Chapter 17: From Gene to Protein
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Transcript of Chapter 17: From Gene to Protein
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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 geneone 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
Class IMutants
Class IIMutants
Class IIIMutantsWild type
Minimal medium(MM)(control)
MM +Ornithine
MM +Citrulline
MM +Arginine(control)
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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 IMutants(mutationin gene A)
Class IIMutants(mutationin gene B)
Class IIIMutants(mutationin gene C)Wild type
Gene A
Gene B
Gene C
Precursor Precursor Precursor Precursor
Ornithine Ornithine Ornithine Ornithine
Citrulline Citrulline Citrulline Citrulline
Arginine Arginine Arginine Arginine
EnzymeA
EnzymeB
EnzymeC
A A A
B B B
C C C
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Figure 17.4 The triplet code
DNAmolecule
Gene 1
Gene 2
Gene 3
DNA strand(template)
TRANSCRIPTION
mRNA
Protein
TRANSLATION
Amino acid
A C C A A A C C G A G T
U G G U U U G G C U C A
Trp Phe Gly Ser
Codon
3 5
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Figure 17.5 The dictionary of the genetic codeSecond mRNA base
U C A G
U
C
A
G
UUU
UUCUUA
UUG
CUU
CUC
CUA
CUG
AUU
AUC
AUA
AUG
GUU
GUC
GUA
GUG
Met orstart
Phe
Leu
Leu
lle
Val
UCU
UCCUCA
UCG
CCU
CCC
CCA
CCG
ACU
ACC
ACA
ACG
GCU
GCC
GCA
GCG
Ser
Pro
Thr
Ala
UAU
UAC
UGU
UGCTyr Cys
CAU
CAC
CAA
CAG
CGU
CGC
CGA
CGG
AAU
AAC
AAA
AAG
AGU
AGCAGA
AGG
GAU
GAC
GAA
GAG
GGU
GGC
GGA
GGG
UGG
UAA
UAG Stop
Stop UGA Stop
Trp
His
Gln
Asn
Lys
Asp
Arg
Ser
Arg
Gly
U
CA
G
UCAG
UCAG
UCAG
Firs
t mR
NA
bas
e (5
end
)
Third
mR
NA
bas
e (3
end
)
Glu
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Figure 17.6 A tobacco plant expressing a firefly gene
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Elongation
RNApolymerase
Non-templatestrand of DNA
RNA nucleotides
3 end
C A E G C A A
U
T A G G T TA
AC
G
U
AT
CA
T C C A A TT
GG
3
5
5
Newly madeRNA
Direction of transcription(“downstream) Template
strand of DNA
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Figure 17.13 Translation: the basic concept
TRANSCRIPTION
TRANSLATION
DNA
mRNARibosome
Polypeptide
Polypeptide
Aminoacids
tRNA withamino acidattachedRibosome
tRNA
Anticodon
mRNA
Trp
Phe Gly
A G C
A A A
CC
G
U G G U U U G G C
Codons5 3
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Figure 17.14 The structure of transfer RNA (tRNA)3ACCACGCUUAA
GACACCU
GC
*G U G U
CUGAG
GU
A
AA G
UC
AGACC
C G A GA G G
G
GACUCAU
UUAGGCG5
Amino acidattachment site
Hydrogenbonds
AnticodonTwo-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.)
(a)
*
*
**
*
**
*
* **
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Amino acidattachment site
Hydrogen bonds
AnticodonAnticodon
(b) Three-dimensional structure
A A G
53
3 5
Symbol used in this book
(c)
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Figure 17.15 An aminoacyl-tRNA synthetase joins a specific amino acid to a tRNA
Amino acid
ATP
Adenosine
Pyrophosphate
Adenosine
Adenosine
Phosphates
tRNA
P P P
P
P Pi
Pi
Pi
P
AMP
Aminoacyl tRNA(an “activatedamino acid”)
AppropriatetRNA covalentlyBonds to aminoAcid, displacing
AMP.
Active site binds theamino acid and ATP. 1
3
Activated amino acidis released by the enzyme.
4
Aminoacyl-tRNAsynthetase (enzyme)
ATP loses two P groupsand joins amino acid as AMP.2
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Figure 17.16 The anatomy of a functioning ribosome
TRANSCRIPTION
TRANSLATION
DNA
mRNARibosome
PolypeptideExit tunnel
Growingpolypeptide
tRNAmolecules
EP A
Largesubunit
Smallsubunit
mRNA
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.
(a)
53
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Figure 17.23 The molecular basis of sickle-cell disease: a point mutation
In the DNA, themutant templatestrand has an A where the wild-type template has a T.
The mutant mRNA has a U instead of an A in one codon.
The mutant (sickle-cell) hemoglobin has a valine (Val) instead of a glutamic acid (Glu).
Mutant hemoglobin DNAWild-type hemoglobin DNA
mRNA mRNA
Normal hemoglobin Sickle-cell hemoglobin
Glu Val
C T T C A T
G A A G U A
3 5 3 5
5 35 3
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Figure 17.26 A summary of transcription and translation in a eukaryotic cell
TRANSCRIPTION RNA is transcribedfrom a DNA template.
DNA
RNApolymerase
RNAtranscript
RNA PROCESSING In eukaryotes, theRNA transcript (pre-mRNA) is spliced andmodified to producemRNA, which movesfrom the nucleus to thecytoplasm.
Exon
Poly-A
RNA transcript(pre-mRNA)
Intron
NUCLEUSCap
FORMATION OFINITIATION COMPLEX
After leaving thenucleus, mRNA attachesto the ribosome.
CYTOPLASM
mRNA
Poly-A
Growingpolypeptide
Ribosomalsubunits
Cap
Aminoacyl-tRNAsynthetase
AminoacidtRNA
AMINO ACID ACTIVATION
Each amino acidattaches to its proper tRNAwith the help of a specificenzyme and ATP.
Activatedamino acid
TRANSLATION A succession of tRNAsadd their amino acids tothe polypeptide chainas the mRNA is movedthrough the ribosomeone codon at a time.(When completed, thepolypeptide is releasedfrom the ribosome.)
Anticodon
A CC
A A AUG GUU UA U G
UACE A
Ribosome
1
Poly-A
5
5
3
Codon
2
3 4
5