What is Protein Synthesis by Ribosomes?
Ribosomes are the molecular machines that build proteins by translating the genetic code carried by messenger RNA (mRNA) into chains of amino acids. They can be found free in the cytoplasm or attached to the rough endoplasmic reticulum.
Ribosomes read mRNA codons and use transfer RNA (tRNA) to link amino acids together in the correct order, a process called translation, which produces functional proteins.
- 1↓InitiationThe small ribosomal subunit binds mRNA at the start codon (AUG).
- 2↓tRNA bindingA tRNA carrying the matching anticodon and amino acid enters the ribosome.
- 3↓Peptide bond formationThe ribosome links the new amino acid to the growing polypeptide chain.
- 4↓TranslocationThe ribosome moves along the mRNA to the next codon, repeating the cycle.
- 5TerminationA stop codon is reached, the polypeptide is released, and the ribosome dissociates.
Step-by-step worked examples
An mRNA strand reads AUG-GGC-UUA-UAG. How many amino acids does the finished polypeptide contain?
AUG is the start codon, coding for the first amino acid (Methionine). GGC and UUA each code for one amino acid. UAG is a stop codon and does not code for an amino acid, so it ends translation. Total amino acids = 3 (from AUG, GGC, UUA).
A ribosome is translating a codon for leucine. Which molecule delivers leucine to the ribosome?
Each amino acid is carried to the ribosome by a specific transfer RNA (tRNA). The tRNA has an anticodon complementary to the mRNA codon for leucine. That tRNA binds the ribosome's A site and hands off leucine to the growing chain.
Where would you expect to find the ribosomes making a protein destined for secretion outside the cell?
Secreted proteins must enter the endomembrane system for processing. This means their ribosomes must be attached to the rough ER, not free in the cytoplasm. As the polypeptide is synthesized, it's threaded directly into the ER lumen.
Flashcards
Quick quiz
Q1.What is the direct template ribosomes use to build a protein?
Q2.What does a codon specify?
Q3.Which molecule brings amino acids to the ribosome?
Q4.What happens when the ribosome reaches a stop codon?
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Common mistakes
Ribosomes read DNA directly. — Correct: Ribosomes read mRNA, which is a copy of DNA made during transcription.
tRNA and mRNA are the same molecule. — Correct: mRNA carries the code; tRNA carries amino acids and matches codons via anticodons.
Every codon codes for an amino acid. — Correct: Stop codons (like UAG) signal the end of translation and code for no amino acid.
All ribosomes are attached to the ER. — Correct: Ribosomes can be free in the cytoplasm or bound to the rough ER, depending on the protein's destination.
FAQ
What are ribosomes?
Cellular machines made of RNA and protein that translate mRNA into polypeptide chains during protein synthesis.
What is the formula/process for protein synthesis?
Transcription copies DNA into mRNA in the nucleus; translation at the ribosome reads mRNA codons and links amino acids via tRNA.
What are examples of protein synthesis by ribosomes?
Free ribosomes making cytoplasmic enzymes, and rough-ER-bound ribosomes making insulin or antibodies for secretion.
How do you calculate the number of amino acids from an mRNA sequence?
Count the codons between the start codon (AUG) and the stop codon; each one (excluding the stop) codes for one amino acid.