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Chapter 17: From Gene to Protein. Figure 17.1 A ribosome, part of the protein synthesis machinery. EXPERIMENT. RESULTS. Class I Mutants. Class II Mutants. Class III Mutants. Wild type. Minimal medium (MM) (control). MM + Ornithine. MM + Citrulline. MM + Arginine (control).

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Chapter 17: From Gene to Protein

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Chapter 17 from gene to protein

Chapter 17:

From Gene to Protein


Figure 17 1 a ribosome part of the protein synthesis machinery

Figure 17.1 A ribosome, part of the protein synthesis machinery


Figure 17 2 do individual genes specify different enzymes in arginine biosynthesis

EXPERIMENT

RESULTS

Class I

Mutants

Class II

Mutants

Class III

Mutants

Wild type

Minimal

medium

(MM)

(control)

MM +

Ornithine

MM +

Citrulline

MM +

Arginine

(control)

Figure 17.2 Do individual genes specify different enzymes in arginine biosynthesis?

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.

The wild-type strain required only the minimal medium for growth. The three classes of mutants had different growth requirements


Chapter 17 from gene to protein

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

Mutants

(mutation

in gene A)

Class II

Mutants

(mutation

in gene B)

Class III

Mutants

(mutation

in gene C)

Wild type

Precursor

Precursor

Precursor

Precursor

Enzyme

A

Gene A

A

A

A

Ornithine

Ornithine

Ornithine

Ornithine

Enzyme

B

Gene B

B

B

B

Citrulline

Citrulline

Citrulline

Citrulline

Enzyme

C

Gene C

C

C

C

Arginine

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

TRANSCRIPTION

G

U

G

G

U

G

C

A

U

U

U

C

5

3

mRNA

Codon

TRANSLATION

Gly

Phe

Protein

Ser

Trp

Amino acid

Figure 17.4 The triplet code


Figure 17 5 the dictionary of the genetic code

Second mRNA base

U

C

A

G

U

UAU

UUU

UCU

UGU

Tyr

Cys

Phe

UAC

UUC

UCC

UGC

C

U

Ser

UUA

UCA

UAA

Stop

Stop

UGA

A

Leu

UAG

UUG

UCG

Stop

UGG

Trp

G

CUU

CCU

U

CAU

CGU

His

CUC

CCC

CAC

CGC

C

C

Arg

Pro

Leu

CUA

CCA

CAA

CGA

A

Gln

CUG

CCG

CAG

CGG

G

Third mRNA base (3 end)

First mRNA base (5 end)

U

AUU

ACU

AAU

AGU

Asn

Ser

C

lle

AUC

ACC

AAC

AGC

A

Thr

A

AUA

ACA

AAA

AGA

Lys

Arg

Met or

start

G

AUG

ACG

AAG

AGG

U

GUU

GCU

GAU

GGU

Asp

C

GUC

GCC

GAC

GGC

G

Val

Ala

Gly

GUA

GCA

GAA

GGA

A

GUG

GCG

GAG

GGG

G

Figure 17.5 The dictionary of the genetic code

Glu


Figure 17 6 a tobacco plant expressing a firefly gene

Figure 17.6 A tobacco plant expressing a firefly gene


Chapter 17 from gene to protein

Non-template

strand of DNA

Elongation

RNA nucleotides

RNA

polymerase

T

A

C

C

A

T

A

T

3

C

U

3 end

T

G

A

U

G

G

A

G

E

A

C

C

C

A

5

A

A

T

A

G

G

T

T

Direction of transcription

(“downstream)

5

Template

strand of DNA

Newly made

RNA


Figure 17 13 translation the basic concept

DNA

TRANSCRIPTION

mRNA

Ribosome

TRANSLATION

Polypeptide

Amino

acids

Polypeptide

tRNA with

amino acid

attached

Ribosome

Trp

Phe

Gly

tRNA

C

C

C

G

G

Anticodon

A

A

A

A

G

G

G

U

G

U

U

U

C

5

Codons

3

mRNA

Figure 17.13 Translation: the basic concept


Figure 17 14 the structure of transfer rna trna

3

A

Amino acid

attachment site

C

C

5

A

C

G

C

G

C

G

U

G

U

A

A

U

U

A

U

C

G

*

G

U

A

C

A

C

A

*

A

U

C

C

*

G

*

U

G

U

G

G

*

G

A

C

C

G

*

C

*

A

G

U

G

*

*

G

A

G

C

Hydrogen

bonds

G

C

U

A

G

*

A

*

A

C

*

U

A

G

A

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.)

Figure 17.14 The structure of transfer RNA (tRNA)


Chapter 17 from gene to protein

Amino acid

attachment site

5

3

Hydrogen

bonds

A

A

G

3

5

Anticodon

Anticodon

(c)

(b) Three-dimensional structure

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)

Active site binds the

amino acid and ATP.

1

4

3

2

P

P

P

Adenosine

ATP

ATP loses two P groups

and joins amino acid as AMP.

Adenosine

P

Pyrophosphate

P

Pi

Pi

Pi

Phosphates

tRNA

Appropriate

tRNA covalently

Bonds to amino

Acid, displacing

AMP.

Adenosine

P

AMP

Activated amino acid

is released by the enzyme.

Aminoacyl tRNA

(an “activated

amino acid”)

Figure 17.15 An aminoacyl-tRNA synthetase joins a specific amino acid to a tRNA


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

3

mRNA

(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.

Figure 17.16 The anatomy of a functioning ribosome


Figure 17 23 the molecular basis of sickle cell disease a point mutation

Wild-type hemoglobin DNA

Mutant hemoglobin DNA

In the DNA, the

mutant template

strand has an A where

the wild-type template

has a T.

3

5

3

5

T

T

C

A

T

C

mRNA

mRNA

The mutant mRNA has

a U instead of an A in

one codon.

G

A

A

U

A

G

5

3

5

3

Normal hemoglobin

Sickle-cell hemoglobin

The mutant (sickle-cell)

hemoglobin has a valine

(Val) instead of a glutamic

acid (Glu).

Val

Glu

Figure 17.23 The molecular basis of sickle-cell disease: a point mutation


Figure 17 26 a summary of transcription and translation in a eukaryotic cell

DNA

TRANSCRIPTION

RNA is transcribed

from a DNA template.

5

2

1

3

4

3

Poly-A

RNA

transcript

RNA

polymerase

5

Exon

RNA PROCESSING

In eukaryotes, the

RNA transcript (pre-

mRNA) is spliced and

modified to produce

mRNA, which moves

from the nucleus to the

cytoplasm.

RNA transcript

(pre-mRNA)

Intron

Aminoacyl-tRNA

synthetase

Cap

NUCLEUS

Amino

acid

FORMATION OF

INITIATION COMPLEX

AMINO ACID ACTIVATION

tRNA

CYTOPLASM

After leaving the

nucleus, mRNA attaches

to the ribosome.

Each amino acid

attaches to its proper tRNA

with the help of a specific

enzyme and ATP.

Growing

polypeptide

mRNA

Activated

amino acid

Poly-A

Poly-A

Ribosomal

subunits

Cap

5

TRANSLATION

C

A succession of tRNAs

add their amino acids to

the polypeptide chain

as the mRNA is moved

through the ribosome

one codon at a time.

(When completed, the

polypeptide is released

from the ribosome.)

C

A

U

A

E

A

C

Anticodon

A

A

A

U

G

G

U

G

U

U

U

A

Codon

Ribosome

Figure 17.26 A summary of transcription and translation in a eukaryotic cell


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