Download MBBS (Bachelor of Medicine and Bachelor of Surgery) Latest Translation Lecture PPT
TRANSLATION
Translation--
is a process in which a polypeptide is synthesized
according to the nucleotide sequence of m RNA.
It is carried out by a translation complex comprising m RNA,
tRNA, ribosomes.
Occurs at the ribosome in the cytoplasm.
Uses anticodon to the tRNA to bring an aminoacid to the
ribosomes.
PROTEIN SYNTHESIS.
TRANSLATION.
Takes place in ribosomes.
tRNA brings aminoacid to ribosomes.
tRNA forms a complimentary message ? anticodon.
The aminoacid is read.
Aminoacid sequence continue until reaching a STOP codon.
PROTEIN SYNTHESIS.
STEPS FOR PROTEIN SYNTHESIS.
STARTS WITH DNA.
Transcribe to mRNA.
mRNA speccifies aminoacid sequence in polypeptide.
Brought by tRNA to r RNA.
Is translated into protein.
GENETIC CODE.
GENETIC CODE-
Genetic code is the language used to
convert the sequence of nucleotides in mRNA into the
sequence of amino acids of a protein.
A codon is a combination of three consecutive nucleotides
present on DNA or mRNA. Hence, it is commonly called a
triplet i.e. a codon is a triplet of nucleotides.
Three codons do not code for any amino acid and the signal
the termination of protein synthesis. These are called
termination codons or stop codons. These are UAA, UGA and
UAG.
I
AUG is used as the first or initiation codon in all the proteins
synthesized 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
in final fully functional mature protein because this amino acid may
be removed during processing and maturation of protein.
Codons on mRNA or DNA(from which mRNA is transcribed) are
read from 5` to 3` direction by the anti-codon loop of the tRNA.
TRANSLATION.
TRANSLATION IS COMPLETED IN 4 MAJOR STEPS.
Charging of tRNA.
Initiation.
Elongation and translocation.
Termination.
Folding and processing.
Transport.
ACTIVATION OF AMINOACID AND CHARGING OF TRNA.
Components required.
20 aminoacid.
20 aminoacyl tRNA synthesis.
ATP.
Mg+
INITIATION.
Component required.
mRNA.
fmet-Trna/met-tRNA
30s/50s ? 70s
40s/60s--80s
GTP
Mg+
Initiation factor.IF-1,IF-2,and IF-3.
ELONGATION AND TRANSLOCATION.
Component required.
Initiation complex.
Aminoacyl tRNA.
Mg++
Elongation factor.
Peptidyl transferase.
TERMINATION.
Component required.
ATP.
Termination codon.
Release factor, RF-1, RF-2,RF-3.
TRANSLATION.
INITIATION--
several initiation factors , AMP, and GMP are required and occurs in
the following stages.
ACTIVATION OF AMINO ACID
SPLITING OF RIBOSOMES
PREINITIATION COMPLEX FORMATION
FORMATION OF 48S PRE-INITIATION COMPLEX
FORMATION OF FULLY ACTIVE 80S INITIATION COMPLEX.
ELONGATION
TERMINATION
TRANSLATION IN PROKARYOTES.
INITIATION--
The 70s ribosome dissociated to form 30s and
50s subunits. A set of three proteins called initiation factor (IF-
1, IF-2, and IF-3 ) take part in initiation. Bacterial ribosome
contain three specific sites that bind aminoacyl-t RNAs. They
are the A site(aminoacyl site), the P site(peptidyl site) and the E
site(exit site).
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
fashion that the initiating codon (AUG) comes in to the P site on
the ribosome. AUG is the codon for methionine which has two t
RNAs.
ELONGATION.
ELONGATION?
similar to initiation , a set of three elongation factor
( EF-1, EF-2, EF-3 ) participate in the elongation .
1) Binding of an incoming aminiacyl ?t RNA.
2) peptide bond formation.
3) Translocation.
TRANSLATION.
TERMINATION-
Elongation continues until last aminoacid is added as
coded by the m RNA. One of the three stop codons ?UAA, UAG,UGA occurs
immediately after the codon for the last aminoacid .
TRANSLATION IN EUKARYOTES.
INITIATION-
DISSOCIATION OF RIBOSOME -- The 80s intact
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.
