The Genetic Code (Leaving Cert Biology): Revision Notes
📚 Revision Notes
The Genetic Code
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The Genetic Code: The sequence of DNA bases determines the sequence of amino acids which determines the proteins made.
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Recap of DNA Bases:
The four nitrogen-containing bases in DNA are:
- Adenine (A)
- Guanine (G)
- Cytosine (C)
- Thymine (T)
Base Pairing Rules:
- Adenine (A) always pairs with Thymine (T).
- Cytosine (C) always pairs with Guanine (G).
Hydrogen Bonds:
- A and T are joined by 2 hydrogen bonds.
- C and G are joined by 3 hydrogen bonds.
What is the Genetic Code?
- Genes are made up of long sequences of DNA bases.
- A gene causes a specific sequence of amino acids to be arranged to form a protein.
- The genetic code ensures that the correct amino acids are assembled in the correct order to form the protein.
How the Genetic Code Works:
- The code operates as a sequence of 3 DNA bases, called a triplet or codon.
- Each triplet codes for a specific amino acid.
- Example:
- The triplet CAA codes for the amino acid valine.
- The triplet CGA codes for the amino acid alanine.
- Many triplets together form a gene.
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Analogy: The Genetic Code and Language
- The 4 DNA bases (A, T, C, G) are like the letters of the alphabet.
- Each triplet (codon) forms a word.
- A sequence of triplets forms a gene, which is like a paragraph giving instructions for how to make a protein.
- All the genes in a cell (the genome) form the book of life for that cell.
Non-Coding DNA
- Up to 97% of DNA in human cells does not code for the production of proteins. This is called non-coding DNA.
- Non-coding DNA was once called 'junk' DNA because its purpose was unknown.
- Scientists now know that non-coding DNA plays important roles, such as regulating the expression of genes (acting like a switch to turn genes 'on' or 'off').