From Gene to Protein:
Chpt. 17
How does the DNA lead to
specific traits???
Archibald Garrod (1909)
hypothesized: “proteins are the link
between genotype &
phenotype”
1909 - Archibald Garrod
Suggested
genes
control
enzymes, & enzymes
catalyze chemical
processes in cells.
Inherited Diseases are “inborn
errors of metabolism” where a
person can’t make an enzyme.”
Example
Alkaptonuria (Al ka toe nuria)-
where urine turns black after
exposure to air b/c of chemical
alkapton (Al kae ton).
these
individuals must lack an
enzyme to metabolize (break down)
alkapton (the chemical).
enzyme
enzyme
enzyme
enzyme
George Beadle American(1930)
*“ mutations in eye color
(Drosophila) are a result of
blocks in pigment
production”
*“no enzyme is made therefore pathway to
produce color pigment is
not complete… white
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
G. Beadle & Edward Tatum
Beadle and Tatum set out to provide experimental
proof of the connection between genes and
enzymes.
G. Beadle & Edward Tatum
Cal Tech.
Rockefeller Inst. NY, NY
Nobel Prize in Physiology or Medicine 1958
"for their discovery that genes act by regulating definite chemical events"
G. Beadle & Edward Tatum
test organism = Neurospora
Neurospora
(pink bread mold)
G. Beadle & Edward Tatum
mutant molds (not wild type) had a variety of
special nutritional needs. Unlike their wild type
counterparts, they could not live without the
addition of particular vitamins or amino acids to
their food.
G. Beadle & Edward Tatum
*“ various mutations must be
abnormal variations of
genes”
One Gene, One Enzyme Hypothesis
-Beadle & Tatum
this is the normal metabolic pathway
wild type mold survived in minimal
medium agar.
The wild type mold can produce all of
the enzymes it needs to produce the
necessary amino acids to live.
One Gene, One Enzyme Hypothesis
-Beadle & Tatum
Mutants did not survive in minimal
medium agar.
The mutants must not produce
all of the enzymes needed
The enzymes would synthesize
particular amino acids. But they
could survive in complete agar…
supplemented with all 20 amino
acids.
One Gene, One Enzyme Hypothesis
-Beadle & Tatum
One Gene, One Enzyme Hypothesis
-Beadle & Tatum
Tried to identify where the
mutants’ metabolic defects
(mutations) were
One Gene, One Enzyme Hypothesis
-Beadle & Tatum
All strains were grown on
complete media.
This contained all a.a. &
vitamins made by the wild type,
they all thrived…
By the way, this mold MUST
make Arginine in order to grow.
One Gene, One Enzyme Hypothesis
-Beadle & Tatum
Took mutants out of the
complete media
They did not all grow when
put on “minimal media”
They figured that mutants were
unable to produce compounds
essential for growth… perhaps their
genes did not code for an enzyme…
this ultimately did not let the organism
produce arginine??
One Gene, One Enzyme Hypothesis
-Beadle & Tatum
Put each mutant type in
different vials containing
minimal + one amino acid.
(in this pix., the a.a. is
ornathine)
Because Class I grew on minimal
medium supplemented with
ornithine, citrulline, or argine, it
had to be missing enzyme A. This
would be required to form all
three compounds
One Gene, One Enzyme Hypothesis
-Beadle & Tatum
Class II could not produce
citruline, and that is needed
for the rest of the pathway to
occur... Ornathine “piled up”
One Gene, One Enzyme Hypothesis
-Beadle & Tatum
The media that allowed
growth, would show where
the metabolic defect
(mutation) occurred there
was always a “pile up” of the
amino acid before
One Gene, One Enzyme Hypothesis
-Beadle & Tatum
If we inactivate one gene, that would code
for an enzyme to carry out X -> Y
conversion, no product X or Y is produced.
If product Y is needed to ultimately produce
arginine, no growth will occur
One Gene, One Enzyme Hypothesis
-Beadle & Tatum
The supplement that
allowed growth, would
show where the metabolic
defect (mutation) occurred
One Gene, One Enzyme Hypothesis
-Beadle & Tatum
One Gene, One Enzyme Hypothesis
-Beadle & Tatum
ex. this mutant grew (was able to complete the
pathway) in the vial supplemented with
Citrulline.
therefore, it must be defective in
synthesizing Citrulline~ornithine piled up
One Gene, One Enzyme Hypothesis
-Beadle & Tatum
each gene dictates the
production of one enzyme
each mutant lacks the
ability to produce an
*Some proteins are not enzymes
(ex. insulin)
*Some proteins are made of more than
one polypeptide chain (hemoglobin)
*Each chain specified by its own gene
One Gene, One Polypeptide
Hypothesis
-Beadle & Tatum (revisited)
How are enzymes
(proteins/polypeptides)
made?
How do we get
from DNA to
protein?
QuickT ime™ and a
T IFF (Uncompressed) decompressor
are needed to see this picture.
RNA is the
bridge
between
DNA and
Protein
mRNA
tRNA
This is the
basic process:
TRANSCRIPTION
TRANSLATION
No nucleus, protein
synthesis occurs same
time as transcription.
Transcription occurs in
nucleus-> pre -mRNA
Translation occurs on ribos.
DNA -->
RNA -->
PROTEIN!
Triplets - series of
three nucleotidescode for Amino Acids
U not T
(1961)
Marshall
Nirenberg
deciphered first
codon.
(1961)
Marshall
Nirenberg
deciphered first
Won Nobel prize in Physiology
codon.
and Medicine 1968 for the
interpretation of the genetic
code and its function in protein
synthesis.
1965 Dictionary of Amino Acids
1965 Dictionary of Amino Acids
Code
Redundancy
• Third base in a codon shows
"wobble”.
• First two bases are the most
important in reading the code
and giving the correct AA. The
third base often doesn’t matter.