We developed from a single cell, the zygote. After many rounds of mitosis, we are the result. Barring genetic change such as mutations, each of our diploid cells are identical to each other and to the zygote from which we developed. This means that every cell contains the genes for hemoglobin production, but only red blood cells contain hemoglobin. Many genes are active in only certain cells, and for only certain periods of time.
Sexual reproduction involves meiosis - two consecutive cell divisions.
When a diploid cell with two copies of each chromosome divides by meiosis, four haploid cells result, each cell bearing only one copy of each chromosome. Haploid cells can fuse in fertilization, forming a new diploid cell. Genetic information is stored in chromosomes, which are composed, in part, of DNA molecules - each chromatid contains one DNA molecule. The complex formed by DNA and proteins is called chromatin . Chromatin is found in packages called chromosomes .