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In this schematic diagram of a human metaphase chromosome, (1) refers to a chromatid: one-half of the two identical double helices of the already replicated chromosome, both joined together by the centromere (2). During cell division, the identical copies (together called a sister chromatid pair) are divided over the two daughter cells, each double helix then constituting a chromosome in its own right. The short (3) and long arm (4) of the chromatid to the right are also marked.
