Protein Synthesis
Revision
The Genetic Code
A gene is a sequence of bases in DNA that
codes for the sequence of amino acids in a
polypeptide (protein)
The ‘language’ of a gene has only 4 letters
- these are?
A T C and G
The Genetic Code
The ‘language’ of a protein has 20 letters
- these are?
The 20 different amino acids that make up
proteins
The Genetic Code



If 1 base coded for one amino
acid in a protein then, only 4
amino acids could be coded for
If 2 bases coded for one amino
acid in a protein then, only 16
amino acids could be coded for
If 3 bases coded for one amino
acid in a protein then, 64 amino
acids could be coded for – more
than enough
41 = 4
42 = 16
43 = 64
The genetic code is a triplet code
The Genetic Code


There are 20 amino acids to be coded
for and 64 base triplets to use to code
them
Each amino acid has more than one
code word – that is the genetic code is
degenerate.
The Genetic Code
The genetic code is non-overlapping
ATTCGAGGCGGT is ‘read’ as
ATT CGA GGC GGT
Each base is a part of only one triplet.
The Genetic Code is:
A
triplet code
 Degenerate
 Non-overlapping
 Universal
Protein synthesis
2
major processes involved
 Transcription
 Translation
Transcription
 The
relevant gene in the DNA in the
nucleus is ‘copied’ into a molecule
of RNA called mRNA or messenger
RNA
Transcription



A
T
G
C
C
G
T
A
DNA double helix unzips as
hydrogen bonds between complementary bases break
and the two polynucleotide strands separate
Transcription


A
A
T
G
G
C
C
C
G
T
U
A
One strand called the sense strand acts as a template, free
RNA nucleotides complementary base pair to the exposed
bases on this strand by forming hydrogen bonds
RNA polymerase forms sugar-phosphate bonds between
nucleotides
Transcription



A
A
T
G
G
C
C
C
G
T
U
A
The mRNA detaches from the sense strand
The two DNA strands join together by complementary base
pairing
The DNA molecules winds back up into a helix
Transcription


The sequences of 3 bases on the mRNA coding
for amino acids are called CODONS.
Not all the bases in the DNA code for amino
acids so the mRNA just transcribed contains
non-coding regions known as INTRONS
Transcription
exon
intron
exon
intron
exon
enzymes
These introns are removed by enzymes before the mRNA
leaves the nucleus
This leaves just EXONS or coding regions of mRNA
Transcription
intron
exon
intron
exon
exon
enzymes
These introns are removed by enzymes before the mRNA
leaves the nucleus
This leaves just EXONS or coding regions of mRNA
Transcription to translation
mRNA
Following the removal of
introns the mRNA moves out
through a nuclear pore and
attaches to a ribosome
nucleus
ribosome
aa1
Translation
aa2
tRNA
tRNA
UAC
GGG
anticodon
AUG CCC GGG CGC ACA CGU UUC UGA
start codon
stop codon
peptide bond formed
aa1
tRNA
aa2
tRNA
UAC GGG
AUG CCC GGG CGC ACA CGU UUC UGA
aa1
tRNA
aa2
tRNA
UAC GGG
AUG CCC GGG CGC ACA CGU UUC UGA
‘empty’ tRNA leaves
to pick up another
specific amino acid
aa3
aa1
tRNA
aa2
tRNA
CCC
GGG
AUG CCC GGG CGC ACA CGU UUC UGA
Ribosome moves along mRNA by one codon
peptide bond formed
aa1
tRNA
aa2
aa3
tRNA
GGG CCC
AUG CCC GGG CGC ACA CGU UUC UGA
‘empty’ tRNA leaves
to pick up another
specific amino acid
aa1
aa2
aa3
aa4
aa5
aa6
aa7
aa8
tRNA
ACU
AUG CCC GGG CGC ACA CGU UUC UGA
This process is repeated until the ribosome
reads a stop codon