Chromosomes, Chromatids, Arms, & C Value

    Over the course of a mitotic or meiotic cell cycle, replication of DNA in combination with the movement of chromosomes during karyokinesis (division of the nucleus) causes the DNA content and chromosome complement of a cell  to vary. In particular, it is important to appreciate that a typical metaphase karyotype does not show the chromosomes as they would appear in a somatic diploid cell.
    Consider a species with an XY chromosomal sex determination system. In a diploid somatic cell, the female (XX) karyotype [top right] comprises a pair of metacentric sex chromosomes, plus one pair of acrocentric and one pair of telocentric autosomes. There are thus six chromosomes with a total of ten arms [the telocentrics have only one arm each], and the karyotype is described as 2N=6, XX. The DNA content is 2C, where C is defined as the mass of DNA present in a haploid chromosome set.  The male (XY) karyotype [top left] comprises a pair of sex chromosomes, one metacentric and one telocentric (with a single arm), along with the same autosome complement as the female. The numbers of chromosomes is the DNA content are the same as in the female, however their are only 9 arms (the C value is effectively the same). The karyotype is described as 2N=6, XY. At this stage, the number of chromosomes equals the number of chromatids, and these are the same between males and females.

    When this cell undergoes mitotic division, metaphase chromosomes are maximally compact and have duplicated their DNA (4C), however the two DNA strands (chromatids) remain attached to a single centromere. In a standard metaphase spread [middle set of diagrams], each chromosome appears as a single "X" shape with two chromatids joined at the centromere, therefore twice the number of arms, but the same number of centromeres as in the diploid cell. The convention in a metaphase karyotype is to count the chromosome number as the number of centromeres, even though chromatids and their arms are doubled.

    [If the chromosomes could be visualized during anaphase, we would see the centromeres separate as they moved towards the cell poles, so that the cell nucleus would temporarily have twice the usual number of chromosomes (4N). At the completion of telophase, the two sets have separated to either pole, karyokinesis is complete, and there are two daughter cells each with a 2N chromosome complement].

    In haploid gametes [bottom set], only one of each pair of chromosomes is present, so the number of chromosomes and arms is half that of the diploid cell, and the DNA content C. The heterogametic males produce two kinds of gametes, with either an X or a Y chromosome, with 5 or 4 arms respectively, whereas the homogametic females produce only one kind, with an X chromosome and 5 arms. [In some groups, notably birds, females are heterogametic and males are homogametic].

    "X" and "Y" are just names. They do not refer to the shapes of the sex chromosomes: all chromosomes are X- or V-shaped in a metaphase karyotype, and the "Y" chromosome is not an X chromosome with "one leg missing."

Figure & Text ©2016 by Steven M. Carr