Biology Reference
In-Depth Information
Figure 3.2
DNA packing in eukaryotic chromosomes must be efficient to achieve a dramatic reduc-
tion in DNA length during mitosis and meiosis. This figure illustrates the method by which DNA is
thought to be packed, although the organization of elements in D-F are controversial. Naked DNA
(A), DNA in nucleosomes (B), 30-nm chromatin fiber (C), the 300-nm fiber made up of looped
30-nm fibers (D), the 700-nm supercoiled structure that comprises the arms of a metaphase chro-
mosome (E), and the metaphase chromosome (F).
Nucleosomes reduce the length of DNA by a factor of
≈
6 to a flexible beaded
fiber.
Additional condensation of the DNA occurs when nucleosomes are condensed
into a shorter thicker fiber, called the
30-nm iber
(
Figure 3.2C
). The structural
organization of this 30-nm fiber seems to be an irregular left-handed superhe-
lix with six nucleosomes per turn. Another level of condensation is thought to
occur in metaphase chromosomes (
Figure 3.2D-F
), although the details of this
