Translation

Translation: Gene Expression Mechanism

  • How are genes expressed? Discusses the process of translation, where RNA is converted into proteins.

Where and When Does Translation Occur?

  • Prokaryotes:

    • Location: In the cytosol.

    • Can begin translation before transcription (Txn) is completed.

  • Eukaryotes:

    • Location: In the cytosol after the completion of transcription, which includes RNA processing and export of mRNA from the nucleus.

Genetic Code

  • The genetic code is the mechanism that converts nucleotides to amino acids.

Codon

  • Definition: A codon consists of three nucleotides in a row, referred to as the triplet code.

    • There are 61 codons that specify one amino acid.

    • The order of nucleotides is critical, as it determines the specific amino acid encoded by each codon.

Start and Stop Codons

  • Start Codon: AUG codifies for methionine (MET, M).

  • Stop Codons:

    • Three stop codons include UGA, UAG, UAA.

    • These do not specify any amino acid and instead signal termination of translation.

Characteristics of the Genetic Code

  • Redundant: Some amino acids are specified by more than one codon.

    • 61 amino-acid-specifying codons corresponding to 20 amino acids.

  • Not Ambiguous: Each codon uniquely specifies either one amino acid or represents a stop codon.

  • Universal: All organisms on Earth, except a few atypical prokaryotes, utilize the same genetic code.

Reading Frame

  • A reading frame determines how codons are read:

    • Example sequences include: CGU ACU GAC UUG ACG UCA UAC …

    • Translation begins at the start codon (AUG), reading three bases at a time (codon) without skipping any bases.

Components of Translation

  1. mRNA (Messenger RNA)

    • In eukaryotes, mature and processed mRNA serves as the nucleotide sequence that is translated into an amino acid sequence.

  2. tRNAs (Transfer RNAs)

    • Function: Decoding units that link codons to their corresponding amino acids following the genetic code.

    • Anticodon: A group of three nucleotides on the tRNA that base-pairs with the codon on the mRNA.

    • Amino Acid: Attached to the 3' end of the tRNA.

  3. Aminoacyl-tRNA Synthetase

    • Definition: Enzymes responsible for attaching the appropriate amino acids to their respective tRNAs.

    • There is one synthetase for each amino acid corresponding to a unique tRNA.

  4. Ribosomes

    • Composition: Composed of several proteins (20-50) and a few rRNAs (1-3).

    • Function: Acts as the site of translation. Ribosomal proteins primarily provide structural support, while some enhance the catalytic function of rRNA involved in peptide bond formation.

    • Large Subunit and Small Subunit: Each is assembled separately in the nucleus and exported to the cytosol where they combine during translation.

Ribosomal Structure and Function

  • mRNA Binding Site: Located on the small ribosomal subunit.

  • tRNA Binding Sites: Located on the large ribosomal subunit:

    • A Site (Aminoacyl-tRNA): The first site where new tRNAs attach; corresponds to the codon in mRNA.

    • P Site (Peptidyl-tRNA): The second site where the growing polypeptide is linked to the tRNA.

    • E Site (Exit): Third site where tRNA exits the ribosome after releasing its amino acid.

Process of Translation

  1. Initiation: Formation of the translation initiation complex involves:

    1. Initiation factors (proteins).

    2. mRNA molecule.

    3. Small ribosomal subunit.

    4. Initiator tRNA (AUG/Met).

    5. Large ribosomal subunit (at which point initiation factors are released).

    6. Hydrolysis of GTP by initiation factors (proteins/enzymes).

  2. Elongation: Process of adding amino acids:

    • Codon Recognition: Complementary base-pairing between mRNA codon and tRNA anticodon.

    • Peptide Bond Formation: Catalyzed by ribozymes, linking the growing amino acid chain by transferring it from the tRNA in the P site to the tRNA in the A site.

    • Translocation: The ribosome shifts to the next codon, which frees up the A site:

    1. The A-site tRNA moves to the P-site.

    2. The P-site tRNA moves to the E-site and exits the ribosome.

    • GTP hydrolysis by elongation factors is necessary during this phase.

  3. Termination: Concludes translation:

    • Stop Codon present in the A site activates a release factor (protein).

    • The release factor binds to the A site, triggering a hydrolysis reaction that releases the polypeptide from the tRNA.

    • This is catalyzed by rRNA (acting as a ribozyme).

    • Disassembly: Involves hydrolysis of two GTP molecules after translation is complete.

    • The resulting amino acid sequence represents the primary structure of the protein.

Post-Translational Modifications

  • Protein Folding: After translation, proteins undergo folding which includes:

    • Secondary, tertiary, and potential quaternary structures.

    • Chaperonins: Proteins that assist in correctly folding other proteins.

  • Post-translational Modifications: Include:

    • Phosphorylation: Addition of phosphate groups to certain amino acids.

    • Glycosylation: Addition of sugar molecules.

    • Addition of cofactors, other molecules, or ions as needed by the protein.

Protein Targeting in Eukaryotes

  • The process of directing proteins to their specific functional locations within the cell involves:

    • Two Types of Ribosomes:

    • Free Ribosomes: Float within the cytosol, producing cytosolic proteins.

    • Bound Ribosomes: Attached to the rough endoplasmic reticulum (ER), producing proteins directed to:

      • Endoplasmic reticulum (ER)

      • Mitochondria

      • Chloroplasts

      • Peroxisomes

      • Nuclear envelope

      • Golgi apparatus

      • Lysosomes

      • Plasma membrane

      • Secreted from the cell altogether.