DNA is a double-stranded molecule that contains the genetic instructions for all living organisms. It is located in the cell nucleus and has a double helix structure. DNA is made up of nucleotides containing the bases adenine, thymine, guanine and cytosine which pair up in a specific way. Genes are segments of DNA that code for traits, with every person having two copies of each gene, one from each parent. Chromosomes are DNA molecules packaged into structures in the nucleus, with humans having 23 pairs of chromosomes in each cell. Sex is determined by the sex chromosomes, with females having two X chromosomes and males having one X and one Y chromosome.