Genetic Code and Translation — Revision Notes

⚡ 30-Second Revision

Genetic Code: — Triplet, Degenerate, Unambiguous, Non-overlapping, Comma-less, Universal.
Start Codon: — AUG (Met)
Stop Codons: — UAA, UAG, UGA (Nonsense codons)
mRNA: — Carries genetic message.
tRNA: — Adaptor molecule (anticodon + amino acid).
rRNA: — Ribosomal structure & catalytic (peptidyl transferase).
Ribosome Sites: — A (aminoacyl), P (peptidyl), E (exit).
Enzymes: — Aminoacyl-tRNA synthetase (tRNA charging, uses ATP).
Energy: — GTP hydrolysis for initiation, elongation (codon recognition, translocation), termination.
Prokaryotic vs. Eukaryotic: — 70S vs 80S ribosomes, fMet vs Met, Shine-Dalgarno vs 5' cap/Kozak, coupled vs separated transcription-translation.

2-Minute Revision

The genetic code is the set of rules dictating how mRNA nucleotide sequences are translated into protein amino acid sequences. It's a triplet code, with each three-nucleotide sequence (codon) specifying an amino acid. The code is degenerate (multiple codons for one amino acid) but unambiguous (one codon for one amino acid). It's non-overlapping, comma-less, and nearly universal. AUG is the start codon (Methionine), and UAA, UAG, UGA are stop codons.

Translation is protein synthesis on ribosomes. mRNA carries the message. tRNA acts as an adaptor, bringing specific amino acids to the ribosome, matching its anticodon to the mRNA codon.

Aminoacyl-tRNA synthetases 'charge' tRNAs with correct amino acids (using ATP). Ribosomes have A, P, E sites for tRNA binding. The process has three stages: Initiation (ribosome assembly at start codon), Elongation (sequential amino acid addition via peptide bond formation by rRNA's peptidyl transferase activity, and translocation), and Termination (at stop codon by release factors).

GTP hydrolysis powers most steps of initiation, elongation, and termination. Prokaryotic and eukaryotic translation differ in ribosome size (70S vs 80S), initiator amino acid (fMet vs Met), and initiation mechanisms.

5-Minute Revision

The genetic code is the fundamental language of life, translating nucleotide sequences into protein sequences. It's characterized by being a triplet code, where each three-nucleotide unit (codon) on mRNA specifies one of the 20 amino acids.

With 64 possible codons, the code exhibits degeneracy, meaning most amino acids are coded by more than one codon, often differing only in the third base (explained by the wobble hypothesis). Crucially, it's unambiguous, meaning each codon specifies only one particular amino acid.

The code is non-overlapping (no shared nucleotides between codons), comma-less (no gaps), and remarkably universal across species, with minor exceptions in mitochondria and some protozoa. AUG serves as the start codon (coding for Methionine), while UAA, UAG, UGA are stop codons, signaling termination.

Translation, the process of protein synthesis, occurs on ribosomes. It involves three key RNA types: mRNA carries the genetic blueprint from DNA; tRNA acts as an adaptor, possessing an anticodon to recognize mRNA codons and an acceptor arm to carry a specific amino acid; rRNA forms the structural and catalytic core of the ribosome, notably catalyzing peptide bond formation (peptidyl transferase activity).

The accuracy of tRNA charging with the correct amino acid is ensured by aminoacyl-tRNA synthetases, which use ATP.

Translation proceeds in three stages:

1
Initiation: — The small ribosomal subunit, mRNA, and initiator tRNA (carrying Met in eukaryotes, fMet in prokaryotes) assemble at the start codon (AUG). In prokaryotes, this involves the Shine-Dalgarno sequence; in eukaryotes, the 5' cap and scanning.
2
Elongation: — Amino acids are sequentially added. An incoming aminoacyl-tRNA binds to the A-site (aminoacyl site), matching its anticodon to the mRNA codon. A peptide bond forms between the amino acid in the A-site and the growing polypeptide in the P-site (peptidyl site). The ribosome then translocates one codon along the mRNA, moving the A-site tRNA to the P-site and the P-site tRNA to the E-site (exit site) for release. This cycle is powered by GTP hydrolysis and elongation factors.
3
Termination: — When a stop codon enters the A-site, release factors bind, causing the hydrolysis of the polypeptide from the tRNA in the P-site and the dissociation of the ribosomal complex. This entire process is energy-intensive, primarily using GTP for dynamic steps and ATP for tRNA charging.

