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Translation: From mRNA to Protein – Study Notes

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Translation: mRNA to Protein

Overview of Translation

Translation is the process by which the nucleotide sequence of an mRNA is converted into an amino acid sequence, forming a protein. This process occurs at ribosomes and is fundamental to gene expression.

  • In prokaryotes, transcription and translation are coupled; ribosomes can bind to mRNA as it is being transcribed.

  • In eukaryotes, the nuclear envelope separates mRNA production (transcription) from translation.

Translation and Codons

The Genetic Code

  • The genetic information is organized as a series of sequential, nonoverlapping, three-letter "words" called codons.

  • Each codon specifies only one amino acid.

  • AUG codes for methionine and serves as the start codon.

  • UAA, UAG, and UGA are stop codons, signaling the end of translation.

tRNA and Ribosomes in Translation

Role of tRNA

  • tRNAs link information in mRNA codons with specific amino acids.

  • Two key events must occur for accurate translation:

    1. tRNAs must read mRNA codons correctly.

    2. tRNAs must deliver the correct amino acids corresponding to each codon.

  • For each amino acid, there is at least one specific type of tRNA.

Structure and Function of tRNA

  • tRNAs have three main functions:

    1. Bind to a specific amino acid and become "charged".

    2. Bind to mRNA at the anticodon—a triplet complementary to the mRNA codon for the particular amino acid.

    3. Interact with ribosomes noncovalently.

Wobble Hypothesis

  • Wobble: Specificity for the base at the 3' end of the codon is not always observed.

  • Example: Codons for alanine—GCA, GCC, and GCU—are recognized by the same tRNA.

  • Wobble allows cells to produce fewer tRNA species but does not allow the genetic code to be ambiguous.

Codons

Amino Acid

UCU, UCC, UCA, UCG

Serine

CCU, CCC, CCA, CCG

Proline

ACU, ACC, ACA, ACG

Threonine

GCU, GCC, GCA, GCG

Alanine

Charging tRNAs

  • tRNAs are charged by specific aminoacyl-tRNA synthetases.

  • This process attaches the correct amino acid to its corresponding tRNA, using ATP as an energy source.

Ribosomes and Translation

Structure of Ribosomes

  • Both prokaryotic and eukaryotic ribosomes consist of one large and one small subunit.

  • The subunits exist separately when not translating.

  • Ribosomes are not specific; they can synthesize any type of protein.

tRNA Binding Sites on the Ribosome

  • The large ribosomal subunit has three tRNA binding sites:

    1. A (amino acid) site: Binds with the anticodon of a charged tRNA.

    2. P (polypeptide) site: Where tRNA adds its amino acid to the growing polypeptide chain.

    3. E (exit) site: Where tRNA sits before being released from the ribosome.

Fidelity of Translation

  • Ribosomes have a fidelity function: when proper binding occurs, hydrogen bonds form between the base pairs.

  • The small subunit validates the match—if hydrogen bonds have not formed between all three base pairs, the tRNA is rejected.

Steps of Translation

Overview

Translation occurs in three main steps:

  1. Initiation

  2. Elongation

  3. Termination

Transcription

Translation

Initiation: Promoter DNA

AUG start codon in the mRNA

Termination: Terminator DNA

UAA, UAG, or UGA in the mRNA

Step 1: Initiation

  • The initiation complex forms when the small ribosomal subunit binds to its recognition sequence on mRNA.

  • A methionine-charged tRNA then binds to the AUG start codon.

  • The large subunit joins the complex, positioning the charged tRNA in the P site.

  • Initiation factors (proteins) help assemble the initiation complex.

Step 2: Elongation

  • Another charged tRNA enters the A site; the large subunit catalyzes two reactions:

    1. Breaks the bond between the tRNA in the P site and its amino acid.

    2. Forms a peptide bond between that amino acid and the amino acid on the tRNA in the A site.

  • The large subunit has peptidyl transferase activity, which is a ribozyme (an RNA molecule with catalytic activity).

  • After peptide bond formation, the ribosome moves so that the first tRNA is at the E site and is released.

  • Elongation repeats as new tRNAs bring amino acids to the growing chain.

Step 3: Termination

  • Termination occurs when a stop codon (UAA, UAG, or UGA) enters the A site.

  • A protein release factor binds to the stop codon, triggering hydrolysis of the bond between the polypeptide chain and the tRNA in the P site.

  • The newly completed polypeptide then separates from the ribosome.

Polyribosomes (Polysomes)

Simultaneous Translation

  • Several ribosomes can simultaneously translate a single mRNA molecule, producing multiple polypeptides at the same time.

  • A strand of mRNA with associated ribosomes is called a polyribosome or polysome.

Key Terms and Concepts

  • Codon: A sequence of three nucleotides in mRNA that specifies an amino acid or stop signal.

  • Anticodon: A sequence of three bases in tRNA that is complementary to an mRNA codon.

  • Aminoacyl-tRNA synthetase: An enzyme that attaches the correct amino acid to its corresponding tRNA.

  • Ribozyme: An RNA molecule capable of acting as an enzyme.

  • Peptidyl transferase: The catalytic activity of the ribosome that forms peptide bonds between amino acids.

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