Protein Synthesis: The Central
Dogma
Protein Synthesis
• The information content of DNA is in the form
of specific sequences of nucleotides along
the DNA strands
• The DNA inherited by an organism leads to
specific traits by directing the synthesis of
proteins (chains of amino acids)
• The process by which DNA directs protein
synthesis (gene expression) includes two
stages called transcription and translation
Transcription and Translation
• Cells are governed by a cellular “chain of
command”
– DNA RNA protein
• Transcription
– Is the synthesis of RNA under the direction of DNA
– Produces messenger RNA (mRNA)
• Translation
– Is the actual synthesis of a polypeptide (amino
acids), which occurs under the direction of mRNA
– Occurs on ribosomes
Transcription and Translation
• In prokaryotes transcription and
translation occur together
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
(a) Prokaryotic cell. In a cell lacking a nucleus, mRNA
produced by transcription is immediately translated
without additional processing.
Transcription and Translation
• In a eukaryotic cell the nuclear envelope separates
transcription from translation
• Extensive RNA processing occurs in the nucleus
Nuclear
envelope
DNA
TRANSCRIPTION
Pre-mRNA
RNA PROCESSING
mRNA
Ribosome
TRANSLATION
Polypeptide
Eukaryotic cell. The nucleus
provides a separate compartment
for transcription. The original
RNA transcript, called premRNA, is processed in various
ways before leaving the nucleus
as mRNA.
Transcription
• Transcription is the DNAdirected synthesis of RNA
• RNA synthesis
– Is catalyzed by RNA
polymerase, which pries the
DNA strands apart and
hooks together the RNA
nucleotides
– Follows the same basepairing rules as DNA,
except that in RNA, uracil
substitutes for thymine
RNA
• RNA is single stranded, not double stranded like DNA
• RNA is short, only 1 gene long, where DNA is very long and
contains many genes
• RNA uses the sugar ribose instead of deoxyribose in DNA
• RNA uses the base uracil (U) instead of thymine (T) in DNA.
Synthesis of an RNA Transcript
• The stages of
transcription
are:
– Initiation
– Elongation
– Termination
Promoter
Transcription unit
5
3
3
5
Start point
RNA polymerase
DNA
Initiation. After RNA polymerase binds to
the promoter, the DNA strands unwind, and
the polymerase initiates RNA synthesis at the
start point on the template strand.
1
5
3
Unwound
DNA
3
5
Template strand of
DNA
transcript
2 Elongation. The polymerase moves downstream, unwinding the
DNA and elongating the RNA transcript 5  3 . In the wake of
transcription, the DNA strands re-form a double helix.
Rewound
RNA
RNA
5
3
3
5
3
5
RNA
transcript
3 Termination. Eventually, the RNA
transcript is released, and the
polymerase detaches from the DNA.
5
3
3
5
5
Completed RNA
transcript
3
Transcription Overview
Alteration of mRNA Ends
• Each end of a pre-mRNA molecule is modified
in a particular way
– The 5 end receives a modified nucleotide cap
– The 3 end gets a poly-A tail
A modified guanine nucleotide
added to the 5 end
TRANSCRIPTION
RNA PROCESSING
50 to 250 adenine nucleotides
added to the 3 end
DNA
Pre-mRNA
5
mRNA
Protein-coding segment
Polyadenylation signal
3
G P P P
AAUAAA
AAA…AAA
Ribosome
TRANSLATION
5 Cap
Polypeptide
5 UTR
Start codon Stop codon
3 UTR
Poly-A tail
Translation
• Translation is the RNAdirected synthesis of a
polypeptide
• Translation involves
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
Amino
acids
Polypeptide
– mRNA
– Ribosomes - Ribosomal RNA
(rRNA)
– Transfer RNA (tRNA)
– Genetic coding - codons
tRNA with
amino acid
Ribosome attached
Gly
tRNA
Anticodon
A A A
U G G U U U G G C
Codons
5
mRNA
3
The Genetic Code
• Genetic information is encoded as a sequence of nonoverlapping
base triplets, or codons
• The gene determines the sequence of bases along the length of an
mRNA molecule
Gene 2
DNA
molecule
Gene 1
Gene 3
DNA strand 3
A C C A A A C C G A G T
(template)
5
TRANSCRIPTION
mRNA
5
U G G U U U G G C U C A
Codon
TRANSLATION
Protein
Trp
Amino acid
Phe
Gly
Ser
3
The Genetic Code
• Codons: 3 base code for the production of a specific amino acid,
sequence of three of the four different nucleotides
• Since there are 4 bases and 3 positions in each codon, there
are 4 x 4 x 4 = 64 possible codons
• 64 codons but only 20 amino acids, therefore most have more
than 1 codon
• 3 of the 64 codons are used as STOP signals; they are found at
the end of every gene and mark the end of the protein
• One codon is used as a START signal: it is at the start of every
protein
• Universal: in all living organisms
The Genetic Code
Second mRNA base
U
C
A
UAU
UUU
UCU
Tyr
Phe
UAC
UUC
UCC
U
UUA
UCA Ser UAA Stop
UAG Stop
UUG Leu UCG
CUU
CUC
C
CUA
CUG
CCU
CCC
Leu CCA
CCG
Pro
AUU
AUC
A
AUA
AUG
ACU
ACC
ACA
ACG
Thr
GUU
G GUC
GUA
GUG
lle
Met or
start
GCU
GCC
Val
GCA
GCG
Ala
G
U
UGU
Cys
UGC
C
UGA Stop A
UGG Trp G
U
CAU
CGU
His
CAC
CGC
C
Arg
CAA
CGA
A
Gln
CAG
CGG
G
U
AAU
AGU
Asn
AAC
AGC Ser C
A
AAA
AGA
Lys
Arg
G
AAG
AGG
U
GAU
GGU
C
GAC Asp GGC
Gly
GAA
GGA
A
Glu
GAG
GGG
G
Third mRNA base (3 end)
First mRNA base (5 end)
• A codon in messenger RNA is either translated into an
amino acid or serves as a translational start/stop signal