Translation is the process by which the genetic information stored in DNA and passed to mRNA is ultimately expressed as proteins. It involves the use of ribosomes, mRNA, tRNAs, and amino acids. The genetic code consists of three nucleotide sequences called codons that correspond to specific amino acids. There are 61 codons that code for 20 amino acids, and 3 termination codons that signal the end of protein synthesis. Translation occurs through initiation, elongation, and termination stages on the ribosome, and involves codon-anticodon recognition between mRNA and tRNA to add amino acids in the correct sequence specified by the mRNA.

1. TRANSLATION

2. INTRODUCTION
The biosynthesis of a protein or a polypeptide in the
Living cell is referred to as translation.
The genetic information stored in DNA is passed on to
RNA (through transcription), and ultimately expressed in
the language of proteins
Erythrocytes (red blood cells) lack the machinery for
translation, and therefore cannot synthesize proteins.

3. Genetic code
The three nucleotide (triplet) base sequences in mRNA
that act as code words for amino acids in protein
constitute the genetic code or simply codons.
The codons are composed of the four nucleotide bases,
namely the purines-adenine (A) and guanine (C), and the
pyrimidines-cytosine (C) and uracil (U). These four bases
produce 64 different combinations (4*3) of three base
codons, The nucleotide sequence of the codon on mRNA
is written from the 5'-end to 3'end. Sixty one codons code
for the 20 amino acids found in protein.

4. The three codons UAA, UAG and UGA do not code for
amino acids. They act as stop signals in protein synthesis.
These three codons are collectively known as termination
codons or nonsense codons. The codons UAC, UAA and
UGA are often referred to, respectively, as amber, ochre
and opal codons.

5. Characteristics of genetic codes
 Universal :The same codons are used to code for the same
amino acids in all the living organisms
 Specificity: : A particular codon always codes for the same
amino acid, hence the genetic code is highly specific or
unambiguous e.g. UCC is the codon for tryptophan.
 Non-overlapping: i.e the adjacent codons do not overlap
 Degenerate :Most of the amino acids have more than one
codon. The codon is degenerate or redundant, since there are
61 codons available to code for only 20 amino acids. For
instance, glycine has four codons.

7. Protein synthesis
The protein synthesis which involves the Translation of nucleotide bases
sequence of mRNA into the language of amino acid sequence may be
divided into the following stages for the Convenience of understanding
l. Requirement of the components
II. Activation of amino acids
lll. Protein synthesis proper
lV. Chaperones and protein folding
V. Post-translation modifications

8. 1.Requirement of the components
 Amino acids
 Ribosomes: The functionally active ribosomes are the
centres or factories for protein synthesis.
 Messenger RNA (mRNA)
 Transfer RNAs (tRNAs)
 Energy sources : Both ATP and GTP are required for the
supply of energy in protein synthesis

9. The codon of the mRNA is recognized by the
Anticodon of tRNA . They pair with each
other in antiparallel direction ( 5' -> 3' of mRNA
with 3' -> 5' of tRNA). The usual conventional
Complementary base pairing( A = U, C = G)
occurs between the first two bases of codon and
the last two bases of anticodon. The third base
of the codon is rather lenient or flexible with
regard to the complementary consists of seven
nucleotides and it recognize the three letter
codon in mRNA.
CODON – ANTICODON RECOGNITION

10. 2. Activation of amino acids

11. 3.Protein synthesis proper
 The protein or polypeptide synthesis occurs on the ribosome. The mRNA is
read in the 5'->3' direction and the polypeptide synthesis proceeds from N-
terminal end to C-terminal end.
 Translation proper is divided into three stages i.e
 Initiation , elongation and termination
 1. initiation
 this is further divided into four steps
 Ribosomal dissociation.
 Formation of 43S preinitiation complex.
 Formation of 48S initiation complex.
 Formation of 80S initiation complex.

13. 2. Elongation
Ribosomes elongate the polypeptide chain by a sequential addition of amino
acids.The amino acid sequence is determined by the order of the codons in
the specific mRNA . Elongation , a cyclic process involving certain elongation
factors( Efs,) may be divided into three steps.
i. Binding of aminoacyl tRNA to A-site.
ii. Peptide bond formation.
iii. Translocation

15. 4.Chaperones and protein folding

16. 5.Post-translation modifications
 The proteins synthesized in translation are, as such, not functional. Many
changes take place in the polypeptides after the initiation of their synthesis
or, most frequently, after the protein synthesis is completed. These
modifications include protein folding , trimming by proteolytic
degradation, intein splicing and covalent changes which are collectively
known as post-translational modifications.

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