Tag: Steps Of Protein Synthesis (Translation)

Steps Of Protein Synthesis (Translation)

Steps Of Protein Synthesis (Translation)

Q. 1 Which of the following is required for certain types of eukaryotic protein synthesis but not for prokaryotic protein synthesis?
 A Ribosomal RNA
 B Messenger RNA
 C Signal recognition particle
 D Peptidyl transferase
 
Q. 1 Which of the following is required for certain types of eukaryotic protein synthesis but not for prokaryotic protein synthesis?
 A Ribosomal RNA
 B Messenger RNA
 C Signal recognition particle
 D Peptidyl transferase
 
Ans. C

Explanation:

Signal recognition particles (SRPs) recognize the signal sequence on the N-terminal end of proteins destined for the lumen of the endoplasmic reticulum (ER). SRP binding arrests translation and an SRP receptor facilitates import of the SRP, ribosome, and nascent protein into the ER lumen. A signal peptidase removes the signal sequence from the protein, which may remain in the membrane or be routed for secretion.

Common to both eukaryotic and prokaryotic protein synthesis is the requirement for ATP to activate amino acids. The activated aminoacyltRNAs then interact with ribosomes carrying mRNA. Peptidyl transferase catalyzes the formation of peptide bonds between the free amino group of activated aminoacyl-tRNA on the A site of the ribosome and the esterified carboxyl group of the peptidyl-rRNA on the P site; the liberated rRNA remains on the P site.


Q. 2

GTP is required by which of the following steps in protein synthesis?

 A

Aminoacyl—tRNA synthetase activation of amino acids

 B

Attachment of mRNA to ribosomes

 C

Attachment of ribosomes to endoplasmic reticulum

 D

Translocation of tRNA—nascent protein complex from A toP sites

Q. 2

GTP is required by which of the following steps in protein synthesis?

 A

Aminoacyl—tRNA synthetase activation of amino acids

 B

Attachment of mRNA to ribosomes

 C

Attachment of ribosomes to endoplasmic reticulum

 D

Translocation of tRNA—nascent protein complex from A toP sites

Ans. D

Explanation:

he entry of the aminoacyl-tRNA into the A site results in the hydrolysis of one GTP to GDP.

Translocation of the newly formed peptidyl-tRNA in the A site into the P site by EF 2 similarly results in hydrolysis of GTP to GDP and phosphate.

Thus, the energy requirements for the formation of one peptide bond include the equivalent of the hydrolysis of two ATP molecules to ADP and of two GTP molecules to GDP, or the hydrolysis of four high-energy phosphate bonds.

Ref: Weil P. (2011). Chapter 37. Protein Synthesis & the Genetic Code. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds),Harper’s Illustrated Biochemistry, 29e. 

 


Q. 3

Protein synthesis in eukaryotes is different from prokaryotes. Which of the following is NOT TRUE about steps in initiation of eukaryotic protein synthesis?

 A

Dissociation of the ribosome into its 40S and 60S subunits

 B

Binding of a ternary complex to the 40S ribosome

 C

Binding of mRNA to the 60S preinitiation complex to form the 43S initiation complex

 D

Combination of the 48S initiation complex with the 60S ribosomal subunit

Q. 3

Protein synthesis in eukaryotes is different from prokaryotes. Which of the following is NOT TRUE about steps in initiation of eukaryotic protein synthesis?

 A

Dissociation of the ribosome into its 40S and 60S subunits

 B

Binding of a ternary complex to the 40S ribosome

 C

Binding of mRNA to the 60S preinitiation complex to form the 43S initiation complex

 D

Combination of the 48S initiation complex with the 60S ribosomal subunit

Ans. C

Explanation:

Initiation of protein synthesis can be divided into four steps: 

  • Dissociation of the ribosome into its 40S and 60S subunits; 
  • Binding of a ternary complex consisting of the initiator methionyl-tRNA, (met-tRNAi), GTP, and eIF-2 to the 40S ribosome to form the 43S preinitiation complex; 
  • Binding of mRNA to the 40S preinitiation complex to form the 48S initiation complex; and 
  • Combination of the 48S initiation complex with the 60S ribosomal subunit to form the 80S initiation complex.
 
