Topic 2 Molecular Genetics: Translation

Question 1

• The assembly of polypeptides based on reading of new RNA in the cytoplasm with GTP used as the energy source. In general, an amino acid attaches to the 3’ end of a tRNA (termed an aminoacyl-tRNA) and one 1 ATP is converted to AMP per amino acid added the polypeptide chain.

Answer 1

Note

Question 2

• The small ribosome subunit attaches to the 5’ end of mRNA; a tRNA methionine attaches to the start sequence of mRNA (AUG), and the large ribosomal subunit attaches to form a complete complex. Requires 1 GTP.

Answer 2

1. initation

Question 3

• Next tRNA binds to the A site, peptide bond formation occurs, and the tRNA without methionine is released. The tRNA currently in the A site moves to the P site (translocation) and the next tRNA comes into the A site to repeat the process. This requires 2 GTP per link.

Answer 3

2. Elongation

Question 4

• When the ribosome encounters the stop codon (either UAG, UAA, or UGA), the polypeptide and the two ribosomal subunits all release due to a release factor breaking down the bond between tRNA and the final amino acid of the polypeptide
• While the polypeptide is being translated, amino acid sequences are determining the folding conformation, which is a process that requires assistance from chaperone proteins and 1 GTP

Answer 4

3. Termination

Question 5

• Translation begins on a free floating ribosome; a signal peptide at the beginning of the translated polypeptide may direct the ribosome to attach to the endoplasmic reticulum, in which case the polypeptide is injected into the ER lumen. If injected, the polypeptide may be secreted from the cell via the Golgi apparatus.
• In general, post-translational modifications (addition of sugars, lipids, phosphate groups to the amino acids) may occur. However, the protein may be subsequently processed by the Golgi before it is functional
• Amino acids are placed starting from the 5’ end of the mRNA and move all the way down to the 3’ end. Corresponding tRNA codons are 3’ to 5’
• Translation can occur simultaneously with transcription in prokaryotes, but not in eukaryotes. Multiple ribosomes may, however, simultaneously translate 1 mRNA

Answer 5

4. Post-translation

Question 6

• The amino acid for start codons in eukaryotes is methionine, whereas in bacteria, the amino acid for start codons is n-formylmethionine
• One important phenomenon that occurs in our genetic code is that while it is universal for nearly all organisms, most amino acids have more than one codon specifying them, which is known as redundancy or degeneracy.

Answer 6

Note