Translation Study Guide

Translation

Chapter 15

Lesson 8 -Part 2
1. Protein - An Introduction

  • Overview of the significance of proteins in biological processes, being crucial macromolecules composed of amino acids.

2. Translation System

  • Explanation of the elements and components involved in the translation of proteins from mRNA sequences to polypeptide chains.

3. Translation Process

Step 1: tRNA Links with Amino Acids (tRNA Charging)

  • All tRNAs have the sequence CCA at the 3' end.

  • The 3' end of adenine in tRNA attaches to the carboxyl group (COO⁻) of an amino acid to carry the amino acid.

  • A set of 20 different Aminoacyl-tRNA synthetase enzymes bind amino acids to tRNA:

    • One synthetase enzyme corresponds to each amino acid.

    • These enzymes recognize the specific amino acids based on their R groups, and also recognize specific tRNAs based on different nucleotide sequences at the acceptor arm.

  • The process of amino acid loading onto tRNA involves two main steps, typically referred to as tRNA charging.

Step 2: Prokaryotic Translation Initiation

Requirements:

  • mRNA - template for translation.

  • Small ribosomal subunit (30S) and large ribosomal subunit (50S).

  • Initiation factors: IF1, IF2, and IF3.

  • Initiator tRNA with N-formylmethionine (fMet) attached.

  • Guanosine triphosphate (GTP) - provides energy.

Three Major Steps in Initiation:

  1. Small ribosomal subunit binds to the mRNA molecule.

  2. Initiator tRNA binds to the mRNA.

  3. Large ribosomal subunit joins the initiation complex.

Prokaryotic Translation Initiation Continuation

  • Upstream of the AUG start codon, a consensus sequence called the Shine-Dalgarno sequence is recognized by 16S rRNA in the small ribosomal subunit, ensuring proper positioning of the small ribosome on the mRNA.

Prokaryotic Translation Initiation Continuation (Diagrammatic Representation)

  1. Ribosome Structure:

    • Comprised of two subunits: Large subunit (50S) and Small subunit (30S).

  2. Initiation Complex Formation:

    • IF-3 binds to the small subunit, preventing large subunit binding and allowing attachment to mRNA.

    • A tRNA charged with N-formylmethionine forms a complex with IF-2 and GTP.

    • Upon hydrolysis of GTP to GDP, the large subunit joins, creating a 70S initiation complex.

Conclusion:

  • At the end of initiation, the ribosome is assembled on the mRNA, and the first tRNA is attached to the initiation codon.

Step 3: Translation Elongation

  • Elongation is the process of extending the peptide chain by adding amino acids.

  • The next codon in the mRNA dictates the type of tRNA that will bind next to methionine.

Binding Sites within the Ribosome:

  • P (Peptidyl) Site: The first site, initially bound to fMet.

  • A (Aminoacyl) Site: The second site where the next amino acid is received.

Elongation Factors:

  • Primary factors involved include Tu, Ts, and G.

Process of Elongation:

  • Once both P and A sites are occupied by tRNA, amino acids are linked by a peptide bond (catalyzed by peptidyl transferase).

  • The tRNA in the P site releases its amino acid.

    • This movement allows the ribosome to translocate along the mRNA.

    • The tRNA in the A site moves to the P site and the (now vacant) A site opens for the next tRNA, thus continuing elongation.

Step 4: Translation Termination

  • Elongation of the polypeptide chain continues until the A site encounters a stop codon (UAG, UAA, UGA).

    • Stop codons do not code for an amino acid, hence tRNA does not have an anticodon for these codons.

  • Release factors (RF):

    • RF1 binds to UAA and UAG, while RF2 binds to UGA. Both facilitate the termination of elongation.

    • RF1/RF2 bind to the A site to release the polypeptide chain from the P site.

    • Release factor 3 binds to the ribosome to release RF1/RF2 and translocate tRNA to the E site.

    • Additional factors help release mRNA and dissociate the ribosome.

Components Required for Protein Synthesis in Bacterial Cells

Stage

Component

Function

tRNA Charging

Amino acids

Building blocks of proteins


tRNAs

Deliver amino acids to ribosomes


Aminoacyl-tRNA synthetases

Attach amino acids to tRNAs


ATP

Provides energy for binding amino acids to tRNAs

Initiation

mRNA

Carries coding instructions


fMet-tRNAfMet

Provides the first amino acid in peptide


30S ribosomal subunit

Attaches to mRNA


50S ribosomal subunit

Combines with 30S ribosomal subunit


Initiation factors 1, 2, 3

Stabilize tRNAs and amino acids,…

Elongation

70S initiation complex

Functional ribosome with A, P, and E sites


Charged tRNAs

Bring amino acids to ribosome and help assemble them


Elongation factor Tu, Ts, G

Functions in tRNA delivery and regeneration of active factors


GTP

Provides energy


23S rRNA in large ribosomal subunit

Catalyzes peptide bond formation

Termination

Release factors 1, 2, and 3

Bind to ribosome when stop codon is reached, terminating translation

Simultaneous Translation in Prokaryotes

  • In prokaryotes, transcription and translation happen simultaneously.

  • A single mRNA can be translated by several ribosomes, producing multiple copies of a protein rapidly.

  • The mRNA molecule has a short lifespan of a few minutes to hours, allowing quick adjustments in gene expression.

Translation in Eukaryotes

  • The overall translation mechanism in eukaryotes is similar to that in prokaryotes but has key differences:

    • Pre-mRNA must be processed to become mature mRNA before translation.

    • The 5' cap and 3' poly(A) tail in mRNA are crucial for initiating translation.

    • Eukaryotic mRNA does not contain a Shine-Dalgarno sequence; the 5' cap serves this purpose, assisted by the Kozak sequence.

    • Eukaryotes utilize 80S ribosomes instead of 70S, except in organelles.

    • Eukaryotic translation uses Methionine (Met) instead of N-formylmethionine (fMet), which is specific to prokaryotic translation.

Prokaryotic Translation Process Diagram

  • A schematic representation of the entire translation process showing the roles of various components, from mRNA and ribosomes to tRNAs and amino acids.