Download video slide - Saginaw Valley State University

Chapter 17:
From Gene to Protein
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 17.1 A ribosome, part of the protein
synthesis machinery
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 17.2 Do individual genes specify different
enzymes in arginine biosynthesis?
EXPERIMENT
Working with the mold Neurospora crassa, George Beadle and Edward Tatum had isolated mutants requiring
arginine in their growth medium and had shown genetically that these mutants fell into three classes, each
defective in a different gene. From other considerations, they suspected that the metabolic pathway of arginine
biosynthesis included the precursors ornithine and citrulline. Their most famous experiment, shown here, tested
both their one geneone enzyme hypothesis and their postulated arginine pathway. In this experiment, they grew
their three classes of mutants under the four different conditions shown in the Results section below.
RESULTS
The wild-type strain required only the minimal medium for growth. The three classes of mutants had different
growth requirements
Wild type
Minimal
medium
(MM)
(control)
MM +
Ornithine
MM +
Citrulline
MM +
Arginine
(control)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Class I
Mutants
Class II
Mutants
Class III
Mutants
CONCLUSION
From the growth patterns of the mutants, Beadle and Tatum deduced that each mutant was unable
to carry out one step in the pathway for synthesizing arginine, presumably because it lacked the
necessary enzyme. Because each of their mutants was mutated in a single gene, they concluded
that each mutated gene must normally dictate the production of one enzyme. Their results
supported the one gene–one enzyme hypothesis and also confirmed the arginine pathway.
(Notice that a mutant can grow only if supplied with a compound made after the defective step.)
Class I
Class II
Class III
Mutants
Mutants
Mutants
(mutation
(mutation
(mutation
in gene A)
Wild type
in gene B)
in gene C)
Precursor
Gene A
Enzyme
A
Ornithine
Gene B
Enzyme
B
Citrulline
Gene C
Enzyme
C
Arginine
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Precursor
Precursor
Precursor
A
A
A
Ornithine
Ornithine
Ornithine
B
B
B
Citrulline
Citrulline
Citrulline
C
C
C
Arginine
Arginine
Arginine
Figure 17.4 The triplet code
DNA
molecule
Gene 2
Gene 1
Gene 3
DNA strand
(template)
5
3
A
C
C
A
A
A
C
C
G
A
G
T
U G
G
U
U
U
G G
C
U
C
A
TRANSCRIPTION
mRNA
5
Codon
TRANSLATION
Protein
Trp
Amino acid
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Phe
Gly
Ser
3
Figure 17.5 The dictionary of the genetic code
Second mRNA base
C
UUU
UUC
U
UUA
First mRNA base (5 end)
A
UAU
UCU
Phe
UCC
UCA
UAC
Ser
U
UGU
Tyr
UGC
Cys
C
UCG
CUU
CCU
CAU
CUC
CCC
CAC
CUA
Leu
Leu CCA Pro
CAA
CUG
CCG
CAG
AUU
ACU
AAU
ACC
AAC
AUC
lle
AUA
ACA
Thr
AAG
GUU
GCU
GAU
GUC
GCC
GAC
GUA
GUG
Val
GCA
Ala
GCG
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
His
Gln
Asn
AAA
AUGMet or ACG
start
G
G
UAA Stop UGA Stop A
UAG Stop UGG Trp G
UUG
C
A
Lys
CGC
CGA
Arg
Asp
CGG
G
AGU
U
AGC
Ser C
AGA
A
AGG Arg G
GGC
GGA
Glu
C
A
U
GGU
GAA
GAG
U
CGU
GGG
Gly
C
A
G
Third mRNA base (3 end)
U
Figure 17.6 A tobacco plant expressing a firefly gene
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Non-template
strand of DNA
Elongation
RNA nucleotides
RNA
polymerase
A
T
C
C
A
A
3
3 end
U
5
A
E
G
C
A
T
A
G
G
T
T
Direction of transcription
(“downstream)
5
Newly made
RNA
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Template
strand of DNA
Figure 17.13 Translation: the basic concept
