Download Lecture #3 Genes and Proteins

BIOLOGY 207 - Dr.McDermid
Lecture #3 Genes and Proteins
Readings:
Griffiths et al 7th Edition: Ch. 9 pp 267-281 - includes review of protein structure
Ch 4 pp 106-109 “A diagnostic test for alleles”
Supplement #1 (Symbols) and #2 (Complementation) – on web
Problems: Griffiths et al, 7th Edition: Ch. 9 # Tier 1: 3,12,13,14,16
Supplement #2 Questions
Tier 2: 2, 15, 17, 19a,b
Concepts:
How are genes and proteins related? How does a gene exert its effect?
1. Genes can be defined by their mutability
2. Changes in a gene ‡ changes in a protein !‡ changes in a phenotype
3. Mutations can alter a particular biochemical function in an organism. - one gene ‡ one
enzyme
4. Mutation in the same or different genes can be identified by complementation tests.
Some Basic Genetics Definitions:
- phenotype - gene - allele - locus (plural loci) - gene locus - genotype Genes specify the structure of proteins,
Archibald Garrod, 1902
- concluded human disease is inherited according to Mendelian rules
- mutation in gene ‡ change in basic body chemistry (biochemistry) ‡ heritable illness
- studied Alkaptonuria
- "inborn errors in metabolism"
1-gene 1-enzyme hypothesis (Beadle & Tatum 1941) Biosynthetic pathways
Substrate
Product/
Product/
Substrate
Substrate
1 ---------------------> 2 ------------------> 3 --------------> 4
Enzyme
!!!!!!A !!!!!!!!!!!! B
C
Gene a !!!!!!!!!!!!! b !!!
c
Suppose we inactivate (mutate) gene b
Result: Metabolic Block
If we add back intermediate compounds?
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Product
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Compound:
Growth:
Block:
1
2
3
4
No
before
No
before
Yes
after
Yes
after
PROBLEM:
- 5 independent mutants that can't synthesize compound G
- 6 components in the pathway
- add each to the medium, test for growth
- What is the order of the pathway?
Compound tested
Mutant
1
2
3
4
5
A
grow
B
grow
grow
grow
C
grow
grow
D
grow
grow
grow
grow
E
-
G
grow
grow
grow
grow
grow
Experiments of Beadle & Tatum
Neurospora - a fungus with a haploid genome. (Figure 9-1)
1- irradiated wildtype Neurospora -> produce mutations
2 - collect individual ascospores from fruiting bodies
- each spore is a potentially different mutant
3 - culture each ascospore in tube with complete medium (rich)
4 - grow up 100's -> 1000's of different spores
5 - test each culture to see if it could grow on 'minimal medium'
Beadle & Tatum found:
1- Many strains that were unable to grow on the
minimal medium (MM)
-> called auxotrophs
2- They did test to examine inheritance pattern
- inherited as 1:1 ratio
- concluded each was a simple mutation in a single gene
3- They did a test with amino acids or vitamins
- identified many arg- mutants
Arg mutants provided the focus for Beadle & Tatum's further work
- 3 loci : arg-1, arg-2, arg-3
Mutant
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ornithine
citrulline
arginine
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arg-1
yes
yes
yes
arg-2
no
yes
yes
arg-3
no
no
yes
- proposed biochemical model for Neurospora
precursor------> ornithine ------> citrulline ------->arginine
Enzyme
X !!!!!!!!!!!Y
Z
Gene
arg-1 !!!!!!!!arg-2!!!!!!!!!arg-3
Assume:
1- that a mutation in a particular gene interferes with the production of a single enzyme
2- defective enzyme creates a block in the biosynthetic pathway
3- the block can be circumvented by adding the compound that comes after the block
The one-gene-one-enzyme hypothesis
- two mutations in different genes in pathway
‡ affect different enzymes in pathway
Neurospora - haploid
Diploid organism has 2 copies of each gene
Complementation in Diploids
Complementation studies in diploids can be used to demonstrate two mutations are in either:
1. the same gene, or
2. two different genes
Nomenclature of genes and mutations
- usually, different genes are represented by different letters
- the normal copy of a gene is known as wild type
- the mutant form of the gene:
+ - "genotype" of a / a organism
+ - + - a /a ; b / b means :
Complementation
e.g. Yeast
- can also exist as haploid cells
Assume the following pathway:
Substrate
Product/
Substrate
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Product
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1 ----------------> 2 ----------------> 3
Enzyme!!!
A !!!!!!!!!!B
+
+
Gene !!!!!a !!!!!!!!!!b
Then in haploid Yeast:
+ +
a b
=> wildtype enzymes
+ a b
=> mutant enzyme b
- +
a b
=> mutant enzyme a
- a b
=> mutant enzymes a &b
=>
Yeast can also exist as a diploid
First Strain / Second Strain Diploid strain
+ +
+ +
a b !!!!!!!!!!!!!! a b
-two copies / cell ‡ permits complementation
Substrate
Product/
Substrate
!!!!!1 ----------------> 2 ----------------> 3
Enzyme
A !!!!!!!!!!!B
+!
+
Gene
a !!!!!!!!! b
=>
=>
=>
+ + + +
a b /a b
Product
Therefore:
+ a b and
+ a b
+ +
a /a ;
- +
a b and
- +
a b
- a /a ;
- +
a b and
+ a b
+ a /a ;
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No growth
No complementation
No growth
No complementation
Growth
Complementation
- b /b
+ +
b /b
+ b /b
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Complementation:
If the mutations fail to complement then they are mutant in the same gene
- they are allelic mutations.
If the mutations do complement then they are mutant in different genes
- they are non-allelic mutations.
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