Overview
This lecture explains how to read codon charts to determine which amino acids are coded for by mRNA codons during protein synthesis.
Codons and Their Importance
- mRNA bases are read in groups of three called codons.
- Each codon corresponds to a specific amino acid brought in by tRNA.
- Understanding codons helps explain how genetic information leads to traits in all living organisms.
- Changes in amino acids due to codon changes can alter traits.
Using Codon Charts
- The two most common codon chart types are rectangular and circular charts.
- Always use the mRNA codon, not the tRNA anticodon or DNA template, with codon charts.
- In a rectangular chart: first base (left side), second base (top), third base (right side).
- In a circular chart: start from the center and move outward for first, second, and third bases.
Example Codon Chart Readings
- AUG (mRNA codon) → Anticodon: UAC → Amino acid: Methionine (start codon).
- CCA → Anticodon: GGU → Amino acid: Proline.
- GUC → Anticodon: CAG → Amino acid: Valine.
- UUC → Anticodon: AAG → Amino acid: Phenylalanine.
- UAA → Anticodon: AUU → “Stop” codon; signals end of polypeptide chain.
Common Pitfalls in Reading Codon Charts
- Mistaking the mRNA codon for the tRNA anticodon or DNA template.
- Struggling with rectangular charts—go slowly and isolate each base carefully.
- Not practicing enough with different codon chart types.
Practice Questions & Answers
- Lysine codons (from circular chart): AAA and AAG.
- Serine codons (from rectangular chart): UCU, UCC, UCA, UCG, AGU, and AGC.
Key Terms & Definitions
- Codon — a sequence of three mRNA bases that codes for an amino acid.
- Anticodon — a three-base sequence on tRNA that pairs with the mRNA codon.
- Amino Acid — building block of proteins, specified by codons.
- Start Codon — AUG; signals the start of translation.
- Stop Codon — UAA, UAG, UGA; signals end of protein synthesis.
Action Items / Next Steps
- Download and complete the video companion handout for additional practice.
- Practice identifying amino acids using both rectangular and circular codon charts.