The Genetic Code
Understanding how DNA sequences are translated into proteins through the universal language of life
1. The Concept of a Gene
What is a Gene?
A gene is a specific sequence of nucleotides in DNA that serves as the fundamental unit of heredity. Each gene contains the instructions for building one or more functional products, typically proteins.
Key Definition
Gene: A segment of DNA that contains the coded instructions for synthesizing a specific polypeptide chain (protein) or functional RNA molecule.
Components of a Gene:
- 1Promoter Region: Signals where transcription begins
- 2Coding Sequence: Contains exons (expressed) and introns (removed)
- 3Terminator: Signals where transcription ends
- 4Regulatory Elements: Control when and how much protein is made
3D Visualization: Gene Structure on DNA Helix
The Central Dogma of Molecular Biology
DNA → Transcription → mRNA → Translation → Protein
The genetic code is the set of rules by which information encoded in mRNA sequences is converted into amino acid sequences (proteins).
2. Triplet Nature of the Genetic Code
Codons: The Three-Letter Words
The genetic code is read in groups of three nucleotides called codons. Each codon specifies a particular amino acid or a stop signal during protein synthesis.
Why Triplets?
With 4 nucleotide bases (A, U, G, C in mRNA):
- Singlets (4¹ = 4): Only 4 combinations - not enough for 20 amino acids
- Doublets (4² = 16): Only 16 combinations - still not enough
- Triplets (4³ = 64): 64 combinations - more than enough for 20 amino acids!
Mathematical Basis
4 bases × 4 bases × 4 bases = 64 possible codons
These 64 codons code for 20 amino acids + 3 stop signals
Codon Combinations Chart
The Codon Table (mRNA Codons)
| First Base | Second Base | Third Base | |||
|---|---|---|---|---|---|
| U | C | A | G | ||
| U | UUU Phe | UCU Ser | UAU Tyr | UGU Cys | U |
| UUC Phe | UCC Ser | UAC Tyr | UGC Cys | C | |
| UUA Leu | UCA Ser | UAA STOP | UGA STOP | A | |
| UUG Leu | UCG Ser | UAG STOP | UGG Trp | G | |
| C | CUU Leu | CCU Pro | CAU His | CGU Arg | U |
| CUC Leu | CCC Pro | CAC His | CGC Arg | C | |
| CUA Leu | CCA Pro | CAA Gln | CGA Arg | A | |
| CUG Leu | CCG Pro | CAG Gln | CGG Arg | G | |
Note: This is a partial codon table. STOP codons (UAA, UAG, UGA) signal the end of translation.