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,
each chromosome consists of a single
double-stranded DNA molecule (a chromatid), and
the number of chromosomes equals the number of
chromatids, which are the same in males and
females.
When this cell undergoes mitosis, the DNA molecule in each chromosome replicates, such that the cell's DNA content is now 4C. At metaphase of mitosis, chromosomes are maximally compact. However the two double-stranded DNA molecules (chromatids) remain attached to a single centromere. In a standard metaphase karyotype [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.
During anaphase, the centromeres separate as they moved towards the cell poles. The dividing cell nucleus temporarily includes twice the usual number of centromeres. Because half of the centromeres, with one chromatid each, are attached to each of the two poles, two new nuclei are recognizable, each with 2N chromosomes. 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].
There are two points of confusion common to students encountering this material for the first time. (1) "X" and "Y" are just names for the sex chromosomes. 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." (2) The two sides of a metaphase chromosome ("left" and "right" sides of the X) are not the two separate strands of a single DNA molecule. Each is a complete double-stranded molecule.
When this cell undergoes mitosis, the DNA molecule in each chromosome replicates, such that the cell's DNA content is now 4C. At metaphase of mitosis, chromosomes are maximally compact. However the two double-stranded DNA molecules (chromatids) remain attached to a single centromere. In a standard metaphase karyotype [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.
During anaphase, the centromeres separate as they moved towards the cell poles. The dividing cell nucleus temporarily includes twice the usual number of centromeres. Because half of the centromeres, with one chromatid each, are attached to each of the two poles, two new nuclei are recognizable, each with 2N chromosomes. 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].
There are two points of confusion common to students encountering this material for the first time. (1) "X" and "Y" are just names for the sex chromosomes. 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." (2) The two sides of a metaphase chromosome ("left" and "right" sides of the X) are not the two separate strands of a single DNA molecule. Each is a complete double-stranded molecule.