Chapter 14
Genetic 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.
Figure 14.5
DNA
template
strand
3
5
A
C
C
A
A
A
C C
G
A
G
T
T
G
G
T
T
T
G G
C
T
C
A
5
3
TRANSCRIPTION
U G
mRNA
G
U U
U G
G
C
U
C
3
5
Codon
TRANSLATION
Protein
A
Trp
Amino acid
Phe
Gly
Ser
Second mRNA base
A
C
U
UUU
U
UUC
First mRNA base (5 end of codon)
UUA
C
Leu
UAU
UCC
UCA
UAC
Ser
UGU
Tyr
UGC
U
Cys
C
UAA Stop UGA Stop A
UCG
UAG Stop UGG Trp
G
CUU
CCU
CAU
U
CUC
Leu
CCC
CCA
Pro
CAC
CAA
CUG
CCG
CAG
AUU
ACU
AAU
AUC IIe
ACC
AUA
ACA
AUG
Met or
start
GUU
G
UCU
UUG
CUA
A
Phe
G
GUC
GUA
GUG
AAC
Thr
ACG
AAG
GCU
GAU
GCC
Val
AAA
GCA
GCG
GAC
Ala
GAA
GAG
CGU
His
Gln
CGC
CGA
C
Arg
CGG
AGU
Asn
Lys
Asp
Glu
AGC
AGA
A
G
Ser
Arg
U
C
A
AGG
G
GGU
U
C
GGC
GGA
GGG
Gly
A
G
Third mRNA base (3 end of codon)
Figure 14.6
The genetic code is nearly universal!
Three stages
of
Transcription
Transcription unit
Promoter
5
3
1 Initiation
3
5
Start point
RNA polymerase
5
3
3
5
Template strand of DNA
RNA
transcript
Unwound
DNA
2 Elongation
Rewound
DNA
5
3
3
5
3
5
RNA
transcript
3 Termination
Direction of
transcription
(“downstream”)
5
3
3
5
5
Completed RNA transcript
3
Figure 14.9
Promoter
Nontemplate strand
DNA
5
3
3
5
TAT A A A A
ATAT T T T
TATA box
Start point
Transcription
factors
3
5
Transcription
factors
RNA polymerase II
5
3
promoter
Template
strand
5
3
5
3
2 Several transcription
factors bind to DNA.
3 Transcription
3
5
RNA transcript
Transcription initiation complex
1 A eukaryotic
initiation
complex forms.
Figure 14.10
Nontemplate
strand of DNA
RNA nucleotides
RNA
polymerase
A T
3
C
C A A
5
3 end
A U C
C A
5
T A G G T T
5
Direction of transcription
3
Template
strand of DNA
Newly made
RNA
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.
5
Polyadenylation
signal
3
AAUAAA
Figure 14.UN03
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
mRNA
TRANSLATION
Ribosome
Polypeptide
RNA Processing in Eukaryotes
A modified guanine
nucleotide added to
the 5 end
5
G
P
P
Protein-coding segment
P
5 Cap 5 UTR
50–250 adenine
nucleotides added
to the 3 end
Polyadenylation
signal
3
AAUAAA
Start
codon
Stop
codon
3 UTR
AAA …AAA
Poly-A tail
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
Figure 14.12
Exons
Pre-mRNA
Intron
Intron
Poly-A tail
5 Cap
Introns cut out and
exons spliced together
mRNA
5 Cap
5 UTR
Poly-A tail
Coding
segment
3 UTR
AAUAAA
Figure 14.13
Small RNAs
Spliceosome
5
Pre-mRNA
Exon 2
Exon 1
Intron
Spliceosome
components
mRNA
5
Exon 1
Exon 2
Cut-out
intron
Concept check
• How can human cells make 75,000-100,000
different proteins, given that there are about
20,000 human genes?