Transcript Document

Chapter 4
Gene Function
Copyright © 2010 Pearson Education Inc.
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Genes encode proteins, including enzymes.
Genes work in sets to accomplish biochemical
pathways.
Genes often work in cooperation with other genes.
These discoveries are the foundation of modern
molecular genetics.
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Alkaptonuria is a human trait characterized
by urine blackening on exposure to air and
arthritis in later life.
Archibald Garrod and William Bateson (1902)
concluded alkaptonuria is genetically
determined because:
◦ a. Families with alkaptonuria often have several
affected members.
◦ b.Alkaptonuria is much more common in firstcousin marriages than marriages with unrelated
partners.
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Garrod showed that alkaptonuria
results from homogentisic acid
(HA) in the urine. Garrod
reasoned that normal people
metabolize HA, but those with
alkaptonuria do not because
they lack the necessary enzyme.
He termed this an inborn error
of metabolism .
The responsible mutation is
recessive. The gene was later
shown to be on chromosome 3.
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Genes act by regulating definite chemical
events.
George Beadle and Edward Tatum (1942)
showed a direct relationship between genes
and enzymes in the haploid fungus Neurospora
crassa. This led to their one-gene–one-enzyme
hypothesis, and a share of the 1958 Nobel
Prize in Physiology or Medicine.
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Neurospora propagates
asexually by dispersal of
bits of mycelium or by
conidia.
◦ b. It also propagates sexually
by means of two mating types,
a and a.
◦ c. Wild-type Neurospora
needs only simple minimal
media with:
 i.Inorganic salts (including a
nitrogen source).
 ii.An organic carbon source
(such as glucose or sucrose).
 iii.Biotin (a vitamin).
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Beadle and Tatum isolated
auxotrophic mutants by
mutating with X-rays and then
crossing with a wild-type strain.
The cross ensured that effects
were due to inheritance, rather
than to direct damage from the
radiation. In their experiment:
◦ a. One progeny spore per ascus
was germinated in a complete
medium so that growth would
occur regardless of nutritional
mutations. Then cells were
transferred to minimal media,
where auxotrophs won’t grow.
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Each mutant was then tested
on an array of minimal
media, each with a different
single supplement, to
determine the type of
nutritional mutation.
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Reasoning: metabolism proceeds by a series of
reactions, each catalyzed by an enzyme, and
organized into pathways. Methionine biosynthesis
as example
◦ a. started with a set of methionine auxotrophs and found that four
genes are involved: met-2+, met-3+, met-5+, met-8+.
◦ b. Check each mutant on a series of minimal media, each
supplemented with a different chemical believed to be involved in the
pathway.
◦ c. They were able to deduce the pathway of methionine synthesis and
to correlate mutations with enzymes used in the pathway.
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Beadle and Tatum’s famous conclusion from this
experiment is that one gene encodes one enzyme. Later
work showed that some proteins consist of more than one
polypeptide, and that not all proteins are enzymes. The
principle is now usually stated as one-gene–onepolypeptide.
The enzymes involved in biochemical pathways within cells
are under genetic control. The sum of all chemical
intermediates and products in these biochemical pathways
is the cell’s metabolome. Metabolomics is the study of the
metabolome.
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Single gene mutations are responsible for many
human genetic diseases. Some mutations create a
simple phenotype, while others are pleiotropic.
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Phenylketonuria (PKU) is commonly caused by a mutation on
chromosome 12 in the phenylalanine hydrolase gene.
Phenylalanine is an essential amino acid, excess is harmful
and is normally converted to tyrosine. Excess phenylalanine
affects the CNS, causing mental retardation, slow growth,
and early death.
PKU’s effect is pleiotropic. Symptoms result from excess
phenylalanine and from inability to make tyrosine;
retardation,fair skin and blue eyes (even with brown-eye
genes) and low adrenaline levels.
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Diet: controlled intake of phenylalanine for
protein synthesis but not enough that it
accumulates.
The special diet must commence in the first
two months after birth, continue at least
throughout childhood,
PKU women have to resume the diet before
pregnancy in to avoid phenylalanine levels
that would affect the fetus.
NO Nutra-Sweet for all PKU individuals
(Aspartam is aspartic-acid-phenylalanine)
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Classic albinism is an autosomal recessive mutation in the
gene for tyrosinase.
Tyrosinase converts tyrosine to DOPA in the melanin
pathway.
Without melanin, individuals have white skin and hair, and
red eyes, due to lack of pigmentation in the iris.
Two other forms of albinism are known, resulting from
defects in other genes in the melanin pathway. A cross
between parents with different forms of albinism can
produce normal children.
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Several genes can be mutated to cause this
autosomal recessive disease
Mutated genes code parts of dynein motors of
flagella and cilia, which slide the microtubules to
produce motion.
Pleiotrophic effect:
◦ a. Sinus and lung abnormalities. Due to loss of cilia
function, respiratory diseases are common.
◦ b.Sterility. Sperm cannot swim and cilia cannot move the
oocyte within the reproductive tract.
