DNA stands for deoxyribonucleic acid. DNA is a large molecule that holds genetic information inside a cell. It controls every aspect of your development and reproduction. The DNA makes up more than 80% of your cells. The DNA is stored in chromosomes. There are 46 chromosomes in each human cell. There are 23 pairs of these chromosomes, giving you 23 matching pairs. Some chromosomes are shorter than others. These are called heterochromatin. The heterochromatin makes up only about 5% of the chromosomes. Chromosomes are long, threadlike strands of DNA wrapped around a protein that contains histones. Histones hold the chromosomes together. Most of the chromosome is not composed of DNA. The histones provide space for the DNA and hold the DNA in place. Most of the DNA in your cells is stored in the nucleus. There is about 100 billion nucleotides of DNA in each person. A nucleotide is a unit of DNA. There are four DNA bases, adenine, cytosine, thymine, and guanine. Adenine, cytosine, thymine, and guanine are the four components that make up DNA. Each of these nucleotides comes in two varieties. The base has a hydrogen on one side and a carbon atom with an oxygen on the other side.