Figure 1. Elements of the chromosome. Simple representation of a metacentric eukaryotic chromosome during mitosis. The ends of the linear chromosome are capped by telomeres (from the Greek telos, meaning the end), chromosomal elements that are essential for maintaining the integrity of the genetic material and for preventing fusions between the ends of different chromosomes. The primary constriction marks the position of the centromere. This is the site where the sister chromatids remain attached to one another until anaphase and where the chromosome attaches to the mitotic spindle through a structure known as the kinetochore. In many organisms the centromere is also a site of heterochromatin (black). In many vertebrates the chromosome arms can be differentiated into dark or lightly-staining chromosome bands (hatched and white areas, respectively) using a variety of staining techniques. Replication origins are scattered along the length of the chromosome arms. DNA in different chromosome bands is replicated at different times.

Figure 2. Chromatin packaging within the chromosome. The length of a DNA molecule is shortened by 10 000-fold during the formation of a metaphase chromosome. This is brought about by levels of chromatin packing, built one upon another. The first level of packing is brought about by the wrapping of DNA around nucleosomes to form a structure that has been likened to beads on a string. A folding or coiling of this level of chromatin together with additional chromosomal proteins, such as linker histones, produces the 30-nm diameter chromatin fibre. It is still not certain how chromatin is compacted beyond this stage. The diagram illustrates one particular model, known as the radial loop/scaffold model. In this model it is envisaged that the 30-nm fibre is arranged into loops containing ~100 kb of DNA. These loops are anchored at their bases to the chromosome scaffold. Chromatin is finally condensed both by a shortening in length of the chromosome scaffold and by a twisting of lateral loops in toward the chromosome axis/scaffold.

Figure 3. Natural and artificial human chromosomes: spread metaphase chromosomes from a human cell line that also harbours an artificial chromosome. In the left panel the chromosomal DNA is stained in blue and hybridized with a probe for centromeric alpha-satellite DNA (red) from the acrocentric chromosomes nos 13 and 21. This probe also lights up a small chromosome (arrowed). The green signal superimposed on the red on this small chromosome demonstrates it to be derived from a yeast artificial chromosome-based artificial chromosome. The size of this mammalian artificial chromosome (MAC) is more easily seen in the right panel where the DNA staining is shown in black and white. Note the brightly stained heterochromatin visible at the natural centromeres of the human chromosomes. Images courtesy of Dr B. Grimes, MRC Human Genetics Unit, Edinburgh, UK.