Download Gene function

Chapter 4: Gene Function
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Genes code for enzymes, non-enzymatic proteins, and other
functional molecules like RNA.
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Beadle & Tatum’s “One Gene-One Enzyme Hypothesis”
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Life cycle of Orange Bread Mold (Neurospora crassa)
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Deduction of biochemical pathways using auxotrophic
mutants
Human genetic disorders caused by mutations
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enzyme deficiency disorders
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protein disorders (non-enzymatic deficiencies)
Examples of adaptive evolution (on Tuesday!)
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Some examples from my research
1942: George Beadle and Edward Tatum
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Discovered that genes act by regulating definite chemical events.

Studied relationships between genes and enzymes in the haploid
fungus Neurospora crassa (orange bread mold).

One Gene-One Enzyme Hypothesis
Each gene controls synthesis/activity of a single enzyme.
“one gene-one polypeptide”
1958: George Beadle (Cal Tech) & Edward Tatum (Rockefeller Institute)
Life cycle of the haploid fungus Neurospora crassa (orange bread mold):
1.
Neurospora reproduces in a web-like pattern with asexual spores
called conidia, which are orange in color.
2.
Neurospora is haploid, so mutations are spotted easily (unlike
diploid organisms).
3.
Life cycle is short, and grows on simple media (N, C, biotin).
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Sythesizes all other substances from these basic nutrients.
4.
Propagates vegetatively (asexually) by dispersal of mycelium and
asexual spores (conidia), OR
5.
Sexually, by means of two mating types, A and a (only A will mate
with a and result in a gamete fusion and meiosis).
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Meiosis leads to 4 haploid nuclei (2A, 2a)
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After one round of mitosis, 8 ascospores (4A, 4a).
Life cycle of the haploid fungus Neurospora crassa (orange bread mold)
Fig. 4.2:
Beadle’s and Tatum’s Experimental Method
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Isolated mutant auxotropic Neurospora by irradiating conidia
with X-rays and crossing with wild-type strain of the opposite
mating type.
auxotrophic mutant = unable to make a required nutrient
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Mutant ascospores were germinated on complete media.
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Progeny conidia (asexual spores) were tested on an array of
minimal media, each with a different supplement to determine
the type of nutritional mutation.
e.g., amino acids and vitamins
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Assuming that many genes interact in pathway for synthesis of
amino acids and other cellular products, Beadle and Tatum then
dissected the biochemical pathways.
e.g., synthesis of the amino acid Methionine
Fig. 4.3:
Growth responses of methionine auxotrophs on minimal
media and…
Mutant
strain
Wild type
met-5
met-3
met-2
met-8
Nothing
O-Acetyl
Homoserine
Cystathionine
Homocysteine
Methionine
+
+
+
+
+
-
+
+
+
+
-
-
+
+
+
-
-
-
+
+
-
-
-
-
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Pathway can be deduced using the following logic:
The mutant strain is blocked farther along in a pathway if fewer intermediate
compounds permit the strain to grow.
OR
The mutant strain is blocked at earlier steps in the pathway if a larger number
of intermediates enable the strain to grow.
Mutant
strain
Wild type
met-5
met-3
met-2
met-8
Nothing
O-Acetyl
Homoserine
Cystathionine
Homocysteine
Methionine
+
+
+
+
+
-
+
+
+
+
-
-
+
+
+
-
-
-
+
+
-
-
-
-
+
Fig. 4.4
Beadle and Tatum proposed: “One Gene-One Enzyme Hypothesis”
However, it quickly became apparent that…
1.
More than one gene can control each step in a pathway
(enzymes can be composed of two or more polypeptide chains,
each coded by a separate gene).
2.
Many biochemical pathways are branched.
“One Gene-One Enzyme Hypothesis”
“One Gene-One Polypeptide Hypothesis”
Post-modern formulation:
One gene can actually code several different polypetides or RNA
subunits, by the process of alternative splicing.
Alternative splicing of different exon sequences in different
combinations creates different types of related proteins.
