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Figure 9.1: Fluorescence in situ hybridization of mitotic chromosomes from a human cell.

Previous chapters described chromosomes as simple linear DNA molecules on which genes are located. For example, your largest chromosome, chromosome 1, has about 3536 genes. To ensure that each of your cells possesses these genes the chromosome has features that allow it to be passed on during cell division. Origins of replication found along its length provide places for DNA replication to start, telomeres protect each end of the chromosome, and a single centromere near the middle provides a place for microtubules to attach and move the chromosome during mitosis and meiosis. If the chromosome is damaged but still retains these three features it will continue to be inherited during cell divisions but the daughter cell may not be receiving exactly 3536 genes. For example, if a portion of the chromosome has been lost the cell will be missing some genes. If something goes wrong during cell division the entire chromosome may be lost and the cell will be missing all of these genes. How chromosome abnormalites arise and the consequences they have for the cell and the organism is the subject of this chapter.