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Biology revision – DNA, genes and protein synthesis (Unit 8)
Genes and the genetic code
What is a gene?
- A gene is a section of DNA located at a particular position, a locus, on a DNA
molecule
- Contains the coded info for making polypeptides and functional RNA
- The coded info is in the form of a specific sequence of bases along the DNA molecule
- Polys make up proteins and so genes determine the proteins of an organism
- As enzymes control chemical reactions, they are responsible for an organism’s
development and activities
- Genes, along with environmental factors, determine the nature and development of all
organisms
- The gene is a base sequence of DNA that codes for:
o Amino acid sequence of polypeptide
o Or a functional RNA – includes ribosomal and transfer RNA
- One DNA molecule carries many genes
The genetic code
- Scientists suggested there must be a minimum of 3 bases that coded for each amino
Reasons are as follows:
- Only 20 different aminos regularly occur in proteins
- Each amino must have its own code of bases on the DNA
- Only 4 different bases (A, G, C, T) are present in DNA
- If each base coded for a different amino, only 4 different aminos could be coded for
- Using a pair of bases, 16 (42) different codes are possible – still inadequate
- Three bases produce 64 (43) different codes – satisfy requirements of 20 aminos
- As the code has 3 bases for each amino, each one is called a triplet
- As there are 64 possible triplets and only 20 aminos, it follows that some aminos are
coded for by more than one triplet
Features of genetic code
Further experiments revealed the following features of the genetic code:
- A few aminos are coded for by only a single triplet
- The remaining aminos are coded for by between 2 and 6 triplets each
- Code is known as a ‘degenerate code’ as most aminos are coded for by more than
one triplet
- Triplet is always read in one direction along the DNA strand
- The start of a DNA sequence that codes for a polypeptide is always the same triplet
o This codes for the amino methionine
o If this first methionine molecules doesn’t form part of the final polypeptide, it is
later removed
- 3 triplets don’t code for any amino
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