The nucleus of a cell contains chromosomes. Each chromosome is made of a long molecule of DNA, tightly coiled up so that it fits inside the tiny nucleus.
Each chromosome carries a large number of genes. A gene is a short section of DNA that holds the instructions for making one protein, so the genes on the chromosomes control how the cell and the whole organism develop.
In body cells the chromosomes are normally found in pairs. One chromosome of each pair came from the mother and the other came from the father. A human body cell has 23 pairs of chromosomes, which is 46 chromosomes in total.
