Download Genetic Fine Structure

Genetic Fine Structure
Nature of the Gene at
the Molecular Level
Bead Theory
The gene is the fundamental unit of
1. Structure
The gene is indivisible by crossing over.
Crossing over occurs only between
genes.
Bead Theory
The gene is the fundamental unit of
2. Change
The whole gene must change from one
allelic form to another, there are no
smaller components within gene can
change by mutation.
Bead Theory
The gene is the fundamental unit of
3. Function
The gene functions as a unit, parts of a
gene cannot function on their own.
Revised Bead Theory
The nucleotide pair is the fundamental unit
of
1. Structure
2. Change
The gene is the fundamental unit of
3. Function
How Can the Expression
of a Gene by Altered By:
1. Intragenic recombination?
2. Mutation?
3. Complementation?
Intragenic Recombination
Recombination within a gene is shown
by recombination between two mutants
to give a wild type (non-mutant) form of
the gene.
Mutant 1
O
X
O
Mutant 2
Wild type
O
O
Double Mutant
Application:
Deletion Mapping
Deletions prevent recombination.
1. If no wild type recombinants can be
produced in a cross between two
deletion mutants, the deletions are
overlapping.
2. Regions of a gene can be defined by
deletion mutations, and point mutations
can be located within those regions.
Application:
Deletion Mapping
Non-overlapping deletions
Wild Type
X
Double Mutant
Overlapping deletions
Unable to achieve
recombination to restore
wild type
Application:
Deletion Mapping
Deletion and Point Mutation
do not overlap
O
X
Wild Type
O
Double Mutant
Deletion and Point Mutation
overlap
O
Unable to achieve
recombination to restore
wild type
Deletion mapping of the rII region
of Bacteriophage T4.
Application: Deletion Mapping
Problem 1, page 3-4
Deletion Mutants
Deletion Mutants
In a particular bacteriophage,
four deletion mutants are
crossed in pairwise
combinations to test for their
ability to produce wild-type
recombinants. The results are
given beside where + indicates
that recombinants were found.
Draw a deletion map for these
mutations and divide it into
subdivisions according to
overlapping mutations.
1
2
3
4
1
--
2
---
3
+
---
4
+
+
--
--
Application:
Deletion Mapping
Solution
Problem 1, page 3-4
1
3
2
4
Application: Deletion Mapping
Problem 1, page 3-4
Site-Specific
Mutations
There are several site-specific point mutations (A, B and
C) that map in the region covered by the deletions. By
coinfection of phage with one of the deletions and phage
with each of the site-specific mutations, recombinant
phage are observed in the following cases. Assign each
site-specific mutation to one of the subdivisions of the
deletion map.
Deletion Mutants
A
B
C
1
+
-+
2
--+
3
-+
--
4
+
+
--
Application:
Deletion Mapping
Solution
Problem 1, page 3-4
1 & 2 overlap
1
2&3
2
B
3&4
3
4
A
C
How Can the Expression
of a Gene by Altered By:
1. Intragenic recombination?
Recombination between two mutant
forms gives a wild type version of the
gene --- changes in both genotype
and phenotype occur.
Mutations
Change in a nucleotide can lead to change
in amino acid found in the protein.
Silent
Synonymous
Missense
Nonsense
Frameshift
AGG CGG
Arg
Arg
AAA AGA
Lys Arg
AAA GAA
Lys
Glu
CAG UAG
Gln
Stop
AA(A)GACUUACCAA
Lys-asp-leu-pro 
Lys-thr-tyr-gln
How Can the Expression
of a Gene by Altered By:
2. Mutation?
Change in DNA triplet can alter amino
acid sequence of protein.
Complementation
Production of the wild type phenotype
when two different mutations are
combined in a diploid or heterokaryon.
mutant 1
A
b
enzyme A
PRECURSOR
INTERMEDIATE
PRODUCT
enzyme B
mutant 2
a
B
Application:
Complementation Tests
1. If a wild type phenotype cannot be
produced in a cross between two
mutants, the mutations are in the same
gene (cistron).
2. If wild type phenotype can be produced,
the mutations are in different genes.
Application: Complementation Tests
Problem 2, page 3-4
Five mutant strains of Neurospora give the
following results in complementation tests
where a plus signifies complementation and
a minus shows no complementation.
Determine how many cistrons are
represented by these mutations and
indicate which mutants belong to each
cistron.
Application: Complementation Tests
Problem 2, page 3-4
Mutant Strain
Mutant Strain
1
2
3
4
5
1
--
2
+
--
3
+
+
--
4
+
+
---
5
-+
+
+
--
Application:
Complementation Tests
Solution
Problem 2, page 3-4
(1, 5)
(2) (3, 4)
How Can the Expression of a
Gene by Altered By:
3. Complementation?
Production of the wild type phenotype
when two different mutations are
combined in a diploid or
heterokaryon—genotypes are
unchanged.
Application: Determining the
Order in a Biochemical Pathway
Application: Observing Complementation
for Genes within the Same Pathway
Suppose we have two different haploid cells, each with
mutations in two of the genes in the pathway, and
these haploid cells fuse to form a diploid cell. Which of
the following diploid cells can grow on minimal
medium?
_
_
_
_
1. ARG-E_, ARG-G_ combined with ARG-F_ , ARG-H_
2. ARG-E , ARG-F combined with ARG-F , ARG-H
_
_
_
_
1. ARG-E , ARG-G combined with ARG-F , ARG-H
X
X
X
X
_
_
_
_
2. ARG-E , ARG-F combined with ARG-F , ARG-H
X
X
X
X
X
SOLUTION SUMMARY
Suppose we have two different haploid cells, each
with mutations in two of the genes in the pathway,
and these haploid cells fuse to form a diploid cell.
Which of the following diploid cells can grow on
minimal medium?
_
_
_
_
1. ARG-E , ARG-G combined with ARG-F , ARG-H
YES _
_
_
_
2. ARG-E , ARG-F combined with ARG-F , ARG-H
NO
Ornithine will build up because the ARG-F product is
missing.