Prelims Revision Notes

Genetic Code & Translation: NEET Quick Recall

I. Genetic Code Characteristics:

Triplet: — 3 nucleotides = 1 codon = 1 amino acid. (e.g., AUG)
Degenerate (Redundant): — Most amino acids specified by >1 codon. (e.g., UUA, UUG both code for Leucine). Wobble hypothesis explains this (less stringent pairing at 3rd base of codon/1st base of anticodon).
Unambiguous (Specific): — Each codon specifies only ONE amino acid. (e.g., AUG always Met, never anything else).
Non-overlapping: — Codons read sequentially, no shared nucleotides. (e.g., ABC DEF, not ABC BCD).
Comma-less: — No intervening nucleotides between codons.
Universal: — Same code for almost all organisms (minor exceptions: mitochondria, some protozoa).
Start Codon: — AUG (codes for Methionine, initiates translation).
Stop Codons (Nonsense Codons): — UAA, UAG, UGA (do not code for amino acids, terminate translation).

II. Translation Machinery:

mRNA (Messenger RNA): — Carries genetic message from DNA to ribosome. Read 5' to 3'.
tRNA (Transfer RNA): — Adaptor molecule. Has anticodon (complementary to mRNA codon) and amino acid acceptor arm (binds specific amino acid).
rRNA (Ribosomal RNA): — Structural and catalytic component of ribosomes. Peptidyl transferase activity (forms peptide bonds) is a ribozyme (rRNA).
Ribosomes: — Sites of protein synthesis. Made of rRNA + proteins.

* Prokaryotic: 70S (30S small, 50S large). * Eukaryotic: 80S (40S small, 60S large).

Ribosomal Sites:

* A-site (Aminoacyl): Binds incoming aminoacyl-tRNA. * P-site (Peptidyl): Holds tRNA with growing polypeptide chain. * E-site (Exit): Releases uncharged tRNA.

Aminoacyl-tRNA Synthetases: — Enzymes that 'charge' tRNA with correct amino acid. Highly specific. Requires ATP.
Translation Factors: — Initiation Factors (IFs/eIFs), Elongation Factors (EFs/eEFs), Release Factors (RFs/eRFs).

III. Translation Process (Energy-intensive):

1
Initiation: — Ribosome, mRNA, initiator tRNA assemble at AUG.

* Prokaryotes: Shine-Dalgarno sequence, fMet-tRNA, IFs. * Eukaryotes: 5' cap, scanning for first AUG (Kozak sequence), Met-tRNA, eIFs. * Requires GTP.

1
Elongation: — Sequential addition of amino acids.

* Codon recognition (A-site entry of new tRNA) - GTP. * Peptide bond formation (P-site to A-site transfer, by peptidyl transferase). * Translocation (ribosome moves 1 codon, A-site to P-site, P-site to E-site) - GTP.

1
Termination: — Stop codon in A-site. Release factors bind. Polypeptide released. Ribosome dissociates.

* Requires GTP.

IV. Key Differences (Prokaryotic vs. Eukaryotic Translation):

Ribosomes: — 70S vs 80S.
Initiator AA: — fMet vs Met.
mRNA recognition: — Shine-Dalgarno vs 5' cap/Kozak.
Coupling: — Coupled (simultaneous) vs Separated (nucleus/cytoplasm).
mRNA: — Polycistronic vs Monocistronic.

V. Clinical Relevance: Antibiotics often target prokaryotic translation (e.g., tetracycline, streptomycin, chloramphenicol, erythromycin).

Vyyuha Quick Recall

Three Degenerate Universal Non-overlapping Codons Start Stopping All Proteins.

Triplet, Degenerate, Universal, Non-overlapping, Comma-less, Start (AUG), Stop (UAA, UAG, UGA), Ambiguous (NO!), Punctuation (NO!).