Translation

Translation Processes Overview

• Key Components of Translation:

  • mRNA (messenger RNA): Provides the instructions for synthesizing polypeptides.

  • Ribosomes: Organelles that facilitate the synthesis of polypeptides based on the mRNA instructions.

  • tRNA (transfer RNA): Molecules that transfer individual amino acids to the growing polypeptide chain.

• Definitions:

  • Translation: The biological process involved in the synthesis of polypeptides by translating the codon sequence in mRNA.

Transfer RNA (tRNA)

• Structure:

  • Comprised of a single strand forming a three-dimensional structure.

  • Made of Nucleotides: Each tRNA is composed of ribonucleotides that connect together to create the tRNA molecule.

  • Internal Hydrogen Bonds: tRNA molecules form hydrogen bonds within themselves, leading to their characteristic folded structure.

  • Anticodon: The tRNA contains an anticodon that is complementary to an mRNA codon, allowing for accurate pairing during translation.

  • 3’ site: This site on the tRNA molecule binds a specific amino acid that corresponds to the anticodon.

• Anticodon-Codon Interaction:

  • The anticodon is the “inverse” of the codon found in mRNA, meaning it will pair according to base-pairing rules (A with U, C with G).

Example:

• Codon (from mRNA): 5’ … AUG … 3’

• Anticodon (from tRNA): 3’ … UAC … 5’

• Example Amino Acids:

  • Methionine (Met) – Start codon

  • Valine (Val)

  • Phenylalanine (Phe)

  • Glycine (Gly)

  • Leucine (Leu)

  • Glutamic acid (Glu)

  • Aspartic acid (Asp)

  • Alanine (Ala)

  • Serine (Ser)

  • Tyrosine (Tyr)

  • Cysteine (Cys)

  • Arginine (Arg)

  • Histidine (His)

  • Glutamine (Gln)

  • Lysine (Lys)

  • Threonine (Thr)

  • Tryptophan (Trp)

  • Proline (Pro)

Polypeptide Synthesis

• Initiation of Polypeptide Synthesis:

  • Ribosomal subunits create a complex to begin the polypeptide synthesis.

  • Initiator tRNA: Carries Methionine and is the first tRNA to bind to the ribosome.

  • Ribosome Binding Sequence: The mRNA binds to the small ribosomal subunit, establishing the correct reading frame for translation.

  • Large Ribosomal Subunit Attachment: The large ribosomal subunit attaches to the small one, completing the initiation complex after the initiator tRNA binds.

• Ribosome Sites:

  • E Site: Exit site for tRNAs that have donated their amino acid.

  • P Site: Peptidyl site where the polypeptide is held.

  • A Site: Aminoacyl site where the new tRNA with the next amino acid enters.

• Elongation Phase:

  • The tRNA brings its corresponding amino acid to the polypeptide being synthesized.

  • The next tRNA enters the A site, where its anticodon pairs with the mRNA codon.

  • The polypeptide attached to the tRNA in the P site will form a bond with the amino acid attached to the tRNA in the A site.

  • The ribosome moves along the mRNA, shifting the tRNAs left by one position, effectively allowing the empty tRNA in the E site to be released.

• Termination of Translation:

  • Release Factors: These proteins match the stop codon on mRNA during translation termination.

  • Release factors do not carry amino acids.

  • They facilitate the release of the completed polypeptide and cause the separation of the ribosomal subunits.

• Polypeptide Production Efficiency:

  • Multiple polypeptides can be produced from a single mRNA transcript.

  • In prokaryotic organisms, transcription and translation can occur simultaneously due to the lack of compartmentalization.

Mutations in Translation

• Single Point Mutations:

  • Definition: A change in one nucleotide of a DNA or RNA sequence.

  • Types of Point Mutations:

  • Substitutions: One base is substituted for another, which may or may not impact the resulting protein function.

  • Indels: Insertions or deletions of nucleotides which can result in frameshifts and potentially alter the entire downstream protein sequence.