Process of Translation — Revision Notes

⚡ 30-Second Revision

mRNA: — Template, 5' to 3' direction.
tRNA: — Adaptor, carries amino acid, has anticodon.
Ribosome: — Site of synthesis, A/P/E sites, peptidyl transferase (rRNA).
Start Codon: — AUG (Met/fMet).
Stop Codons: — UAA, UAG, UGA (no tRNA).
Aminoacylation: — tRNA charging, by aminoacyl-tRNA synthetase, uses ATP.
Initiation: — Ribosome assembly at start codon. Prokaryotes: Shine-Dalgarno, IFs, fMet. Eukaryotes: 5' cap, eIFs, Met.
Elongation: — Codon recognition $\rightarrow$ Peptide bond formation $\rightarrow$ Translocation. Uses GTP, elongation factors.
Termination: — Stop codon recognized by release factors, polypeptide released. Uses GTP.
Genetic Code: — Degenerate, unambiguous, universal, non-overlapping, commaless.

2-Minute Revision

Translation is the process of synthesizing proteins from an mRNA template, occurring on ribosomes. It starts with the 'charging' of tRNAs, where each tRNA is attached to its specific amino acid by aminoacyl-tRNA synthetases, using ATP.

The process then proceeds in three main stages. Initiation involves the assembly of the small ribosomal subunit, initiator tRNA (carrying Methionine for eukaryotes, N-formylmethionine for prokaryotes), and mRNA at the start codon (AUG).

In prokaryotes, this is guided by the Shine-Dalgarno sequence; in eukaryotes, by the 5' cap and scanning. Elongation is a cyclical process: first, a new aminoacyl-tRNA binds to the A-site (codon recognition); second, a peptide bond is formed between the amino acid in the A-site and the growing chain in the P-site (catalyzed by rRNA's peptidyl transferase activity); third, the ribosome translocates one codon along the mRNA, shifting tRNAs.

This stage uses GTP and elongation factors. Termination occurs when a stop codon (UAA, UAG, UGA) enters the A-site, which is recognized by protein release factors (not tRNAs). This triggers the release of the completed polypeptide and dissociation of the ribosomal complex, also using GTP.

Post-translational modifications then prepare the protein for its function.

5-Minute Revision

Translation is the cellular mechanism for converting the genetic information from an mRNA sequence into a protein's amino acid sequence. This complex process is carried out by ribosomes and involves several key molecular players and energy inputs.

Prior to actual protein synthesis, each transfer RNA (tRNA) molecule must be 'charged' with its correct amino acid. This crucial step, called aminoacylation, is catalyzed by highly specific aminoacyl-tRNA synthetase enzymes and requires energy from ATP hydrolysis.

The translation process itself is divided into three main stages:

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Initiation: — This stage sets up the ribosome at the start codon (AUG) on the mRNA. In prokaryotes, the small 30S ribosomal subunit binds to a specific Shine-Dalgarno sequence on the mRNA, upstream of AUG, with the help of initiation factors (IFs). The initiator tRNA carries N-formylmethionine (fMet). The large 50S subunit then joins to form the 70S initiation complex. In eukaryotes, the small 40S subunit, along with initiator tRNA (carrying unformylated Methionine) and multiple eukaryotic initiation factors (eIFs), binds to the 5' cap of the mRNA and scans for the first AUG codon. The 60S subunit then joins to form the 80S complex. GTP hydrolysis provides energy for assembly in both.

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Elongation: — This is where the polypeptide chain grows. It's a repetitive cycle involving three steps:

* Codon Recognition: An incoming aminoacyl-tRNA (carrying the next amino acid) binds to the A-site of the ribosome, matching its anticodon to the mRNA codon. This step requires GTP and elongation factors (e.

g., EF-Tu/eEF1A). * Peptide Bond Formation: The peptidyl transferase activity (a ribozyme function of the large ribosomal subunit's rRNA) catalyzes the formation of a peptide bond between the amino acid in the A-site and the growing polypeptide chain attached to the tRNA in the P-site.

* Translocation: The ribosome moves one codon (three nucleotides) along the mRNA in the 5' to 3' direction. This shifts the tRNA from the A-site to the P-site, and the deacylated tRNA from the P-site to the E-site (exit site), from where it leaves.

This step also requires GTP and elongation factors (e.g., EF-G/eEF2). The A-site is now empty for the next aminoacyl-tRNA.