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
Amino
acids
Polypeptide
Ribosome
tRNA with
amino acid
attached
Gly
tRNA
Anticodon
A A A
U G G U U U G G C
Codons
5
mRNA
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
3
Figure 17.14 The structure of transfer RNA (tRNA)
3
A
Amino acid
C
attachment site
C
A 5
C G
G C
C G
U G
U A
A U
A U
U C
*
G
U
AG *
CACA
A CUC
*
G
*
U
G
U
G
G
*
C
CGAG
C
* *
AGG
U
*
* GA
G C
Hydrogen
G C
bonds
U A
* G
* A
A
C
*
U
A
A G
Anticodon
(a) Two-dimensional structure. The four base-paired regions and three loops are characteristic of all tRNAs,
as is the base sequence of the amino acid attachment site at the 3 end. The anticodon triplet is unique to
each tRNA type. (The asterisks mark bases that have been chemically modified, a characteristic of tRNA.)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
5
3
Amino acid
attachment site
Hydrogen
bonds
A
3
Anticodon
(b) Three-dimensional structure
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
A G
Anticodon
5
(c) Symbol used
in this book
Figure 17.15 An aminoacyl-tRNA synthetase joins
a specific amino acid to a tRNA
Amino acid
Aminoacyl-tRNA
synthetase (enzyme)
1 Active site binds the
amino acid and ATP.
P P P Adenosine
ATP
2 ATP loses two P groups
and joins amino acid as AMP.
P Adenosine
Pyrophosphate
Pi
Phosphates
P Pi
Pi
tRNA
3 Appropriate
tRNA covalently
Bonds to amino
Acid, displacing
AMP.
P Adenosine
AMP
4 Activated amino acid
is released by the enzyme.
Aminoacyl tRNA
(an “activated
amino acid”)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 17.16 The anatomy of a functioning ribosome
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
Exit tunnel
Growing
polypeptide
tRNA
molecules
Large
subunit
E
P A
Small
subunit
5
mRNA
3
(a) Computer model of functioning ribosome. This is a model of a bacterial ribosome, showing its overall
shape. The eukaryotic ribosome is roughly similar. A ribosomal subunit is an aggregate of ribosomal
RNA molecules and proteins.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 17.23 The molecular basis of sickle-cell
disease: a point mutation
Wild-type hemoglobin DNA
3
Mutant hemoglobin DNA
5
C
T
T
In the DNA, the
mutant template
strand has an A where
the wild-type template
has a T.
A
The mutant mRNA has
a U instead of an A in
one codon.
3
5
T
C
mRNA
A
mRNA
G
A
A
5
G
3
U
5
3
Normal hemoglobin
Sickle-cell hemoglobin
Glu
Val
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The mutant (sickle-cell)
hemoglobin has a valine
(Val) instead of a glutamic
acid (Glu).
Figure 17.26 A summary of transcription and
translation in a eukaryotic cell
DNA
TRANSCRIPTION
1 RNA is transcribed
from a DNA template.
3
5
RNA
transcript
RNA
polymerase
RNA PROCESSING
Exon
2 In eukaryotes, the
RNA transcript (premRNA) is spliced and
modified to produce
mRNA, which moves
from the nucleus to the
cytoplasm.
RNA transcript
(pre-mRNA)
Intron
Aminoacyl-tRNA
synthetase
NUCLEUS
Amino
acid
tRNA
FORMATION OF
INITIATION COMPLEX
CYTOPLASM 3 After leaving the
nucleus, mRNA attaches
to the ribosome.
mRNA
AMINO ACID ACTIVATION
4
Each amino acid
attaches to its proper tRNA
with the help of a specific
enzyme and ATP.
Growing
polypeptide
Activated
amino acid
Ribosomal
subunits
5
TRANSLATION
A succession of tRNAs
add their amino acids to
the polypeptide chain
Anticodon
as the mRNA is moved
through the ribosome
one codon at a time.
(When completed, the
polypeptide is released
from the ribosome.)
5
E
A
AAA
UG GU U U A U G
Codon
Ribosome
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings