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Lecture 6
Translation
Dr. M. A. Fouad
Definition :
Translation is the transmission of genetic
information from mRNA to protein.
Translation occur in cytoplasm
gene
• Newly synthesized mRNA is
transported from the nucleus to the
cytoplasm.
• The translation process is performed
by ribosomes in the cytoplasm.
DNA
(nucleus)
Transcription
messenger
RNA
(cytoplasm)
Translation
Protein
Structure of Ribosomes
• Ribosomes are composed of
two subunits (small and
large).
• Ribosomes are made up of
four different types of
ribosomal RNA (rRNA)
molecules and many
proteins.
Polyribosomes
Multiple ribosomes
associated with the same
molecule of mRNA are
referred to as
polyribosomes or
polysomes.
Amino Acids and Peptide Bonds
• Amino acids are
the building
blocks of
proteins.
• There are 20
different amino
acids coded in
DNA
Amino Acids Peptide Bonds
• All amino-acids have
• an amino group (-NH2)
group on one end
• an acid group (-COOH) on
the other end.
Peptide Bonds
When polypeptides are synthesized,
the acid group of one amino acid is
linked to 
the amino group of the next amino
acid,
forming a peptide bond.
Peptide bond
Translation
In the ribosomes, the mRNA forms the template that determines the
sequence of amino acids in the generated polypeptide chain.
Translation Process
• The ribosome moves along the
mRNA (5`  3`).
• tRNA bring the correct amino
acids to the ribosome based on
codon-anticodon base pairing.
• Peptidyl Transferase links up
amino acids by the formation of
peptide bonds.
The Genetic Code
The genetic code
• DNA is composed of four different
nucleotides.
• There are Twenty different amino acids
found in proteins.
• The genetic code is a set of rules defining
how the code of DNA nucleotides is
translated into the code of amino acids.
• Genetic code is universal between all
organisms with few exceptions.
4
Obviously a single base cannot
specify one amino acid.
1
3
2
16
If two bases were to specify one
amino acid, possible combinations
would be:
42 = 4x4 = 16.
1
2
.
.
Genetic code is triplet code
• If three bases specified one amino acid 
the possible number of combinations of the
four base types would be:
43 = 4x4x4 = 64.

• This is more than enough to account for all
the 20 known amino acids.
The genetic code
• Genetic code is triplet code
(i.e. contain three nucleotides).
• A three nucleotides sequence
that codes for a particular amino
acid is called codon.
Codon / anticodon
• In mRNA as in DNA, genetic
information is encoded in the
sequence of nucleotides arranged
into codons.
• Each tRNA molecule for a
particular amino acid has a
specific trinucleotide sequence
called the anticodon, which is
complementary to the codon of
the mRNA.
Genetic code
• Each codon codes for a single
amino acid  the code is “nonoverlapping”.
• The same amino acid can be
coded for by more than one
codon  the genetic code is
“degenerate”.
Stop codon
• There are 3 stop codons, also called “nonsense”
codons.
• Proteins synthesis ends when ribosomes reach a stop
codon, which codes for no amino acid.
The Genetic Code II
• There are 64 (4X4X4) possible
triplet codes, but only 20
amino acids.
• 61 codons code for amino-acids
• 3 codons (UAA, UAG, UGA) act
as stop codons.
• More than one codons may
code for the same amino acid.
Quizzes
1. RNA required for the protein synthesis
A.
B.
C.
D.
rRNA
tRNA
mRNA
All of these
2. Anticodon is found on
A.
B.
C.
D.
rRNA
tRNA
mRNA
All of these
3. Codon is found on
A.
B.
C.
D.
rRNA
tRNA
mRNA
All of these
4. The translation Process is performed by:
A.
B.
C.
D.
Ribosomes
Mitochondria
Nucleus
Lysosomes
5. Codons consist of:
A.
B.
C.
D.
3
2
4
5
nucleotides
nucleotides
nucleotides
nucleotides
6. What term is used to describe the process by
which proteins are synthesized from mRNA
template?
A.
B.
C.
D.
reproduction
replication
translation
transcription
7. What amino acid sequence will
be generated based on the
following mRNA codon sequence?
5' AUG.UCU.UCG.UUA.UCC.UUG 3'
A. met-arg-glu-arg-glu-arg
B. met-glu-arg-arg-gln-leu
C. met-ser-leu-ser-leu-ser
D. met-ser-ser-leu-ser-leu
8. What amino acid sequence will
be generated based on the
following mRNA codon sequence?
5' AAA.UGA.UGU. 3'
A. Lys.
B. Ser-Tyr-Cys.
C. Lys-Tyr-Cys.
D. Ser-Lys-Gly.
9. Fill in the spaces:
1:
2:
3:
4:
5:
6:
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