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Termination: — This stage signals the end of protein synthesis. When a stop codon (UAA, UAG, or UGA) enters the A-site, it is recognized not by a tRNA, but by protein release factors (RFs/eRFs). These factors bind to the A-site, causing the peptidyl transferase to hydrolyze the bond between the polypeptide and the tRNA in the P-site, releasing the completed protein. The ribosomal subunits then dissociate from the mRNA, ready for another round of translation, a process also facilitated by GTP hydrolysis.

Post-translational modifications like folding, cleavage, and chemical modifications are often necessary for the protein to become fully functional. The genetic code itself is characterized by being degenerate, unambiguous, universal, non-overlapping, and commaless, ensuring accurate and efficient protein synthesis.

Prelims Revision Notes

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Central Dogma: — DNA $\rightarrow$ RNA $\rightarrow$ Protein. Translation is RNA to Protein.
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Genetic Code Properties:

* Triplet: 3 nucleotides = 1 codon. * Degenerate: Multiple codons for one amino acid (e.g., UUU, UUC for Phe). * Unambiguous: One codon specifies only one amino acid. * Universal: Mostly same code across species (minor exceptions). * Non-overlapping: Each nucleotide is part of only one codon. * Commaless: No intervening nucleotides between codons.

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Key Codons:

* Start Codon: AUG (codes for Methionine; in prokaryotes, N-formylmethionine). * Stop Codons (Nonsense Codons): UAA (Ochre), UAG (Amber), UGA (Opal). Do not code for any amino acid.

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Components of Translation:

* mRNA: Carries genetic message (codons) from DNA to ribosome. * tRNA: Adaptor molecule. Has an anticodon loop (pairs with mRNA codon) and an amino acid acceptor arm (carries specific amino acid).

* Ribosome: Site of protein synthesis. Made of rRNA and proteins. Has A (aminoacyl), P (peptidyl), E (exit) sites. * Prokaryotic: 70S (30S + 50S). * Eukaryotic: 80S (40S + 60S). * Aminoacyl-tRNA Synthetase: Enzyme that 'charges' tRNA with its correct amino acid.

Requires ATP. * Initiation Factors (IFs/eIFs): Proteins that aid in ribosomal assembly at start codon. * Elongation Factors (EFs/eEFs): Proteins that aid in tRNA binding and translocation. * Release Factors (RFs/eRFs): Proteins that recognize stop codons and terminate translation.

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Stages of Translation:

* Aminoacylation (tRNA Charging): Amino acid + tRNA + ATP $\rightarrow$ Aminoacyl-tRNA + AMP + PPi. * Initiation: * Prokaryotes: 30S subunit binds to Shine-Dalgarno sequence on mRNA.

Initiator tRNA (fMet-tRNAfMet) binds to AUG in P-site. 50S subunit joins. (IF1, IF2, IF3 involved). * Eukaryotes: 40S subunit binds to 5' cap of mRNA, scans for first AUG (often in Kozak sequence).

Initiator tRNA (Met-tRNAiMet) binds to AUG in P-site. 60S subunit joins. (Multiple eIFs involved). * Elongation: (Requires GTP, EFs) 1. Codon Recognition: Incoming aminoacyl-tRNA binds to A-site.

2. Peptide Bond Formation: Peptidyl transferase (23S rRNA in prokaryotes, 28S rRNA in eukaryotes - a ribozyme) forms peptide bond between A-site amino acid and P-site polypeptide. 3. Translocation: Ribosome moves 5' to 3' along mRNA.

A $\rightarrow$ P, P $\rightarrow$ E. Deacylated tRNA exits from E-site. * Termination: (Requires GTP, RFs/eRFs) * Stop codon in A-site recognized by release factors. * Polypeptide released by hydrolysis.

* Ribosome dissociates.

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Direction: — mRNA is read 5' $\rightarrow$ 3'. Protein is synthesized N-terminus $\rightarrow$ C-terminus.
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Wobble Hypothesis: — Less stringent pairing at the third base of codon-anticodon, allowing one tRNA to recognize multiple codons.
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Energy: — ATP for aminoacylation. GTP for initiation, elongation, termination.

Vyyuha Quick Recall

To remember the sequence of elongation steps: Come Please Translate!

Codon recognition (tRNA comes to A-site)
Peptide bond formation (peptide bond forms)
Translocation (ribosome moves)