RESULTS

EXPERIMENT

CONCLUSION

Growth:Wild-typecells growing and dividing

No growth:Mutant cellscannot grow and divide

Minimal medium

Classes of Neurospora crassa

Wild type Class I mutants Class II mutants Class III mutants

Minimalmedium(MM)(control)

MM +ornithine

MM +citrulline

Co

nd

itio

n

MM +arginine(control)

Class I mutants(mutation in

gene A)Wild type

Class II mutants(mutation in

gene B)

Class III mutants(mutation in

gene C)

Gene A

Gene B

Gene C

Precursor Precursor Precursor PrecursorEnzyme A Enzyme AEnzyme A Enzyme A

Enzyme B

Ornithine Ornithine Ornithine OrnithineEnzyme B Enzyme B Enzyme B

Citrulline Citrulline Citrulline CitrullineEnzyme C Enzyme C Enzyme C Enzyme C

Arginine Arginine Arginine Arginine

1

TRANSCRIPTION

TRANSLATION

DNA

mRNARibosome

Polypeptide

(a) Bacterial cell

Nuclearenvelope

TRANSCRIPTION

RNA PROCESSINGPre-mRNA

DNA

mRNA

TRANSLATION Ribosome

Polypeptide

(b) Eukaryotic cell

2

DNAmolecule

Gene 1

Gene 2

Gene 3

DNAtemplatestrand

TRANSCRIPTION

TRANSLATION

mRNA

Protein

Codon

Amino acid

3

Second mRNA base

Fir

st

mR

NA

ba

se

(5 e

nd

of

co

do

n)

Th

ird

mR

NA

ba

se

(3 e

nd

of

co

do

n)

4

(a) Tobacco plant expressing a firefly gene

(b) Pig expressing a jellyfish gene

5

Promoter Transcription unit

Start pointDNA

RNA polymerase

5533

Initiation1

2

3

5533

UnwoundDNA

RNAtranscript

Template strandof DNA

Elongation

RewoundDNA

5

55

5

5

33

3

3

RNAtranscript

Termination

5533

35Completed RNA transcript

Newly madeRNA

Templatestrand of DNA

Direction oftranscription(“downstream”)

3 end

RNApolymerase

RNA nucleotides

Nontemplatestrand of DNA

Elongation

6

A eukaryotic promoterincludes a TATA box

3

1

2

3

Promoter

TATA box Start point

Template

TemplateDNA strand

535

Transcriptionfactors

Several transcription factors mustbind to the DNA before RNApolymerase II can do so.

5533

Additional transcription factors bind tothe DNA along with RNA polymerase II,forming the transcription initiation complex.

RNA polymerase IITranscription factors

55 53

3

RNA transcript

Transcription initiation complex

7

Protein-coding segment Polyadenylation signal3

3 UTR5 UTR

5

5 Cap Start codon Stop codon Poly-A tail

G P PP AAUAAA AAA AAA…

8

Pre-mRNA

mRNA

Codingsegment

Introns cut out andexons spliced together

5 Cap

Exon Intron5

1 30 31 104

Exon Intron

105

Exon

146

3Poly-A tail

Poly-A tail5 Cap

5 UTR 3 UTR1 146

9

RNA transcript (pre-mRNA)

Exon 1 Exon 2Intron

ProteinsnRNA

snRNPs

Otherproteins

5

5

Spliceosome

Spliceosomecomponents

Cut-outintronmRNA

Exon 1 Exon 25

10

GeneDNA

Exon 1 Exon 2 Exon 3Intron Intron

Transcription

RNA processing

Translation

Domain 2

Domain 3

Domain 1

Polypeptide

11

Polypeptide

Ribosome

Aminoacids

tRNA withamino acidattached

tRNA

Anticodon

TrpPhe Gly

Codons 35

mRNA

12

Amino acidattachment site

3

5

Hydrogenbonds

Anticodon

(a) Two-dimensional structure

Amino acidattachment site

5

3

Hydrogenbonds

3 5AnticodonAnticodon

(c) Symbol used in this book(b) Three-dimensional structure

13

Amino acid Aminoacyl-tRNAsynthetase (enzyme)