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
complex.
The ternary complex bind to the 40s ribosomal subunit to form
43s pre initiation complex.
INITIATION.
REQUIREMENT--
Initiation factor.
AMP, GMP.
Occurs in following stages---
ACTIVATION OF Amino acid--
First amino acid combined to AMP to
form aminoacyl adenylate which is then attached to the
corresponding t RNA.
SPLITING OF RIBOSOMES----
Ribosomes split into a smaller 40-S
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
association.
PRE INITIATION COMPLEX.
PRE INITIATION COMPLEX--
Methionine is the starting amino
acid , so activated methionine , that is t RNA met first binds eLF
-2 and GTP to form the ternary complex. Then ternary complex
then bind to stablized 40S subunit to form the 43s pre
initiation complex.
FORMATION OF 48S pre-initiation complex ?
Binding of mRNA
to 43s pre-initiation complex leads to the formation of 48s pre-
initiation complex. This is mediated by eLF-3, eLF-4F.
The 48s-pre-initiation complex then scans the mRNA in 5'-3'
direction for the initiation codon.
INITIATION COMPLEX.
FORMATION OF FULLY ACTIVE 80S INITIATION COMPLEX-
eIF-5 fascilitate the binding of 60S
ribosomal subunit to pre-initiation complex to form 80S
initiation complex. At this point ,met-t-RNA(initiator tRNA) is on
the P site of ribosome to start the elongation.
INITIATION COMPLEX.
FORMATION OF INITIATION COMLEX--
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
ribosomal subunit to form the 80s initiation complex.
ELONGATION.
ELONGATION--
once the process of translation is initiated, the
polypeptide chain is expanded in length by the sequential
addition of aminoacids.
Elongation is a cyclic process occurs in three steps.
1.Binding of amino acyl ?t RNA to the A site.
2.Peptide bond formation.
3.Translocation.
TRANSLATION
TERMINATION-----
After several cycles of elongation incorporating
the required number of aminiacids one of the three stop
codons (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
Several antibiotics selectively act on bacterial ribosomes and
other ribosomes and other translation components to inhibit
protein synthesis and kill them.
MECHANISM OF ACTION OF ANTIBIOTICS.
TETRACYCLIN
INHIBIT BINDING AMINOACYL tRNA to
A site.
STREPTOMYCIN
Bind to 30s ribosome.
chloremphenicol
Inhibit peptidyl transferase.
Erythromycin
Binds to 50s ribosomes.
tetracycline
Blocking the a site on the ribosome.
CONTROL OF PROTEIN SYNTHESIS.
CONTROL OF PROTEIN SYNTHESIS-
Control of protein synthesis
occurs at the stage of initiation at two levels.
By eIF-2, preventing 43S PIC formation and
BY eIF-4F complex and preventing 48S PIC formation.
REGULATION.
REGULATION BY eIF-2 -
eIF-2 is a trimeric protein having a, B
and y subunits. It is activated by phosphorylation of a- subunits
by several kinases.
Phosphorylated a-subunits binds to eLF2B and inctivates it.
This prevents 43S PIC formation and stops translation.
REGULATION.
REGULATION BY elF-4F
eIF-4F is inactive when bound by a protein (4E-BP).
On phosphorylation of 4EBP 4E becomes free and takes
part in 4F complex formation.
4F then binds to mRNA cap and ultimately leads to the
formation of 48S PLC.
PROTEIN FOLDING
Nascent protein undergoes folding with the help of chapreons
to acquire an appropriate three dimensional structure to
become active.
Protein that do not fold properly and become non functional,
are degraded . Some may aggregate to produce Prion disease.
POST ? TRANSLATIONAL MODIFICATION.
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.
COVALENT MODIFICATION.
PHOSPHORYLATION.----
Phosphorylation ? Dephosphorylation
depending on cellular needs.This process is under the control of hormones
and is the major mechanism of enzyme regulation.
HYDROXYLATION.---
collagen synthesised as procollagen undergoes
extensive post translational modification to form mature collagen.
GLYCOSYLATION.--
many proteins contain extensive carbohydrate side
chain.
CARBOXYLATION...
ACETYLATION....
Most of the translational processing occurs in the endoplasmic reticulam
and Golgi complex.
This post was last modified on 30 November 2021