translation

1. TRANSLATION IN PROKARYOTES AND
EUKARYOTES
PRESENTED BY:NITIN SHARMA

2. CONTENTS
Introduction
Translation in Prokaryotes
Translation in Eukaryotes

3. INTRODUCTION TO TRANSLATION
Translation of mRNA is the biological polymerization of amino acids into
polypeptide chains.
Triplet codons of mRNA direct specific amino acids into their correct position in
the polypeptide.
Transfer RNA (tRNA) class of molecules adapts genetic information present as
specific triplet codons in mRNA to their corresponding amino acids.
A specific tRNA molecule contains within its nucleotide sequence three
consecutive ribonucleotides complementary to the codon called the anticodon

5. LARGER SUBUNIT SMALLER SUBUNIT LARGER SUBUNIT SMALLER SUBUNIT
23S rRNA
(2904 nucleotides)
+
5S rRNA
(120 nucleotides)
+
31 proteins
16S rRNA
(1541 nucleotides)
+
21 proteins
28S rRNA
(4718 nucleotides)
+
5S rRNA
(120
nucleotides)
+
5.8S rRNA
(160
nucleotides)
+
46 protein
18S rRNA
(1874 nucleotides)
+
33 proteins
Prokaryotes
Monosome 70s
Eukaryotes
Monosome 80s

6. STEPS OF TRANSLATION
Translation proceeds in four phases
Activation
Initiation
Elongation
Termination

7. ACTIVATION
In activation the correct amino acid is covalently bonded to the correct
tRNA also called charging of tRNA
Occurs under the direction of enzymes called aminoacyl tRNA synthetases.
The amino acid joined by its carboxyl group to the 3 OH of tRNA by the help
of ATP
When tRNA is attached to its respective amino acid it is said to be “charged”

8. INITIATION
Prokaryotic initiation requires the small ribosomal subunit, an mRNA molecule, a
specific charged initiator tRNA, GTP, Mg2+, and three proteinaceous initiation factors
(IFs) that enhance the binding affinity of the various translational components
The small ribosomal subunit binds to several initiation factors and this complex in
turn binds to mRNA
In bacteria, this binding involves a sequence of upto to six ribonucleotides (AGGAGG
that precedes the initial AUG start codon of mRNA.This sequence —containing only
purines and called the Shine–Dalgarno sequence
Another initiation protein then enhances the binding of charged formyl methionyl
tRNA to the small subunit in response to the AUG triplet
In whole step a molecule of GTP is released providing energy and initiation factors
are released

10. ELONGATION
With the formation of 70s initiation complex the elongation cycle begins
It requires elongation factors EF-Tu,EF-Ts and EF-G and GTP
Once both subunits of the ribosome are assembled with the mRNA, binding sites for two charged tRNA
molecules are formed.
These are the P (peptidyl) site and the A (aminoacyl) site. The charged initiator tRNA binds to the P site,
provided that the AUG codon of mRNA is in the corresponding position of the small subunit.
The sequence of the second triplet in mRNA dictates which charged tRNA molecule will become positioned at
the A site
The covalent bond between the tRNA occupying the P site and its cognate amino acid is hydrolyzed
(broken).The newly formed dipeptide remains attached to the end of the tRNA still residing in the A site
The tRNA attached to the P site, which is now uncharged, must be released from the large subunit. The
uncharged tRNA moves briefly into a third site on the ribosome called the E (exit) site
The entire mRNA–tRNA–aa2–aa1 complex then shifts in the direction of the P site by a distance of three
nucleotides. This event, called translocation, requires several protein elongation factors (EFs).

12. TERMINATION
The process is signaled by the presence of anyone of the three possible triplet
codons appearing in the A site: UAG, UAA, or UGA. These codons do not specify
amino acid, nor do they call for a tRNA in the A site
They are called stop codons, termination codons, or nonsense codons.
The termination codon signals the action of a GTP dependent release factor, which
stimulates steps leading to the release of the polypeptide chain from the terminal
tRNA and subsequently from the translation complex
The tRNA is released from the ribosome, which then dissociates into its subunits

13. TRANSLATION IN EUKRYOTES
Translation in eukaryotes is much more complex as compare to prokaryotes
In eukaryotic cells transcription occurs in the nucleus and translation in the
cytoplasm.
Three differences center on the mRNA that is being translated. First, the 5 end of
mRNA is capped with a 7-methylguanosine (7-mG) residue at maturation
Many mRNAs contain a purine (A or G) three bases upstream from the AUG
initiator codon, which is followed by a G (A/GNNAUGG) called kozak sequence
Eukaryotic mRNAs require the posttranscriptional addition of a poly-A tail on their
3 end; that is, they are polyadenylated.

15. INITIATION
Initiation of translation in eukaryotes requires 10 initiation factors
A ternary complex containing met Trna and eIF-2 bound to gtp and attaches to 40s
ribosome subunit to form 43s preinitiation complex
Eif-3 and Eif-1A stabilizes this complex
mRNA binds to 43s preinitiation complex result in formation of 48s initiation complex
48S initiation complex binds to 60s ribosomal subunit to form 80s initiation complex.

16. ELONGATION
Elongaton requires the use of the elongation factors EF-1a and
supply of energy by GTP
Translocation requires EF-2 and GTP
GTP get hydrolysed and supplies energy to move mRNA

17. TERMINATION
One of the stop signals (UGA UAA AND UAG) terminates the growing
polypeptide
In eukaryotes Erf1 recognize all three stop codons and Erf3 stimulate the
termination events
Release factor binds and ribosomal unit falls apart releasing the large and
small subunit

18. POST TRANLATIONAL MODIFICATION
The N-terminus amino acid is usually removed or modified
Carbohydrate side chains are sometimes attached
Signal sequences are removed
Polypeptide chains are often complexed with metals