Fitting Inside a Cell

         DNA is a long molecule. For example, a typical bacterium, like E. coli, has DNA molecule with about six,000 genes (A gene is a specific sequence of DNA nucleotides that codes for a protein. We'll speak about this later). If drawn out, this DNA molecule would be about one millimeter long. However, a typical E. coli is only six microns long (six one-thousandths of a millimeter).So to fit inside the cell, the DNA is highly coiled and crooked in to circular chromosome.

        Complex organisms, like plants and animals, have 50,000 to 100,000 genes on lots of different chromosomes (humans have 46 chromosomes). In the cells of these organisms, the DNA is crooked around bead-like proteins called  histones. The histones are also coiled tightly to form chromosomes, which can be present 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 copies of each chromosome that get copied and each daughter cell receives copies (mitosis). In the coursework of meiosis, precursor cells have copies of each chromosome that gets copied and distributed equally to sex cells. The sex cells (sperm and egg) have copy of each chromosome. When sperm and egg unite in fertilization, the offspring have copies of each chromosome (see How Sex Works).