Chapter 14Genetic Code and Transcription

You Must Know

• The differences between replication (from chapter 13), transcription and translation and the role of DNA and RNA in each process. (There will be more on this later.)

• How eukaryotic cells modify RNA after transcription.

Protein Trp

TRANSLATION

Amino acid

Phe Gly Ser

Figure 14.5

DNAtemplatestrand

3

5 3

5CA C A AA C C AG G T

GT G T TT G G TC C A

35mRNA

TRANSCRIPTION

GU G U UU G G UC C A35

CodonCodon

Figure 14.6

UUU

Second mRNA base

UUC

UUA

UUG

UCU

UCC

UCA

UCG

UAU

UAC

UAA

UAG

UGU

UGC

UGA

UGG

CUU

CUC

CUA

CUG

CCU

CCC

CCA

CCG

CAU

CAC

CAA

CAG

CGU

CGC

CGA

CGG

AUU

AUC

AUA

AUG

ACU

ACC

ACA

ACG

AAU

AAC

AAA

AAG

AGU

AGC

AGA

AGG

GUU

GUC

GUA

GUG

GCU

GCC

GCA

GCG

GAU

GAC

GAA

GAG

GGU

GGC

GGA

GGG

Fir

st m

RN

A b

ase

(5

en

d o

f co

do

n)

U

C

A

G

U

C

A

G

U

C

A

G

U

C

A

G

U

C

A

G

U C A G

Phe

Leu

Ser

Tyr Cys

Trp

Met orstart

Stop

Stop Stop

Arg

Gln

His

ProLeu

Val Ala

Asp

Glu

Gly

IIeThr

Lys

Asn

Arg

Ser

Th

ird

mR

NA

bas

e (

3 e

nd

of

cod

on

)

The genetic code is nearly universal!

Completed RNA transcript

Direction oftranscription(“downstream”)

35

35

35

35

5

Transcription unit

RNA polymerase

Promoter

Start point

3

Three stages of

Transcription

35 3

5

RewoundDNA

RNA transcript

35 3

5

35

Termination

Initiation

Elongation

3

2

1

UnwoundDNA

RNA transcript

Template strand of DNA

Figure 14.9

TATA box

Promoter Nontemplate strand

Start point

DNA

A eukaryoticpromoter

35 1

Templatestrand

35 TA T A A A A

A T A T T T T

Transcription factors

Several transcriptionfactors bind to DNA.

35

235

Transcriptioninitiationcomplex forms.

Transcription initiation complex

RNA transcript

35

5 3 35

3

Transcription factors

RNA polymerase II

Figure 14.10

Nontemplate strand of DNA

Direction of transcription

RNA polymerase

3

53

5

RNA nucleotides

Template strand of DNA

Newly made RNA

3 end

5

UC

U

G

A

A

A

A

AA

A

AA

A

T T T

TT

T

T

CC

C

CCC C

G

GG

U

Termination of Transcription

• The mechanisms of termination are different in bacteria and eukaryotes.

• In bacteria, the polymerase stops transcription at the end of the terminator and the mRNA can be translated without further modification.

Termination of Transcription

• In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequence; the RNA transcript is released 10–35 nucleotides past this polyadenylation sequence.

Polyadenylationsignal

35AAUAAA

Figure 14.UN03

DNA

Pre-mRNA

mRNA

Ribosome

Polypeptide

TRANSLATION

TRANSCRIPTION

RNA PROCESSING

RNA Processing in Eukaryotes

These modifications share several functions• Facilitating the export of mRNA to the cytoplasm• Protecting mRNA from hydrolytic enzymes• Helping ribosomes attach to the 5 end

Protein-coding segmentPolyadenylation

signal

G P

A modified guaninenucleotide added tothe 5 end

50–250 adeninenucleotides added to the 3 end

35

5 Cap 5 UTR 3 UTR Poly-A tailStartcodon

Stopcodon

P P AAUAAA …AAA AAA

Figure 14.12

AAUAAA

5 Cap Poly-A tail

Pre-mRNA

Intron Intron

Exons

Introns cut out andexons spliced together

5 Cap

5 UTR 3 UTR

Poly-A tail

Codingsegment

mRNA

Figure 14.13

Spliceosomecomponents

5

Pre-mRNA

5

mRNA

Intron

Spliceosome

Exon 1

Small RNAs

Exon 2

Exon 2Exon 1

Cut-outintron

Concept check

• How can human cells make 75,000-100,000 different proteins, given that there are about 20,000 human genes?