is a process in which a polypeptide is synthesized
according to the nucleotide sequence of m RNA.
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It is carried out by a translation complex comprising m RNA,tRNA, ribosomes.
Occurs at the ribosome in the cytoplasm.
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Uses anticodon to the tRNA to bring an aminoacid to the
ribosomes.
PROTEIN SYNTHESIS.
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TRANSLATION.Takes place in ribosomes.
tRNA brings aminoacid to ribosomes.
tRNA forms a complimentary message ? anticodon.
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The aminoacid is read.Aminoacid sequence continue until reaching a STOP codon.
PROTEIN SYNTHESIS.
STEPS FOR PROTEIN SYNTHESIS.
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STARTS WITH DNA.Transcribe to mRNA.
mRNA speccifies aminoacid sequence in polypeptide.
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Brought by tRNA to r RNA.
Is translated into protein.
GENETIC CODE.
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GENETIC CODE-
Genetic code is the language used to
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convert the sequence of nucleotides in mRNA into thesequence of amino acids of a protein.
A codon is a combination of three consecutive nucleotides
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present on DNA or mRNA. Hence, it is commonly called atriplet i.e. a codon is a triplet of nucleotides.
Three codons do not code for any amino acid and the signal
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the termination of protein synthesis. These are calledtermination codons or stop codons. These are UAA, UGA and
UAG.
I
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AUG is used as the first or initiation codon in all the proteinssynthesized from mRNA. It codes for methionine. Therefore, each
newly synthesized polypeptide chain [nascent chain] has
methionine as the first amino acid. However, it may not be present
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in final fully functional mature protein because this amino acid maybe removed during processing and maturation of protein.
Codons on mRNA or DNA(from which mRNA is transcribed) are
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read from 5` to 3` direction by the anti-codon loop of the tRNA.TRANSLATION.
TRANSLATION IS COMPLETED IN 4 MAJOR STEPS.
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Charging of tRNA.
Initiation.
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Elongation and translocation.Termination.
Folding and processing.
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Transport.
ACTIVATION OF AMINOACID AND CHARGING OF TRNA.
Components required.
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20 aminoacid.
20 aminoacyl tRNA synthesis.
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ATP.Mg+
INITIATION.
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Component required.mRNA.
fmet-Trna/met-tRNA
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30s/50s ? 70s
40s/60s--80s
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GTPMg+
Initiation factor.IF-1,IF-2,and IF-3.
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ELONGATION AND TRANSLOCATION.Component required.
Initiation complex.
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Aminoacyl tRNA.
Mg++
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Elongation factor.Peptidyl transferase.
TERMINATION.
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Component required.ATP.
Termination codon.
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Release factor, RF-1, RF-2,RF-3.
TRANSLATION.
INITIATION--
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several initiation factors , AMP, and GMP are required and occurs in
the following stages.
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ACTIVATION OF AMINO ACIDSPLITING OF RIBOSOMES
PREINITIATION COMPLEX FORMATION
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FORMATION OF 48S PRE-INITIATION COMPLEX
FORMATION OF FULLY ACTIVE 80S INITIATION COMPLEX.
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ELONGATIONTERMINATION
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TRANSLATION IN PROKARYOTES.INITIATION--
The 70s ribosome dissociated to form 30s and
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50s subunits. A set of three proteins called initiation factor (IF-
1, IF-2, and IF-3 ) take part in initiation. Bacterial ribosome
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contain three specific sites that bind aminoacyl-t RNAs. Theyare the A site(aminoacyl site), the P site(peptidyl site) and the E
site(exit site).
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Two initiation factor, IF-1 and IF-3, bind to the 30s ribosomal
subunit .The m RNA now bind to the 30s subunit in such a
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fashion that the initiating codon (AUG) comes in to the P site onthe ribosome. AUG is the codon for methionine which has two t
RNAs.
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ELONGATION.ELONGATION?
similar to initiation , a set of three elongation factor
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( EF-1, EF-2, EF-3 ) participate in the elongation .
1) Binding of an incoming aminiacyl ?t RNA.
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2) peptide bond formation.3) Translocation.
TRANSLATION.
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TERMINATION-Elongation continues until last aminoacid is added as
coded by the m RNA. One of the three stop codons ?UAA, UAG,UGA occurs
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immediately after the codon for the last aminoacid .TRANSLATION IN EUKARYOTES.
INITIATION-
DISSOCIATION OF RIBOSOME -- The 80s intact
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ribosome dissociate to its constituent subunits-40s and 60s. Two
initiation factor (elF-1 and elF-2 ) bind to 40s subunit and delays
its reassociation with the 60s subunit allowing the binding of
other initiation factors to the 40s subunit.
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FORMATION OF THE PREINITIATION COMPLEX-- The elongation
factor -2 (elF-2) binds GTP to form a binary complex which in
turn associates with t RNA (carrying methionine) to form ternary
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complex.The ternary complex bind to the 40s ribosomal subunit to form
43s pre initiation complex.
