Lecture: Features of the Genetic Code

Introduction

• Features of the genetic code crucial for AQA Biology: degenerate, universal, and non-overlapping.
• Focus on start and stop codons.

Start and Stop Codons

• Start Codon: First codon on DNA and mRNA, initiates translation.
  • Bases: TAC on DNA → AUG on mRNA, codes for Methionine (later removed).
• Stop Codon: Final three bases on DNA and mRNA.
  • Does not code for an amino acid.
  • Causes ribosome detachment, halting translation, marking completion of polypeptide synthesis.

Key Features of the Genetic Code

Degenerate

• 20 amino acids can be coded by the genetic code.
• Four DNA bases: Guanine (G), Cytosine (C), Thymine (T), Adenine (A).
  • Uses combinations of 3 bases, 4^3 = 64 combinations.
• Degenerate: More than one triplet code can code for the same amino acid.
  • E.g., Tyrosine coded by ATA and ATG.
• Advantage: Silent mutations → No effect if mutation occurs, same amino acid might be coded.

Universal

• Same triplet codes for the same amino acid in nearly all organisms.
• Enables genetic engineering (e.g., bacterial insulin production).

Non-overlapping

• Each base is part of only one triplet.
  • E.g., ACG, GCT, TCA.
• Advantage: Limits mutation impact to one amino acid, minimizing protein structure alteration.

Additional Concepts

Introns vs. Exons

• Introns: Non-coding DNA sections (over 90% of human DNA).
  • Not in mRNA, only in eukaryotic DNA.
• Exons: Sequences coding for amino acids.

Genome vs. Proteome

• Genome: Complete DNA set in a cell.
• Proteome: Full range of proteins produced in a cell.
• Genome is constant (barring mutation); proteome varies between cells and conditions.

Genetic Variation

• Human genomes: ~3 billion DNA base pairs.
• Bacteria: ~600,000 DNA base pairs.

Conclusion

• Understanding these features aids in grasping genetic code applications and implications.
• Reminder to engage with further content for deeper learning.