Basics of the Genetic Code

• The genetic code is the set of rules by which mRNA sequences are translated into proteins.
• mRNA is made of four bases: Adenine (A), Uracil (U), Cytosine (C), and Guanine (G).
• These bases are read three at a time in units called codons.

Codons

• A codon is a sequence of three mRNA bases that corresponds to one amino acid or a signal.
• There are 64 codons (4³ combinations).
• Some amino acids have more than one codon (redundancy).

Start and Stop Codons

• The Start codon is AUG – it codes for Methionine (Met) and signals the start of translation.
• Stop codons do not code for any amino acid. They signal the end of translation:
  • UAA
  • UAG
  • UGA

Reading Frame and Codon Identification

To read an mRNA sequence:

1. Start scanning from the 5’ end until you find the first AUG.
2. Then read every 3 bases after that as a codon.
3. Continue until a stop codon is encountered.
4. Translation only happens in one reading frame.

How to Translate a Random Sequence

If given a sequence (e.g., bases 671 to 700), follow these steps:

Step 1: Count how many total bases

• Example: 700 – 671 + 1 = 30 bases

Step 2: Find if the sequence starts in the middle of a codon

• Divide the starting base number (e.g., 671) by 3
• Do 671 ÷ 3 = 223 remainder 2

🔹 Remainder 0 → starts at beginning of a codon.

🔹 Remainder 1 → skip 2 bases (start from base 673)

🔹 Remainder 2 → skip 1 base (start from base 672)

Step 3: After adjusting, divide the remaining sequence into groups of 3 (codons)

🔬 Example

Given: Bases 671 to 700:

C U C G A G U U G C A U A A C C G U G A A G U G C A G A G U U

Step 1: 30 bases

Step 2: 671 ÷ 3 = remainder 2 → skip 1 base (start from base 672)

U C G A G U U G C A U A A C C G U G A A G U G C A G A G U U

Step 3: Divide into codons:

• UCG → Ser
• AGU → Ser
• UGC → Cys
• AUA → Ile
• ACC → Thr
• GUG → Val
• AAG → Lys
• UGC → Cys
• AGA → Arg
• GUU →Valine

✅ Amino acids:

Ser - Ser - Cys - Ile - Thr - Val - Lys - Cys - Arg - Val

Written by Kasiban Parthipan