Fitting Inside a Cell
DNA is a long molecule. For example, a typical bacterium, like E. coli, has one DNA molecule with about 3,000 genes (A gene is a specific sequence of DNA nucleotides that codes for a protein. We'll talk about this later). If drawn out, this DNA molecule would be about 1 millimeter long. However, a typical E. coli is only 3 microns long (3 one-thousandths of a millimeter).So to fit inside the cell, the DNA is highly coiled and twisted into one circular chromosome.
Complex organisms, like plants and animals, have 50,000 to 100,000 genes on many different chromosomes (humans have 46 chromosomes). In the cells of these organisms, the DNA is twisted around bead-like proteins called histones. The histones are also coiled tightly to form chromosomes, which are located in the nucleus of the cell. When a cell reproduces, the chromosomes (DNA) get copied and distributed to each offspring, or daughter, cell. Non-sex cells have two copies of each chromosome that get copied and each daughter cell receives two copies (mitosis). During meiosis, precursor cells have two copies of each chromosome that gets copied and distributed equally to four sex cells. The sex cells (sperm and egg) have only one copy of each chromosome. When sperm and egg unite in fertilization, the offspring have two copies of each chromosome (see How Sex Works).
In the next section we'll look at how the DNA replication process works.