Chromosomes

DNA is stored in the nucleus as chromosomes. Each species has a different number of chromosomes, humans have 46. They come in pairs that are called homologous, which means they are the same size and have the same genes in the same locations. There is a pair of each chromosome because each parent gives one.

A cell that has two of each chromosome is referred to as diploid (or 2N). The cells that are a part of functional tissue within an orgnamism are always diploid. These body cells (aka somatic cells) are clones of the zygote, produced through mitosis, so all of the chromosomes are identical. They can become specialized cells and tissue because each kind of cell expresses different genes.

Only gametes (sex cells) have a different number of chromosomes. Human gametes (sperm and egg) have only one of every chromosome, for a total of 23. Gametes are referred to as monoploid (or N) cells, and when they combine with another gamete (fertilization), the resulting zygote is diploid.

Most of the time, the chromosomes are only partially coiled up so that the cell's machinery can read the code. This form of organization is called chromatin. If the cell is about to undergo mitosis, though, the chromosomes become super-coiled into structures called chromatids. This allows the chromosomes to move around the cell without getting tangled, and the chromatids can be viewed under a microscope.

Chromatin: good for reading and controlling cell functions, bad for moving around during mitosis (gets tangled with others)

Chromatids: bad for reading (information is trapped inside coiled-up DNA), good for moving around during mitosis