in
mucus accumulation
Cystic fibrosis (CF) affects
the pancreas, lungs, and
digestive system, and
sometimes the vas deferens
in males
Treatment regimen includes
antibiotics and percussion
of the thoracic cavity
Genetic counseling
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Genetic counseling is advice based on genetic
analysis, focusing either on the probability that an
individual has a genetic defect or the probability
that prospective parents will produce a child with a
genetic defect
Information for genetic counseling is obtained from
two sources:
 Pedigree
Analysis (prior history of disease in a family?)
 Carrier Detection (identify genotype of parents - Aa?)
 Fetal Analysis (fetal biochemical or genetic assay)
Fetal Analysis
Amniocentosis
 Involves removal of a
sample of amniotic fluid
using a syringe needle
inserted through the
uterine wall (post 12 wk)
 Fetal cells are cultured
 Biochemical and
genetic analyses are
performed
Chorionic villus sampling
 Involves removal of
chorionic villus tissue
either through the
abdomen or vagina
(wk 8-12)
 No need for further
culture of fetal cells
 Biochemical and
genetic analyses are
performed
Caveats
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Amniocentesis is costly and cannot be performed
until the second trimester, eliminating early
abortion as an option in cases of severe genetic
defect
Chorionic villus sampling can be done earlier, but
it carries a higher risk of fetal death and inaccurate
diagnosis due to the presence of maternal cells