DNA is a long polymer of nucleotides and encodes the sequence of the amino acid residues in proteins using the genetic code, a triplet code of nucleotides. In complex cells (eukaryotes), such as those from plants, animals, fungi and protists, most of the DNA is located in the cell nucleus. By contrast, in simpler cells (prokaryotes) such as bacteria, DNA is not separated from the cytoplasm by a nuclear envelope. The energy-generating organelles known as chloroplasts and mitochondria also carry DNA. DNA is often referred to as the molecule of heredity as it is responsible for the genetic propagation of most inherited traits. These traits can range from hair colour to disease susceptibility. During cell division, DNA is replicated and can be transmitted to offspring during reproduction. Lineage studies can be done based on the facts that the DNA in mitochondria (mitochondrial DNA) only comes from the mother, and the male "Y" chromosome only comes from the father. Every person's DNA is inherited from both parents. The mother's mitochondrial DNA together with twenty-three chromosomes from each parent combine to form the genome of a fertilized egg. As a result, with certain exceptions such as red blood cells, most human cells contain 23 pairs of chromosomes, together with mitochondrial DNA inherited from the mother.