The Genetic Code and Translation

Translation & the Genetic Code Study Notes

Key Questions and Answers

  • Question 1:

    • The 3’ end of an mRNA molecule corresponds to the carboxyl terminus of the polypeptide it encodes.

  • Question 2:

    • If you align a polypeptide sequence next to the gene that encodes it, the amino terminus of the polypeptide would line up closest to the promoter.

  • Question 3:

    • 3’ UTR and 5’ UTR Definitions:
      • UTR (Untranslated Region): Portions of the mRNA that are not translated.
      • The 5’ UTR is the region between the 5’ end and the start codon.
      • The 3’ UTR is the region after the stop codon and until the 3’ end.

  • Question 4:

    • The part of a tRNA that forms base pairs with mRNA is the anticodon.

  • Question 5:

    • During translation, amino acids are added to the carboxyl-terminal end of the growing polypeptide.

  • Question 6:

    • If you isolate the DNA of a gene and want to predict the type of protein encoded, you will see the codons for the amino acids of the polypeptide in the sense strand of the DNA sequence.

  • Question 7:

    • Given bacterial mRNA sequence: 5'-AUGUUUAAGUAAGGAGGUGACAUGCUAGGAUUUUGUUAGCCAUAA-3'
    • A. The open reading frame (ORF) starts with the 21st base:
      • ORF: 5’-AUG/CUA/GGA/UUU/UGU-3’ (The specific AUG is the start of the open reading frame.)
    • B. The amino acid sequence produced from this mRNA is:
      • Amino Acid Sequence: Amino-met-leu-gly-phe-cys-carboxyl

  • Question 8:

    • One possible mRNA open reading frame sequence to encode this polypeptide:
      • Open Reading Frame Sequence: 5’-AUGUUUCUACUCUGA-3’ (indicate the 5’ and 3’ ends and include the stop codon)
    • Reason: There are multiple correct answers due to the degeneracy in the genetic code (more than one codon sequence per amino acid and more than one stop codon).

  • Question 9:

    • An mRNA sequence contains AUG in many places; however, only one is the correct start codon.
      • In Bacteria: The ribosome locates the correct AUG using the Shine-Dalgarno sequence to position the small subunit at the start codon.
      • In Eukaryotes: The small subunit binds at the 5’ cap and scans downstream to the first AUG.

  • Question 10:

    • In an aminoacyl tRNA (charged tRNA), the amino acid is attached to the 3’ end of the tRNA.

  • Question 11:

    • At the initiation of translation, the start codon is located in the P site of the ribosome.

  • Question 12:

    • During elongation in translation, the aminoacyl tRNA enters the A site of the ribosome.

  • Question 13:

    • For the mRNA codon sequence 5’-CGA-3’, the corresponding anticodon in tRNA is:
      • Anticodon: 3’-GCU-5’.

  • Question 14:

    • Possible sequences in the anticodon loop of a tRNA that has proline (abbreviated pro) attached:
      • 3’-GGA-5’
      • 3’-GGG-5’
      • 3’-GGU-5’
      • 3’-GGC-5’

  • Question 15:

    • The process by which a bacterial ribosome assembles onto the mRNA to initiate translation involves:
      • The Shine-Dalgarno sequence positioning the small ribosomal subunit at the AUG start codon.

  • Question 16:

    • Differences between eukaryotic and bacterial translation:
      • Eukaryotic mRNA has the Kozak sequence surrounding the start codon while bacterial mRNA has the Shine-Dalgarno sequence.
      • Eukaryotic translation does not use N-formylmethionine (f-met).
      • Translation occurs in a separate compartment (cytoplasm, or on the rough endoplasmic reticulum) as opposed to in bacteria where transcription and translation can occur concurrently.
      • Eukaryotic initiation factors bind to the 5’ cap, assisting in the small ribosomal subunit binding.
      • In eukaryotes, the process of start codon selection is different; the small subunit scans to find the first AUG after the 5’ cap.
      • PolyA binding proteins bind to the polyA tail, interacting with initiation factors to help the 40S ribosomal subunit bind.

  • Question 17:

    • An example demonstrating the degeneracy of the genetic code includes:
      • Codons CCC and CCU, both encoding proline. Any two codons that encode the same amino acid illustrate this concept.

Key Terms

  • Translation: Process of synthesizing proteins from mRNA.
  • Protein: Long chains of amino acids that perform various functions in the body.
  • Amino Acid: Organic compounds that serve as the building blocks of proteins.
  • Polypeptide: A polymer of amino acids linked by peptide bonds.
  • R group: Variable side chain that defines the characteristics of different amino acids.
  • Peptide Bond: A covalent bond formed between amino acids.
  • Amino Group: The -NH₂ group of an amino acid.
  • Carboxyl Group: The -COOH group of an amino acid.
  • Primary Structure: The sequence of amino acids in a polypeptide chain.
  • Secondary Structure: Local folded structures that form within a polypeptide due to interactions between atoms of the backbone.
  • Tertiary Structure: The overall three-dimensional structure of a polypeptide.
  • Quaternary Structure: The structure formed by the combination of multiple polypeptide chains.
  • 3’ UTR: The untranslated region at the 3’ end of mRNA.
  • 5’ UTR: The untranslated region at the 5’ end of mRNA.
  • Genetic Code: The set of rules that defines how codons specify which amino acids will be added during protein synthesis.
  • Triplet Code: The code in which three nucleotides correspond to one amino acid.
  • Start Codon: The codon (AUG) that signals the start of translation.
  • Stop Codon: Codons that terminate the translation process.
  • Degenerate Genetic Code: The principle that multiple codons can code for the same amino acid.
  • Reading Frame: The way in which the nucleotides in mRNA are read in triplets (codons).
  • tRNA (transfer RNA): Type of RNA that carries amino acids to the ribosome for protein synthesis.
  • Anticodon Loop: The region of tRNA that is complementary to the mRNA codon.
  • Acceptor Arm: The region of tRNA that attaches to the amino acid.
  • Ribosome: The cellular machinery responsible for protein synthesis.
  • Small Subunit/Large Subunit: The two components of the ribosome responsible for translating mRNA into protein.
  • Ribosomal Protein/Ribosomal RNA: Components of the ribosome that help synthesize proteins.
  • A Site/P Site/E Site: The three sites on the ribosome where tRNA molecules bind during translation.
  • Initiation of Translation: The phase where the ribosome assembles around the mRNA to begin translating it into proteins.
  • Shine-Dalgarno Sequence: The ribosomal binding site in bacterial mRNA.
  • Initiator tRNA: The tRNA molecule that carries the first amino acid of the polypeptide chain (typically fMet in bacteria).
  • Elongation Phase of Translation: The phase where the polypeptide chain is elongating as the ribosome moves along the mRNA.
  • Translocation: The process where the ribosome moves along the mRNA during elongation.
  • Termination of Translation: The process in which the completed polypeptide is released from the ribosome when a stop codon is reached.
  • Release Factor: A protein that helps terminate translation by recognizing stop codons.
  • Kozak Sequence: A sequence that surrounds the start codon in eukaryotic mRNA that helps initiate translation.
  • Initiation Factor: Proteins that help the ribosome bind to the mRNA and start translation.
  • Poly A Binding Protein: A protein that binds to the poly A tail of eukaryotic mRNA and assists in translation initiation.

Note

  • These notes cover critical aspects of translation and the genetic code, with detailed answers and definitions relevant for thoroughly understanding the concepts discussed.