Ref: Weil P. (2011). Chapter 37. Protein Synthesis & the Genetic Code. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper’s Illustrated Biochemistry, 29e.

Quiz In Between


Q. 4

Chain initiation in protein synthesis is by‑

 A

AUG

 B

GLA

 C

UGA

 D

UAG

Q. 4

Chain initiation in protein synthesis is by‑

 A

AUG

 B

GLA

 C

UGA

 D

UAG

Ans. A

Explanation:

A i.e. AUG


Q. 5

The cellular component for protein synthesis is :

 A

Smooth endoplasmic reticulum

 B

Rough endoplasmic reticulum

 C

Ribosomes

 D

Mitochondria

Q. 5

The cellular component for protein synthesis is :

 A

Smooth endoplasmic reticulum

 B

Rough endoplasmic reticulum

 C

Ribosomes

 D

Mitochondria

Ans. C

Explanation:

C i.e. Ribosomes


Q. 6

Termination process of protein synthesis is performed by:

 A

Releasing factor

 B

Stop codon

 C

UAA codon

 D

All

Q. 6

Termination process of protein synthesis is performed by:

 A

Releasing factor

 B

Stop codon

 C

UAA codon

 D

All

Ans. D

Explanation:

A, B, C i.e. Releasing factor, Stop codon, UAA codon

Quiz In Between


Q. 7

Which of the following is not required for protein synthesis of eukaryotes:   

 A

RNA polymerase

 B

Ribosomes

 C

Peptidyl transferase

 D

Amino acyl tRNA synthetase

Q. 7

Which of the following is not required for protein synthesis of eukaryotes:   

 A

RNA polymerase

 B

Ribosomes

 C

Peptidyl transferase

 D

Amino acyl tRNA synthetase

Ans. A

Explanation:

Ans. a. RNA polymerase (Ref Harper 28/e p362) RNA polymerase enzyme is involved in transcription process, not in translation.

The a-amino group of the new aminoacyl-tRNA in the A site carries out a nucleophilic attach on the esterified carboxyl group of the peptidyl-tRNA occupying the P site (peptidyl or polypeptide site).

Peptidyl transferase: Catalyses two reactions, peptide bond formation between amino acids and together with release factor, peptide release.


Q. 8

False about eukaryotic protein synthesis is:

 A

N formyl Met is the first-RNA to come into action

 B

mRNA read from 5′ to 3′

 C

Eft shift between GDP to GTP

 D

Capping helps in attachment of mRNA to 40 S ribosome

Q. 8

False about eukaryotic protein synthesis is:

 A

N formyl Met is the first-RNA to come into action

 B

mRNA read from 5′ to 3′

 C

Eft shift between GDP to GTP

 D

Capping helps in attachment of mRNA to 40 S ribosome

Ans. A

Explanation:

A i.e. N formyl Met is the first – RNA to come into action


Q. 9

Protein synthesis occurs in:  

September 2012

 A

Smooth endoplasmic reticulum

 B

Rough endoplasmic reticulum

 C

Golgi bodies

 D

Nucleus

Q. 9

Protein synthesis occurs in:  

September 2012

 A

Smooth endoplasmic reticulum

 B

Rough endoplasmic reticulum

 C

Golgi bodies

 D

Nucleus

Ans. B

Explanation:

Ans. B i.e. Rough endoplasmic reticulum

Rough ER

  • The surface of the rough endoplasmic reticulum (often abbreviated RER) is studded with protein-manufacturing ribosomes giving it a “rough” appearance (hence its name).

Quiz In Between



Steps Of Protein Synthesis (Translation)

STEPS OF PROTEIN SYNTHESIS (Translation)


TRANSLATION 

  • Translation is the process in which the genetic information stored in DNA is passed on to mRNA where it is translated into proteins.
  • Translation occurs in ribosomes.
  • mRNA is translated from its 5’ end to its 3’- end (51 à 31 )
  • 4 letter language information from nucleic acids to 20 letter language proteins.