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INITIATION.REQUIREMENT--
Initiation factor.
AMP, GMP.
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Occurs in following stages---ACTIVATION OF Amino acid--
First amino acid combined to AMP to
form aminoacyl adenylate which is then attached to the
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corresponding t RNA.
SPLITING OF RIBOSOMES----
Ribosomes split into a smaller 40-S
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subunits and 60-S subunit . eIF -3 and Eif -1a bind to 40S subunit
and eLF-6 bind to 60s subunit to stabilize them and prevent their re
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association.PRE INITIATION COMPLEX.
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PRE INITIATION COMPLEX--Methionine is the starting amino
acid , so activated methionine , that is t RNA met first binds eLF
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-2 and GTP to form the ternary complex. Then ternary complex
then bind to stablized 40S subunit to form the 43s pre
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initiation complex.FORMATION OF 48S pre-initiation complex ?
Binding of mRNA
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to 43s pre-initiation complex leads to the formation of 48s pre-
initiation complex. This is mediated by eLF-3, eLF-4F.
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The 48s-pre-initiation complex then scans the mRNA in 5'-3'direction for the initiation codon.
INITIATION COMPLEX.
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FORMATION OF FULLY ACTIVE 80S INITIATION COMPLEX-eIF-5 fascilitate the binding of 60S
ribosomal subunit to pre-initiation complex to form 80S
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initiation complex. At this point ,met-t-RNA(initiator tRNA) is onthe P site of ribosome to start the elongation.
INITIATION COMPLEX.
FORMATION OF INITIATION COMLEX--
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Activated m RNA is then
transferred to the 43s pre initiation complex to form 48s
initiation complex. The 48s initiation complex bind to the 60s
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ribosomal subunit to form the 80s initiation complex.ELONGATION.
ELONGATION--
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once the process of translation is initiated, thepolypeptide chain is expanded in length by the sequential
addition of aminoacids.
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Elongation is a cyclic process occurs in three steps.1.Binding of amino acyl ?t RNA to the A site.
2.Peptide bond formation.
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3.Translocation.
TRANSLATION
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TERMINATION-----
After several cycles of elongation incorporating
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the required number of aminiacids one of the three stopcodons (UAA, UAG, UGA) appears in the A site. The stop codon
is not recognised by any specific t RNA but a releasing factor
(RF) recognises and binds to the stop codon.
INHIBITORS OF PROTEIN SYNTHESIS
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Several antibiotics selectively act on bacterial ribosomes and
other ribosomes and other translation components to inhibit
protein synthesis and kill them.
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MECHANISM OF ACTION OF ANTIBIOTICS.TETRACYCLIN
INHIBIT BINDING AMINOACYL tRNA to
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A site.STREPTOMYCIN
Bind to 30s ribosome.
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chloremphenicol
Inhibit peptidyl transferase.
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ErythromycinBinds to 50s ribosomes.
tetracycline
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Blocking the a site on the ribosome.
CONTROL OF PROTEIN SYNTHESIS.
CONTROL OF PROTEIN SYNTHESIS-
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Control of protein synthesis
occurs at the stage of initiation at two levels.
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By eIF-2, preventing 43S PIC formation andBY eIF-4F complex and preventing 48S PIC formation.
REGULATION.
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REGULATION BY eIF-2 -eIF-2 is a trimeric protein having a, B
and y subunits. It is activated by phosphorylation of a- subunits
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by several kinases.Phosphorylated a-subunits binds to eLF2B and inctivates it.
This prevents 43S PIC formation and stops translation.
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REGULATION.REGULATION BY elF-4F
eIF-4F is inactive when bound by a protein (4E-BP).
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On phosphorylation of 4EBP 4E becomes free and takes
part in 4F complex formation.
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4F then binds to mRNA cap and ultimately leads to theformation of 48S PLC.
PROTEIN FOLDING
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Nascent protein undergoes folding with the help of chapreonsto acquire an appropriate three dimensional structure to
become active.
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Protein that do not fold properly and become non functional,are degraded . Some may aggregate to produce Prion disease.
POST ? TRANSLATIONAL MODIFICATION.
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most of the newly synthesised proteins are not functional.After attaining a three ? dimensional structure by folding
mechanism, these proteins undergo several structural and
chemical alteration to become fully functional.
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COVALENT MODIFICATION.PHOSPHORYLATION.----
Phosphorylation ? Dephosphorylation
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depending on cellular needs.This process is under the control of hormones
and is the major mechanism of enzyme regulation.
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HYDROXYLATION.---collagen synthesised as procollagen undergoes
extensive post translational modification to form mature collagen.
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GLYCOSYLATION.--
many proteins contain extensive carbohydrate side
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chain.CARBOXYLATION...
ACETYLATION....
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Most of the translational processing occurs in the endoplasmic reticulam
and Golgi complex.
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