# The Genetic Code ## Nature of the Genetic Code - A **gene** is a sequence of nucleotides within a DNA molecule - Forms part of a larger DNA molecule that contains many genes - Codes for the production of a specific polypeptide (protein) - Protein molecules consist of amino acids bonded together in series - The shape and behaviour of a protein depends on the exact sequence of amino acids - The initial amino acid sequence = **primary structure** of the protein

Genes control protein structure and function by determining the exact sequence in which amino acids join together during protein synthesis

## The Triplet Code - The nucleotide base sequence in a gene is determined by a **triplet code** - Each sequence of three bases (triplet) codes for one amino acid - There are 20 different amino acids used by cells to make proteins ### Examples of triplet codes | DNA Triplet | Amino Acid Coded For | |-------------|---------------------| | CAG | Valine | | TTC | Lysine | | GAC | Leucine | | CCG | Glycine | - Some triplets act as **start and stop signals** - Start signal: TAC (codes for methionine) - Stop signals mark where genes end - These signals tell the cell where individual genes begin and end

Start and stop signals ensure the cell reads DNA correctly and produces the right amino acid sequences

### Non-overlapping code - The genetic code is **non-overlapping** - Each base is only read once as part of one codon - Bases aren't reused in multiple overlapping triplets ### Degenerate code - There are four bases in DNA, giving $4^3 = 64$ possible triplet combinations - But only 20 amino acids commonly occur in biological proteins - Therefore, the code is **degenerate**: multiple triplets can code for the same amino acid

The degenerate nature of the genetic code can limit the effects of mutations

### Universal code - The genetic code is **universal** - Almost every organism uses the same code - Only rare and minor exceptions exist - The same triplet codes specify the same amino acids in all living things - This means genetic information is transferable between species

The universal nature of the genetic code makes genetic engineering (transfer of genes between species) possible

## Codons and Anticodons - After mRNA forms and leaves the nucleus, it moves to ribosomes - Acts as a template for protein synthesis - Each triplet within the mRNA code = **codon** - tRNA molecules carry amino acids and possess **anticodons** - Anticodons are complementary to codons on mRNA - This ensures the correct amino acid is delivered to match each mRNA codon ### Relationship between DNA and mRNA | Feature | Details | |---------|---------| | DNA template strand | Used to create mRNA during transcription | | DNA coding strand | Has same base sequence as mRNA (except T instead of U) | | mRNA direction | Synthesized in 5' to 3' direction | | mRNA codons | Read in 5' to 3' direction during translation | ## Summary of Key Properties The genetic code is: 1. **Triplet-based** - three nucleotides code for one amino acid 2. **Non-overlapping** - each base read only once 3. **Degenerate** - multiple codons can code for the same amino acid 4. **Universal** - same code used by nearly all organisms These properties ensure: - Genetic information is accurately stored - Proteins are correctly synthesized - Genetic material can be shared between organisms (basis for genetic engineering) - Some protection against the effects of mutations