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Biochemical
Genetics
“The study of the relationships between genes and
enzymes, specifically the role of genes in controlling the
steps in biochemical pathways.”
Gregor Mendel’s
Pea Studies
Mendel’s Seven Traits
Mendel made a cross between a Tall
plant and a Dwarf plant.
X
Progeny were all Tall >
Then, Mendel allowed the plants to self-pollinate:
Offspring were:
75% Tall
25% Dwarf
How could such a thing happen?
Alleles are often indicated with a single letter.
A CAPITAL letter denotes a dominant allele = T
A lowercase letter denotes a recessive allele = t
Genotype (chemical code):
TT
x
Tt
tt
Phenotype (visual):
Tall
x
Tall
Dwarf
…one parent (female)
T
t
T
TT
Tt
t
Tt
tt
..other
Parent (male)
25% TT (tall)
50% Tt (tall)
25% tt (dwarf)
Dr. James Reid
University of Tasmania
discoverer of the le
mutation
Gibberellin Hormone
Gibberellin 3 beta-hydroxylase
is non-functional in this mutant
The Active form
Of Gibberellin
The le allele is a point mutation (substitution)
that alters the active site of the enzyme.
Which explains the differences seen between
heterozygotes and homozygous-recessives !
Finally, a biochemical
explanation for Dominance!!!
Dr. Reid found it using Arabidopsis mutant
Gibberellin
Mutant
Pea
Arabidopsis
le mutation
Iron-binding cofactor sites
Mutation of Active Site
Pea
Arabidopsis
Probably NOT in these
four “conserved” areas
Where could neutral mutations occur?
Mendel’s Seven Traits
Another example of
using “Biochemical
Genetics” to explain
Mendel’s observations
pea
corn
The wrinkled seed trait
in plants
“Smooth seed” is dominant
over wrinkled seed
Why?
Starch
Grains
Made by Starch Synthase
and Starch-Branching Enzyme
“r” is the gene
described by
Mendel
“r” stands for
“rugosus” = “wrinkled”
Wrinkled seeds
are low in starch
and high in sugar
(starch is a polymer
of glucose)
Starch synthesis
in developing seeds
Wrinkled seeds
are low in starch
and high in sugar
It all has to do with
Osmosis
“diffusion of water across a membrane
from high concentration to
low concentration”
Starch-Branching
Enzyme is mutated
(Loss of Function)
Sugars build-up
because they are
not being used to
make starch.
Later, as seeds mature,
they dry down (so they
can survive long periods).
Dry seeds end up
looking wrinkled
(like a raisin).
Water moves
into cells of
seed due to
osmosis.
Tissue swells
So what?
Could try to overcome mutation in pea or other plants.
Could induce mutation in other species.
Learn about the function of the enzyme.
Learn about similar genes in other species:
The Pea β-Hydroxylase gene for instance is fairly
similar to the ones in Tobacco and Lettuce,
but much more different than the one in Cucumber.
Kiwi (Actinidia deliciosa) = interspecific hybrid from China
Hexaploid = 174 chromosomes (29 pairs)