Translation

Lecture Information

  • Course: Molecular Biology & Biochemistry

  • Lecture Number: 4

  • Topic: Translation

  • Instructor: Louise Jones (louise.jones@york.ac.uk)

Learning Outcomes

At the end of this lecture, students should be able to:

  • LO1: Describe the process of translation including the role of the ribosome and tRNAs.

  • LO2: Understand how translation is initiated in prokaryotes and eukaryotes.

  • LO3: Use the genetic code table.

Flow of Information

  • The flow of genetic information from DNA to proteins involves:

    • DNA (Deoxyribonucleic Acid), which undergoes transcription to produce RNA.

    • RNA (Ribonucleic Acid), which undergoes translation to synthesize proteins (polypeptides).

    • Proteins are polymers of amino acids that serve various functions including transport and catalysis.

Prokaryotic Protein-coding Genes

  • Key elements include:

    • -35 and -10 promoter regions

    • Transcribed Regions: Include the 5' Untranslated Region (5' UTR) and 3' Untranslated Region (3' UTR).

    • Open Reading Frame (ORF): Contains the protein-coding region of the gene.

    • Example: mRNA corresponding to gene A translates into a single protein.

Prokaryotic Operons

  • In prokaryotes, related genes often cluster together and are transcribed as a single mRNA. This is known as an operon.

  • Operons allow the coordination of gene expression for proteins involved in the same metabolic pathway.

Eukaryotic mRNA Processing

  • Eukaryotic mRNAs undergo several modifications:

    • Addition of a 5' cap (7-methylguanosine) to facilitate translation initiation.

    • Addition of a polyA tail (200-250 adenine nucleotides) at the 3' end for stability and export from the nucleus.

Translation Basics

  • Translation: The process of decoding mRNA into proteins involves:

    • The 4-letter RNA code (A, U, C, G) is translated into a 20 amino acid code.

    • mRNA is read as triplet codons, each specifying either an amino acid or a stop signal.

The Genetic Code

  • Key Features:

    • 64 codons: Represent different amino acids and stop signals.

    • Triplet nature: Each amino acid is encoded by a series of 3 nucleotides.

    • Unambiguous: Each group of 3 specifies exactly one amino acid.

    • Degenerate: Most amino acids are encoded by multiple codons, with the third base being less significant.

    • Start Codon: AUG initiates translation; Stop codons include UAA, UAG, UGA.

Transfer RNA (tRNA)

  • tRNAs are responsible for bringing amino acids to the ribosome.

  • Each tRNA has an anticodon that is complementary to the codon on the mRNA allowing for accurate translation.

Aminoacyl-tRNA Synthetase

  • Function: Enzymes that attach the correct amino acid to its corresponding tRNA.

  • Each aminoacyl-tRNA synthetase is specific for one amino acid.

Ribosome Structure

  • Ribosomes, the site of protein synthesis:

    • Composed of proteins and ribosomal RNA (rRNA).

    • Consists of a large subunit and small subunit.

    • Catalytic function provided by rRNA.

Key Ribosome Sites

  • A (Aminoacyl): Incoming tRNA site.

  • P (Peptidyl): Holds the tRNA with the growing polypeptide chain.

  • E (Exit): Where uncharged tRNAs exit the ribosome.

Translation Initiation

Prokaryotes

  • The Shine-Dalgarno sequence in the mRNA pairs with 16S rRNA, positioning AUG in the P site.

  • Initiating amino acid: N-formylmethionine.

Eukaryotes

  • The small ribosomal subunit binds to the mRNA cap, scanning for the first AUG codon.

Translation Elongation Steps

  1. An aminoacyl-tRNA enters the A site (pairing codon with anticodon).

  2. Peptide bond formation occurs while the bond between tRNA and the amino acid in the P site is broken.

  3. The mRNA translocates by 3 bases; the tRNA in the P site moves to the E site, leaving the A site free.

  4. Peptidyl transferase catalyzes peptide bond formation.

Translation Termination

  1. A stop codon enters the A site.

  2. Release factors trigger hydrolysis of the peptide from tRNA; polypeptide is released, ribosomal subunits dissociate, and mRNA & tRNA are freed.

Practice Questions

  • How is the start codon found?

  • How does translation elongation proceed?

  • How is translation terminated?

Wrap-Up

  • Connection to other MBB content: Future lectures will cover protein functions and transcription regulation.

  • Reflection on the lecture: Review learning outcomes, participate in quizzes, and study profiles of key scientists like Ada Yonath.