STEPS OF PROTEIN SYNTHESIS-

  1. Activation of Amino acid
  2. Initiation
  3. Elongation
  4. Termination

1. Activation of amino acid (charging of tRNA)

  • Activation of amino acids takes place cytosol.
  • Each of the 20 amino acids covalently attached to the respective tRNA, by the ATP as two high energy phosphate bond catalyzed by aminoacyl tRNA synthase (AAS) so called as charging tRNA.
  • AAS is identified by DHU arm and is considered as proofreading mechanism of translation.
  • Aminoacyl tRNA synthase – are specific for particular amino acids and tRNA.
  • They are responsible for high fidelity of translation of genetic message.
  • Implements genetic code by acting as molecular dictionaries.
  • 2 ATPs are required for this reaction.

2.  Initiation- is a multi process stage.

  • It is facilitated by accessory proteins called Initiation factors (IF) and for eukaryotic initiation factor (eIF).

a) Ribosomal dissociation

  • Two initiation factors(eIF3 & eIF-1A) binds to 40S subunit of eukaryotic ribosome (80S).
  • 80S ribosomes disassociates into 40S and 60S subunits.

b) Formation of 43S preinitiation complex

  • A ternary complex containing met- tRNA1 and eIF-2(controlling factor in eukaryotes) bound to GTP attaches to 40S subunit to form 43S preinitiation complex.
  •  AUG serves as initiation codon for protein synthesis and codes for methionine

c)Formation of 48S Initiation complex-

  • The binding of mRNA to 43S preinitiation complex results in the formation 48S initiation complex is facilitated by 7- methyguanylate cap at 51 –end of mRNA.
  • In Eukaryotes, Kozak consensus sequence surrounds AUG and determines the initiating codon of mRNA.
  • In Prokaryotes, a sequence of nucleotide bases on mRNA called as Shine- Dalgarno sequence (SD sequence). It is located -6 to -10bp from AUG codon. (purine rich)

d) Formation of 80S initiation complex-

  • 48S initiation complex + 60S subunit = 80S initiation complex
  • 3 sites on 80S Ribosome- A site, P site, E site.

3. Elongation-

  • Catalyzed by proteins called as elongation fators.
  • Has 4 steps-

a) Binding of aminoacyl tRNA to the A-site-

  • Elongation factor EF-1 helps in binding of tRNA.

b) Peptide bond formation-

  • The methionine of tRNA of P- site is transferred to the new amino acid on tRNA of A-site to form peptide bond catalyzed by peptidyl transferase (a ribozyme).

c) Translocation-

  • It requires elongation factor eEF2 (translocase) and hydrolysis of GTP.

4. Termination-

  • Stop codon is in the A site now.
  • In eukaryotes, one single releasing factor, eRF.
  • In prokaryotes, 3 releasing factors- RF- 1, RF-2, RF-3.

Exam Important

  1. Translation occurs in ribosomes.
  2. mRNA is translated from its 5’ end to its 3’- end (51 à 31 )
  3. AAS is identified by DHU arm and is considered as proofreading mechanism of translation.
  4. Aminoacyl synthase implements genetic code by acting as molecular dictionaries.
  5. Two initiation factors(eIF3 & eIF-1A) binds to 40S subunit of eukaryotic ribosome (80S).
  6. AUG serves as initiation codon for protein synthesis and codes for methionine
  7. Shine- Dalgarno sequence located -6 to -10bp from AUG codon. (purine rich).
  8. 3 sites on 80S Ribosome-
  • A site- new aminoacyl tRNA binds
  • P site- growing peptidyl chain present
  • E site- deacylated tRNA present.
  1. 2 ATPs are required for activation of amino acid.
  2.  There is no tRNA for hydroxyproline and hydroxylysine.

 

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