Human chromosomes, coloured light micrograph. Each chromosome comprises two identical chromatids linked at a region called the centromere. The centromere also divides each chromatid into long and short arms. Chromosomes are found in the nuclei of cells. They only appear like this during cell division. Chromosomes contain the DNA (deoxy ribonucleic acid) that provides the instructions for the growth, development and functioning of the organism. Most cell nuclei contain 46 chromosomes, 23 inherited from each parent. Sex (sperm and egg) cells contain only 23. Magnification: x4000 at 6x4.5cm size.