ATP

AdenosineP P P

AdenosineP

PP i

PPi

i

tRNA

tRNA

Aminoacyl-tRNAsynthetase

Computer model

AMPAdenosineP

Aminoacyl-tRNA(“charged tRNA”)

14

Growingpolypeptide Exit tunnel

Largesubunit

Smallsubunit

tRNAmolecules

E PA

mRNA5 3

(a) Computer model of functioning ribosome

P site (Peptidyl-tRNAbinding site)

E site(Exit site)

A site (Aminoacyl-tRNA binding site)

E P A Largesubunit

mRNAbinding site

Smallsubunit

(b) Schematic model showing binding sites

Amino end Growing polypeptide

Next amino acidto be added topolypeptide chain

mRNAtRNAE

3

5 Codons

(c) Schematic model with mRNA and tRNA

15

3355U

UA

ACGMet

GTP GDPInitiator

tRNA

mRNA

5 3Start codon

mRNA binding site

Smallribosomalsubunit

5

P site

Translation initiation complex

3

E A

Met

Largeribosomalsubunit

16

Amino endof polypeptide

mRNA

5

3E

Psite

Asite

GTP

GDP

E

P A

E

P A

GDPGTP

Ribosome ready fornext aminoacyl tRNA

E

P A

17

Releasefactor

3

5Stop codon(UAG, UAA, or UGA)

5

32

Freepolypeptide

2 GDP

GTP

5

3

18

Growingpolypeptides

Completedpolypeptide

Incomingribosomalsubunits

Start ofmRNA(5 end)

Polyribosome

End ofmRNA(3 end)

(a)

Ribosomes

mRNA

(b) 0.1 µm

19

Ribosome

mRNA

Signalpeptide

Signal-recognitionparticle (SRP)

CYTOSOL Translocationcomplex

SRPreceptorprotein

ER LUMEN

Signalpeptideremoved

ERmembrane

Protein

20

Wild-type hemoglobin DNA

mRNA

Mutant hemoglobin DNA

mRNA

33

3

3

3

3

55

5

55

5

C CT T TTG GA A AA

A A AGG U

Normal hemoglobin Sickle-cell hemoglobin

Glu Val

21

Wild-type

3DNA template strand

5

5

53

3

Stop

Carboxyl endAmino end

Protein

mRNA

33

3

55

5

A instead of G

U instead of C

Silent (no effect on amino acid sequence)

Stop

T instead of C

33

3

55

5

A instead of G

Stop

Missense

A instead of T

U instead of A

33

3

5

5

5

Stop

Nonsense No frameshift, but one amino acid missing (3 base-pair deletion)

Frameshift causing extensive missense (1 base-pair deletion)

Frameshift causing immediate nonsense (1 base-pair insertion)

5

5

533

3

Stop

missing

missing

3

3

3

5

55

missing

missing

Stop

5

5533

3

Extra U

Extra A

(a) Base-pair substitution (b) Base-pair insertion or deletion

22

RNA polymerase

DNA

Polyribosome

mRNA

0.25 µmDirection oftranscription

DNA

RNApolymerase

Polyribosome

Polypeptide(amino end)

Ribosome

mRNA (5 end)

23

TRANSCRIPTION

RNA PROCESSING

DNA

RNAtranscript

3

5RNApolymerase

Poly-A

Poly-A

RNA transcript(pre-mRNA)

Intron

Exon

NUCLEUS

Aminoacyl-tRNAsynthetase

AMINO ACID ACTIVATIONAminoacid

tRNACYTOPLASM

Poly-A

Growingpolypeptide

3

Activatedamino acid

mRNA

TRANSLATION

Cap

Ribosomalsubunits

Cap

5

E

P

A

AAnticodon

Ribosome

Codon

E

24