These may be tissue specific.
Many different types of gene products are transcribed to RNA but
not translated to protein (e.g., rRNA, tRNA, snRNA).
Other types of RNA sequences also are important for development
and regulation of gene expression (e.g., miRNAs, siRNA).
Non-coding DNA sequences also function importantly in gene
regulation (e.g., binding sites for transcription etc.).
Ultimately, the # of genes in not so important. This # is
remarkably conserved among species (e.g., Drosophila vs Human).
How the genes are expressed is more essential.
Human genetic disorders with demonstrated enzyme deficiencies:
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Effects may be simple or wide-reaching
pleiotropic = wide-reaching
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Three common examples:
Phenylketonuria (PKA)
Type I Albinism
Tay-Sachs Disease
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Learn more about genetic disorders at NIH’s website:
Online Mendelian Inheritance in Man (OMIM)
http://www.ncbi.nlm.nih.gov/omim/
Phenylketonuria (PKU, OMIM-261600)
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Mutation in the gene for phenylalanine hydoxylase, which prevents the
conversion of phenyalanine to the amino acid tyrosine (phenylalanine is
essential to the human diet).
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Homozygous recessive: ~1 in 12,000 Caucasian births on chromosome 12.
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Children born with PKU accumulate excess phenyalanine, which is converted to
phenylpyruvic acid.
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Results are damage to the nervous system: severe retardation, slow growth
rate, and early death (prior to birth fetus is protected by maternal enzymes).
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Pleiotropic effects: tyrosine is required for the production of the hormones
thyroxine and adrenaline and the synthesis of melanin.
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Can be managed by controlling dietary intake of phenylalanine beginning at 12 months after birth to ~10 years of age.
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Pregnant women with PKU should maintain their diet through pregnancy, or
their children will experience mental retardation independent of their
genotype.
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U.S. requires all newborns to be screened for PKU; the Guthrie test
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“NutraSweet” contains phenylalanine
Type I Albinism (OMIM-203100)
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Mutation affects the gene that codes for tyrosinase, which converts tyrosine to
DOPA (dihydroxylphenylalanine), a precursor to melanin.
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People with albinism produce no melanin.
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Homozygous recessive: ~1 in 33,000 Caucasian births and 1 in 28,000 African
Americans in the U.S.
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No known problems resulting from accumulation of other precursors in the
pathway.
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Two other forms of albinism are known (Type II, Type III)
Tay-Sachs Disease (OMIM-272800)
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Lysosomal storage disease caused by mutation in a lysosomal enzyme.
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Homozygous recessive, occurs on chromosome 15.
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Relatively rare, but common (~1 in 3,600) in Central European origin Jews.
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Caused by a mutation in the HexA gene, which codes for the enzyme Nacetylhexosaminidase.
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Results in the accumulation of unprocessed gangliosides in brain cells.
(ganglioside = complex glycolipid common in nerve membranes)
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Infants experience acute reactions to sharp sounds, a spot on the retina, and
rapid neurological degeneration beginning at one year of age.
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Followed by paralysis, loss of hearing, inability to feed, and death at age 3-4.
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No cure, but carriers (heterozygotes) can be detected by genetic screening.
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High-frequency in Jewish population has led some to speculate that
heterozygotes may have tuberculosis (TB) resistance, but not verified.
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More likely that frequency of Tay-Sachs was driven to high levels by chance
(genetic drift) following a population bottleneck.
https://www.studyblue.com/
Most enzymes are proteins, but not all proteins are enzymes:
Examples of traits and diseases that result from mutations in
genes coding for non-enzymatic proteins.
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Genetics of ABO Blood Groups (trait)
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Cystic fibrosis (disease)
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Sickle-cell Anemia (disease)
Genetics of the Human ABO Blood Group
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ABO blood groups discovered by Karl Landsteiner early 1900s.
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Four phenotypes determined by three alleles, IA, IB, and i.
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ABO locus produces RBC antigens by adding sugars to
membrane glycolipids using enzymes called
glycosyltransferases.
