Genetic Code and Translation

Focus on finishing Chapter 14, which is crucial for the upcoming exam.
Review of Chapters 11 to 14 will be conducted.
Plans for lab completion between the current session and next (Monday).
Expectation of an exam review session and third lab exam taking place two weeks from today, after Thanksgiving break.

Participation opportunity is scheduled for the end of the class to better manage time.
A brief review will be conducted to consolidate prior material.

mRNA Reading Direction: Always read in the 5' to 3' direction, not tRNA.
- Codon AUG is the starting point - always signals methionine as the first amino acid in polypeptide chains.
Codons and Stop Codons:
- There are 64 codons in total.
- 61 of these code for amino acids while 3 are stop codons that do not code for amino acids.
- Importance of the codon table for reference during examinations; memorization of the table is not necessary.
Redundancy in Amino Acid Coding:
- Many amino acids are represented by multiple codons, showing redundancy in the genetic code.
- Leucine has 6 codons.
- Valine, Serine, Proline, Threonine, Alanine, Arginine, Glycine each have 4 codons.
- Isoleucine has 3 codons.
- Methionine and Tryptophan are unique as they have only one codon each.
Universal Genetic Code:
- This genetic code is nearly consistent across all organisms.

Regions in mRNA:
- Example gene has regions labeled a, b, c, d, e, where b and d are introns (non-coding) and a, c, e are exons (coding).
- Mature mRNA retains only the exons after processing: a, c, e. Introns are spliced out in the final product.
Impact of Mutation on Splicing:
- If splicing fails to remove an intron, the result may be a nonfunctional or aberrant protein, potentially resulting in premature termination of translation if a stop codon appears within the intron.
Translation Process:
- Involves mRNA, tRNA, and rRNA.
- Codons in mRNA are matched by tRNAs that deliver specific amino acids.

Structure of tRNA: Each tRNA has:
- A binding region for one specific amino acid.
- An anticodon complementary to the mRNA codon.
Wobble Base Pairing:
- Allows fewer tRNAs to be utilized due to multiple codons coding for one amino acid, especially in the third base position.
Charging of tRNA:
- Process by which the correct amino acid is attached to its tRNA by a specific enzyme.

Ribosome Composition:
- Comprised of a large and small subunit, each made of ribosomal RNA (rRNA) and proteins.
- Eukaryotic ribosomes: Large subunit has 3 rRNA molecules and 49 proteins; Small subunit has 1 rRNA molecule and 33 proteins.
Ribosome's Role in Translation:
- Holds mRNA and tRNA in position for peptide bond formation.
- Does not regulate protein correctness; solely relies on the fidelity check via the small subunit.
Translation Steps:
1. Initiation: Small subunit finds the start codon (AUG) aided by initiation factors.
2. Elongation: tRNAs bring amino acids based on codon recognition, forming peptide bonds.
- Flow of the process: from A site (aminoacyl site) to P site (peptidyl site) to E site (exit site).
1. Termination: Triggered when a stop codon enters the A site, leading to disassembly of the ribosome.

Protein Targeting:
- Ribosomes can be free in cytosol or bound to the rough ER.
- Signal Sequences: Indicate if a protein should be directed to the rough ER or elsewhere in the cell.
- Proteins may undergo modifications post-translation (e.g., glycosylation, phosphorylation, proteolytic cleavage).
Nuclear Localizing Signal (NLS):
- A specific sequence allowing proteins to localize to the nucleus (
- Example: Proline, Proline, Lysine, Lysine, Lysine, Arginine, Lysine, Valine).

Experiment on NLS: Shows the necessity and sufficiency of the NLS in ensuring proper localization of proteins to the nucleus.

This concludes the review of Chapter 14. Prepare for participation activity at the end of class.