Chromosome morphology at the light microscopic level
Chromosomes
are typically visualized at mitotic
metaphase, when the DNA has
replicated and chromosomal proteins are
doubled, but have not yet separated to the
opposite poles. Chromosomes in this state
appear as two chromatids
(sister chromatids) each comprising
a single DNA double helix, attached to a
single centromere.
Chromosomes are classified according to the relative
position of the centromere., which divides
the chromatid into short
(p)
and long (q) arms. Basic
patterns are metacentric
(centromere in the middle), acrocentric (centromere
nearer to the end than the middle), and telocentric (centromere
all the way at one end.). Chromosomes in
which the centromere is closer to the middle than
the end are described as submetacentric.
All chromatids are capped with special DNA sequences
called telomeres.
Careful examination of telocentric chromosomes
show they have telomeric sequences as well as
(very) short DNA sequences. However, the
convention remains to count the number of arms
in a karyotype (the Fundamental Number,
or NF)
as two for each metacentric or
acrocentric chromosome, and one for
each telocentric.
Figure © 2012 TA Brown, Introduction
to Genetics (1st ed.); additional
text © 2015 by Steven M. Carr