◦ c. Dextrocardia (heart shifted to the right), in some cases.
Cilia are involved in orientation of the embryonic heart.
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Tay–Sachs is one of a group of diseases called lysosomal-
storage diseases.
Recessive mutations in genes encoding lysosomal enzymes.
Tay–Sachs disease (infantile amaurotic idiocy) recessive
mutation in the gene hexA, which encodes the enzyme Nacetylhexosaminidase A.
The HexA enzyme cleaves a
terminal Nacetylgalactosamine group
from a brain ganglioside.
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Infants will have nonfunctional HexA enzyme and the
unprocessed ganglioside accumulates in brain cells causing
various clinical symptoms:
◦ a. Enhanced reaction to sharp sounds.
◦ b. A cherry-colored spot surrounded by a
white halo may be visible on the retina.
◦ c. Rapid neurological degeneration begins
about age 1, as brain accumulate of
unprocessed ganglioside.
◦ d. Blindness, hearing loss, and serious
feeding problems lead to immobility by age
2.
◦ e. Death often occurs at 3 to 4 years of age,
often from respiratory infection.
The disease is incurable. Carriers and affected
individuals can be detected by genetic testing.
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Genes also make proteins that are not
enzymes. Structural proteins, such as
hemoglobin, are often abundant, making them
easier to isolate and purify.
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J. Herrick (1910) first described sickle-cell anemia.
◦ found that red blood cells (RBCs) change shape (form a
sickle) under low O2 tension
◦ a. Sickled RBCs are fragile, hence the anemia.
◦ b.Less flexible than normal RBCs, forming blocks in
capillaries.
◦ c. Pleiotropic, damage to extremities, heart, lungs, brain,
kidneys, GI tract, muscles, and joints. Others are: heart
failure, pneumonia, paralysis, kidney failure, abdominal
pain, and rheumatism.
◦ d.Heterozygous individuals have sickle-cell
trait, a much milder form of the disease.
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Linus Pauling and his colleagues (1949) used
electrophoresis and showed:
◦ a. Sickle-cell hemoglobin (Hb-S)
has altered mobility compared
with normal hemoglobin (Hb-A).
◦ b. Sickle-cell trait shows equal
amounts of Hb-A and Hb-S,
heterozygotes make both forms
of hemoglobin.
◦ c. The sickle-cell mutation
changes the form of its
corresponding protein, and
protein structure is controlled
by genes.
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Hemoglobin has four polypeptide chains: two α
polypeptide and two of the β polypeptide, each
associated with a heme group.
V. M. Ingram (1956) found that the sixth amino
acid of the β chain in sickle-cell hemoglobin is
valine (no electrical charge) replacing the
negatively charged glutamic acid of normal
hemoglobin.
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Outline of sickle-cell anemia and trait:
◦ a. Wild-type β chain allele is bA, which is codominant with bS.
◦ b. Hemoglobin of bA/bA individuals has normal b subunits,
hemoglobin of genotype bS/bS has b subunits that sickle at low
O2 tension.
◦ c. Hemoglobin of bA/bS individuals is 1⁄2 normal and 1⁄2 sickling
form. (The two b chains of an individual hemoglobin molecule
will be of the same type, rather than mixed.) Sickle-cell
symptoms after sharp drop in the oxygen content of their
environment.
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One-gene–one-polypeptide hypothesis is a
simplification. Alternative splicing in eukaryotes can
result in more than one polypeptide from a single
gene.
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Screening of hemoglobin for altered electrophoretic
mobility identified over 200 hemoglobin mutants.
Most effects are not as severe as those seen in sicklecell anemia.
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Cystic fibrosis (CF) is a recessive disease with
peliotrophic effects:
◦ affects the pancreas, lungs, and digestive system, and
sometimes the vas deferens in males.
◦ abnormally viscous secreted mucus resulting in lung
complications.
◦ Life expectancy with current treatments is about 40 years
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The gene is on the long arm of chromosome 7.
Encodes a protein called cystic fibrosis
transmembrane conductance regulator (CFTR).
Most common is 3bp deletion at position 508.
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The structure of the protein has
been deduced from its sequence.
CFTR has homology with a large
family of active transport
membrane proteins.
Functional analysis shows that
CFTR normally forms a chloride
channel in the cell membrane. The
mutation prevents chlorine ion
transport and resulting in CF
symptoms.
Mice genetically engineered to
have the same defect in their
CFTR gene are an animal model
for the disease.
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Genetic testing for a disease or carrier status, and
chromosomal abnormalities.
The advice is based on genetic analysis, explaining
diseases, probabilities, and options to affected
individuals or parents.
Effective genetic counseling requires up-to-theminute knowledge and is based on:
◦ a. Pedigree analysis is an important tool of genetic
counseling, considering phenotypes found in both families
over several generations. This is particularly useful for
identifying suspected carriers.
◦ b. Fetal analysis includes assays for enzyme activity or
protein level, or detection of changes in the DNA itself.
Prenatal Diagnoses