Chromosomes, Chromatids, Arms, & C value

Chromosomes, Chromatids, Arms, & C Value

It is important to appreciate that a standard karyotype does not show the chromosomes as they would appear in a "working" diploid cell.

    Consider a species with an XY chromosomal sex determination. 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 small sub-metacentric, along with the same autosome complement as the female. The numbers of chromosomes and arms and the DNA content are the same as the female, and the karyotype is described as 2N=6, XY.

    In a diploid somatic cell undergoing cell division, metaphase chromosomes are maximally compact and have also duplicated their DNA (4C), but the two DNA strands remain attached to a single centromere. In a standard metaphase spread [middle pair of diagrams], each chromosome appears as a single "X" shape with two chromatids joined at the centromere, and 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 number of chromosomes as the number of  centromeres, even though chromatids and their arms are doubled.

    [If the chromosomes could be seen during anaphase, we would see the centromeres separate, so that the cell nucleus woud temporarily have twice the usual number of chromosomes (4N). At the completion of telophase, the two sets have separated to either poll, and there are two daughter cells each have a 2N chromosome commplement].

    In haploid gametes [bottom three], only one of each pair of chromosomes is present, so the number of chromosomes and arms is half that of the diploid cell. The heterogametic males produce two kinds of gametes, with either an X or a Y chromosome, whereas the homogametic females produce only one kind, with an X chromosome.

[NB: The names X and Y are just names, and do not refer to the shapes of the sex chromosomes: all chromosomes are X- or V-shaped in a metaphases karyotype, and the Y chromosome is not an X chromosome with one "arm" "missing."]

Figure & Text ©2014 by Steven M. Carr