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?
Top Related