Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 17.4 Translation Is the RNA-directed Synthesis of a Polypeptide: A Closer Look
Translation is the process where genetic information carried by messenger RNA (mRNA) directs the synthesis of polypeptides (proteins). The translation machinery, involving ribosomes, transfer RNAs (tRNAs), and various enzymes, translates nucleotide sequences into amino acid sequences.
Molecular Components of Translation
- Messenger RNA (mRNA): carries genetic information from DNA to ribosomes
- Transfer RNA (tRNA): carries amino acids and matches them to codons on the mRNA
- tRNAs have an anticodon (complementary to mRNA codon) and an amino acid attachment site
- Ribosomes: sites of protein synthesis, composed of rRNA and proteins, having large and small subunits
Structure and Function of tRNA
- tRNA molecules (~80 nucleotides) form an L-shaped structure
- Each tRNA carries a specific amino acid at the 3′ end
- Anticodon at the opposite end binds to mRNA codon
- Aminoacyl-tRNA synthetases: enzymes that attach amino acids to correct tRNAs (using ATP)
- Wobble effect: flexible pairing at the 3rd nucleotide of a codon allows fewer tRNAs (about 45) to recognize all 61 amino acid codons
Ribosome Structure and Function
- Ribosomes consist of a small and large subunit
- Three binding sites for tRNA:
- A site: holds tRNA with next amino acid
- P site: holds tRNA with growing polypeptide
- E site: exit site for discharged tRNAs
- Ribosomal RNA (rRNA) catalyzes peptide bond formation, acting as a ribozyme
Stages of Translation
Initiation
- Small ribosomal subunit binds mRNA and initiator tRNA (methionine, AUG codon)
- Large subunit joins, forming the translation initiation complex
- Requires energy (GTP) and initiation factors
Elongation
- Amino acids are added one by one, extending the polypeptide chain
- Steps repeated for each amino acid:
- Codon recognition (requires GTP)
- Peptide bond formation (catalyzed by rRNA)
- Translocation of tRNAs and mRNA (requires GTP)
Termination
- Occurs when ribosome reaches a stop codon (UAG, UAA, UGA)
- A release factor binds to the stop codon, adds a water molecule, releasing the polypeptide
- Translation complex disassembles (requires GTP)
Completing and Targeting Proteins
- Polypeptides spontaneously fold during synthesis, forming a functional protein (secondary and tertiary structures)
- Post-translational modifications: chemical modifications (e.g., phosphorylation), removal of amino acids, or cleavage into separate polypeptides
- Proteins targeted for secretion or membranes have signal peptides at the amino end
- A signal recognition particle (SRP) directs these proteins to the endoplasmic reticulum (ER), where translation continues, and the protein is released into the ER lumen or embedded into the membrane
Polyribosomes and Efficiency
- Multiple ribosomes simultaneously translate a single mRNA, forming polyribosomes (polysomes), increasing protein production speed
- Bacterial cells couple transcription and translation; eukaryotes separate the processes by the nuclear envelope, allowing RNA processing
In a Nutshell
Translation converts the genetic code on mRNA into proteins through ribosomes and tRNAs. Initiation, elongation, and termination coordinate amino acid assembly into polypeptides. Protein processing and targeting ensure functional proteins reach their correct cellular destinations.