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Geneticist Definition of Gene
If you ask 10 different geneticist to define
gene, you would get 10 different answers
Genes must involve information
Information must be contained in a
molecule
Need to define nature of information
Then look at molecules
Mutations as a Tool
To geneticist mutations are data
Mutations shows us the effect of
removing or disrupting a process
If you are interested in a process
Then make mutants to disrupt the process
Number of different mutants will tell you
steps in the process
Can be used to determine order of events
Combinations of mutants: double mutants
Functional Test for Allelism
Two mutants with the same phenotype
Are they alleles of one gene?
Do they represent alleles of two different
genes?
We could isolate the genes, sequence
them and compare...
Can we do this with simple genetic
crosses?
Complementation Test
Complementation Test
Only works with recessive alleles
Logic is simple:
Cross homozygous mutants—>heterozygote
Assess phenotype
If wild type (doubly heterozygous)
Then different genes, we say they
complement (provide wild type function)
If mutant (heteroallelic)
Then alleles of same gene, we say they fail
to complement (no wild type function)
Treat with mutagen
Isolate (recessive)
mutants
Make homozygous
Mutant
1
Mutant
2
Mutant
3
How many genes do these represent?
Mutant
4
Consider Single Test
complements (+)
X
w1 w2+
w1 w2+
w1+ w2
w1+ w2
w1 w2+
w1+ w2
fails to complement (-)
X
w1a
w1a
w1b
w1b
w1a
w1b
Test All 4 Mutants
M1
M2
M3
M4
M1
M2
M3
M4
–
+
+
–
–
–
+
–
+
–
Allelic Relationships: 2 genes, each with 2 alleles
Gene 1: with alleles M1, M4
Gene 2: with alleles M2, M3
Genes and Proteins
Relationship of genotype and phenotype
First clue came from Beadle and Tatum:
Studied synthesis of arginine in Neurospora
Isolate mutant alleles of genes
Correlate with biochemical pathway
Mutants identified by failure to make Arg
Call this kind of mutant auxotroph
Supplement media with Arg = growth
No Arg in media = no growth
Supplement with intermediates = pathway
Media Supplement
Ornithine Citrulline Arginine
Mutant
Wild type
Mutant A
Mutant B
Mutant C
precursor
A
+
+
-
ornithine
+
+
+
B
citrulline
+
+
+
+
C
arginine
Conclusions?
Supplement with intermediate after block
in pathway = growth
Can dissect pathways using genetics
Beadle and Tatum concluded each gene
encoded a single enzyme in the pathway:
One gene-one enzyme
One gene one polypeptide is better
Even this is an oversimplification
Begins to get at the relationship
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Gene
1
Chromosome
Gene
2
arg-E
arg-G arg-H
arg-F
Encoded
enzyme Enzyme E
Glutamate
Gene
3&4
Enzyme F
Ornithine
Enzyme G
Citruline
biochemical pathway
Enzyme H
Arginosuccinate
Arginine
The Central Dogma of Molecular
Biology
Is that a cool phrase or what?!
But what does it mean?
A brief description of the most important
overall concept of molecular biology
What is this all about?
Information
Central Dogma describes the flow of
information in cells
CDMB
DNA makes RNA makes Protein
replication
DNA
transcription
RNA
translation
Protein
DNA Stores information, and is replicated
RNA contains information in DNA
RNA is used to direct synthesis of
proteins
CDMB
A more modern version is below:
DNA
RNA
Protein
This encompasses the original idea
DNA can be made from RNA (reverse
transcription)
Gene Expression
Central dogma describes flow of
information
Actual process is called Gene Expression
Transcription
Translation
How genotype is converted into
phenotype
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
DNA
Transcription
mRNA
Translation
Protein