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Most people produce a glycolipid called the H antigen.
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Next, depending on allele types, -N-galactosamine is added
to produce either an A antigen or B anitgen.
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antigen = any molecule recognized as a foreign by an organism
and stimulates the productions of antibodies.
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antibodies = specific molecules that recognize and bind
antigens.
1930: Karl Landsteiner (Rockefeller Institute)
ABO blood groups and their properties:
Phenotype
Genotype
RBC-antigen
Blood-antibody
O
i/i
none (H)
anti-A & B
A
IA/ IA or IA/i
A
anti-B
B
IB /IB or IB /i
B
anti-A
AB
IA/IB
A and B
none
Donor relationships:
1.
2.
3.
4.
5.
Type AB produce A & B antigens and can be transfused only
to blood types AB (universal recipients).
Type B produce B antigen and can donate to type B and AB.
Type A produce A antigen and can donate to type A and AB.
Type O produce no antigen and can donate to all blood types
(universal donors).
Classic example of Codominance
Cystic fibrosis (OMIM-219700):
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Most common genetic disease in people of European descent, occurs in
~1 in 2,000 (1 in 23 are carriers).
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Affects the pancreas, lungs, digestive system, and sometimes the vas
deferens in males, resulting in sterility.
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Characterized by abnormally viscous mucus and frequent lung
infections. Life expectancy is generally ~40 years.
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Autosomal recessive deletion on chromosome 7, commonly 3 base pairs.
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Heterozygous state may confer resistance to diseases involving a loss of
body fluid (i.e., resistance to cholera or TB).
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Three base pair deletion (deletion of a codon) results in the deletion of
an amino acid at an ATP binding site in the CFTR protein = Cystic
Fibrosis Transmembrane Conductance Regulator
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Examples of common allelic variants:
PHE508
ILE507
GLN493
DELETION
DELETION
STOP CODON
ASP110 HIS
ARG117 HIS
ARG347 PRO
Sickle-cell anemia (OMIM-141900):
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First described by J. Herrick (1910): discovered that red blood cells
(RBCs) change shape to form a sickle under low oxygen pressure.
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Sickle-shaped RBCs are fragile and less flexible than normal RBCs,
resulting in anemia, blocking capillaries, and damaging tissues.
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One common form of the disease results from an amino acid
substitution in the 6th amino acid of the  chain of the hemoglobin
molecule (hemoglobins are composed of four polypeptides (2
 chain), each associated with a heme).

and two
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Effects include: anemia, damage to the extremities, heart, lung, brains,
kidney, GI tract, muscles, and joints.
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Heterozygotes produce both normal and sickle-shaped RBCs and show a
sickle-cell trait, but it is a much milder form of the disease.
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Sickle-cell trait also protects heterozygotes against malaria (when the
malarial parasite Plasmodium falciparum infects a sickle-cell, the RBC
and parasite are destroyed, resulting in a lower parasite count).
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In tropical Africa, as many as 20-40% of people are heterozygotes.
Fig. 4.9
Fig. 4.10
Glu = glutamic acid (acidic amino acid with a negative charge)
Val = valine (nonpolar amino acid with no electrical charge)
Fig. 4.8
What happens when the heterozygote confers resistance to a disease?
Heterozygote superiority:
Heterozygote has higher fitness than either homozygotes, and both
alleles are maintained in the population because the heterozygote
genotype is favored (e.g., sickle cell trait).
Also known as: heterosis or overdominance
Distribution of
malaria and Hb-S
allele.
Fig. 4.10a Examples of amino acid substitutions Fig.
found
in 
4.11
polypeptides of various
human hemoglobin variants
Next Tuesday Lecture - Examples of adaptive evolution
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Some examples from my research
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Important to understand that disease is only one aspect of
biological function.
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Humans are just one of millions of species, and relative
newcomers.
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You can learn much more about how proteins function (&
even medicine) by studying the diversity of other species.
Bar-headed Geese crossing the Himalayas
http://enoogle.com/5-best-examples-jaw-dropping-natural-phenomena/