Genes are segments of DNA that code for proteins and RNA. DNA is organized into chromosomes, which are made up of DNA and proteins. Humans have 46 chromosomes in each cell organized into 23 pairs. One chromosome of each pair is inherited from the mother and one from the father. DNA is packaged into chromosomes and chromatin in the cell nucleus. Chromosomes replicate and separate during cell division, forming two identical copies or chromatids connected at the centromere. Changes or mutations in genes that regulate the cell cycle can lead to uncontrolled cell growth and cancer if they cause cells to divide uncontrollably or fail to die.