Transcription and Translation

Codons

Codons are instructions for making proteins.

• A codon is a sequence of 3 RNA bases on mRNA

• Each codon codes for one amino acid or a stop signal

• Codons are read during translation at the ribosome

Key Rules to Remember

• Codons are read on mRNA, not DNA

• RNA bases:

  • A, U, C, G

• Bases are read in groups of three, starting at a start codon

Important Codons

• Start codon: AUG

  • Codes for Methionine (Met)

  • Signals where translation begins

• Stop codons: UAA, UAG, UGA

  • Signal where translation ends

  • Do not code for an amino acid

How to Read a Codon Chart (Step-by-Step)

A codon chart shows which amino acid each mRNA codon codes for.

Step-by-Step Example

Codon: AUG

1. First letter → find A on the left side of the chart

2. Second letter → find U across the top

3. Third letter → find G on the right side

4. The box where they meet tells you the amino acid:
   → Methionine (Met)

• Amino acid: Phenylalanine (Phe)

Important Reminder

• Codon charts are written for mRNA

• If given DNA, convert it to mRNA first before using the chart

Practice example

Codon: UUU

• First letter: U

• Second letter: U

• Third letter: U

What Is a Codon Wheel?

A codon wheel is another way to determine which amino acid a codon codes for.

• Like a codon chart, it is used for mRNA codons

• It is read from the center outward

• Each ring represents one base of the codon

Codon Wheel Rules

• Always start with the 1st base in the center

• Move outward to the 2nd base

• End with the 3rd base on the outside

• The final section names the amino acid

Practice

Example Codon: AUG

1. Start in the center of the wheel
   → Find A

2. Move to the second ring
   → Find U

3. Move to the outer ring
   → Find G

4. The section you land on tells you the amino acid:
   Methionine (Met)