Chapter 17: From Gene to Protein

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Chapter 17:

From Gene to Protein


Figure 17.1 A ribosome, part of the protein synthesis machinery


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)


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


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

35


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


Figure 17.6 A tobacco plant expressing a firefly gene


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


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


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)

*

*

**

*

**

*

* **


Amino acidattachment site

Hydrogen bonds

AnticodonAnticodon

(b) Three-dimensional structure

A A G

53

3 5

Symbol used in this book

(c)


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


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


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


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

Chapter 17: From Gene to Protein

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