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Subject - cell biology, genetics
Chromosomes are a form of organization for eukaryotic genetic information in the nucleus; humans have 23 chromosomes in a haploid set. All together, human nuclear DNA consists of 3,000 mbp1). Each chromosome is composed of a double-stranded DNA molecule and a variety of proteins, including . The length of a chromasomal DNA strand is about 5 cm. This length can be shortened by orders of magnitude through adoption of a more compact structural arrangement. These condensed DNA-protein complexes are present in the metaphase chromosome. At the beginning of mitosis each chromosome consists of two chromatids held together at the centromere. The chromatids each contain an identical DNA double helix. In addition, there is extrachromosomal DNA, which is found in mitochondria, in the chloroplasts of plants, and in bacterial plasmids.
|1)||mbp: mega base pairs, 1,000,000 base pairs|
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Cell reproduction is based on the principle of cell division. In order to divide a cell into two identical daughter cells, all of the genetic material and all cell components must be duplicated. Animated illustrations demonstrate the cell cycle responsible for this, as well as illustrating cell division by mitosis and meiosis.