DNA is wound around proteins called histones, allowing both the DNA and histones to be packaged and tightly condensed to form chromosomes.
DNA is a very large molecule. Yet cells are very small. Thus, DNA in cells is very tightly condensed. In eukaryotes and prokaryotes, DNA is condensed into structures called chromosomes. A chromosome is a tightly condensed molecule of DNA and proteins. A histone is a collection of proteins in a chromosome around which DNA winds. There are several types of histones in the human body. Chromosomes are only visible when the cell is preparing for cell division. The DNA winds several times around each nucleosome. A nucleosome consists of a set of 8 histone proteins around which a short region of DNA double-stranded helix is wound, and the short region of DNA that links it to the next nucleosome. A linear strand of chromatin is only one-third the length of the unwound, naked double-stranded DNA helix and represents the first level of DNA condensation in the cell. At this level of packing, the chromatin fiber resembles beads on a string.
DNA is further condensed by another protein called histone H1. It pulls adjacent nucleosomes together, folding the beads-on-a-string structure back and forth over itself. The packaged DNA that results when the DNA double helical structure is wrapped around histone proteins is called chromatin. Chromatin is further condensed by folding into loops. The mechanism responsible for folding loops of chromatin is still not fully understood. The final result of chromatin packaging is the supercoiled, compact DNA that makes up chromosomes.
During most phases of the cell cycle, DNA is not packed into condensed chromosomes. It remains more loosely coiled. During the M phase, the phase in which the cell divides and separates, the chromosomes condense. During the S phase of the cell cycle, each chromosome is replicated, forming two identical sister chromatids. Each X-shaped chromosome is composed of two sister chromatids. A chromatid is one of two identical halves of a replicated chromosome. They are joined at a centromere, a point on the chromosome that attaches spindle fibers during cell division. The spindle fibers will eventually separate the sister chromatids and pull them into the daughter cells formed by cell division.
The total number of chromosomes in a cell is dependent on the organism. Humans have 23 pairs of chromosomes, for a total of 46 chromosomes. The number and appearance of the chromosomes of an organism is its karyotype.
In prokaryotes, DNA is not packaged into chromosomes in the same way eukaryotic DNA is. Prokaryotes lack a nucleus, so their DNA does not exist inside a distinct organelle. Instead, prokaryotic DNA exists in a nucleoid, a specific region of the cell that contains genetic material. This DNA usually is found in a large, supercoiled loop. In addition, a prokaryote may have one or more plasmids. A plasmid is a small, circular piece of bacterial DNA that replicates on its own and can be transferred between cells. Some prokaryotes contain several plasmids. Both the chromosome and the plasmids make up a prokaryote's genome, the entire genetic sequence of an organism.