Eukaryotic DNA Packaging

Eukaryotic chromosomes contain an enormous length of DNA relative to the size of the nucleus and therefore must be tightly packed in order to be manageable. Chromosomes are made of chromatin, which is a complex of DNA and protein. In each chromosome, a very long piece of DNA is wrapped around proteins called histones, forming bead-like units called nucleosomes. Prior to cell division, DNA replication takes place. After DNA replication, the chromatin is highly condensed and the chromosomes become shorter and thicker. Each duplicated chromosome consists of two sister chromatids, attached to each other by a protein complex called cohesins. The centromere acts as the link between two sister chromatids. In the later stages of cell division, the sister chromatids will separate into two new nuclei. Remember, once the sister chromatids separate, they are no longer called sister chromatids, but become individual chromosomes.

Every eukaryotic species exhibits a typical number of chromosomes. Human somatic cells contain 2 sets of chromosomes, making 46 chromosomes in total. However, human reproductive cells, or gametes, contain only one set of 23 chromosomes.

Three panels. Top panel is a duplicated chromosome with arms connected at the mid point. Area of one arm is zoomed up in second panel below. Many blue spheres are wrapped by a thin yellow string to form a tightly compacted thicker string that loops. Part of the yellow string is zoomed up in the third panel below. Third panel shows cartoon of double stranded DNA.

Figure 1: DNA packaging. Chromosome (top), chromatin coiled around histones forming nucleosomes (middle), DNA double helix (bottom).

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