DNA, genes and protein synthesis · Year 11
DNA is the chemical that carries the instructions for making a living thing. It is a long molecule made of two strands twisted into a double helix. A gene is a short section of DNA that holds the code for one protein. Cells read the code in two stages, transcription and translation, to build proteins from amino acids.
Key facts
- DNA is a polymer made of repeating units called nucleotides. Each nucleotide has a phosphate, a sugar (deoxyribose) and one of four bases: A, T, C or G.
- The two strands are held together by pairs of bases: A always pairs with T, and C always pairs with G. These are complementary base pairs.
- A gene is a small section of DNA on a chromosome. Its sequence of bases codes for a particular sequence of amino acids, which makes a specific protein. The genome is all the genetic material of an organism.
- Three bases (a codon) code for one amino acid. The order of the codons decides the order of the amino acids in the protein.
- Transcription happens in the nucleus: the DNA of a gene unzips and a complementary copy is made as messenger RNA (mRNA). In RNA the base U (uracil) takes the place of T.
- Translation happens at a ribosome: the ribosome reads the mRNA three bases at a time, and tRNA molecules bring the matching amino acids, which are joined into a chain. The chain folds into the protein’s shape.
- A mutation is a change in the base sequence. Most have no effect on the protein; some change its shape, so an enzyme’s active site or a structural protein may no longer work.