Essential Cell Biology 5th edition

The Structure of Eukaryotic Chromosomes
187
looped domain
Figure 5−22 The chromatin in human
chromosomes is folded into looped
domains. These loops are established by
special nonhistone chromosomal proteins
that bind to specific DNA sequences,
creating a clamp at the base of each loop.
matching specific
DNA sequences
chromosome
loop-forming
clamp proteins
histone called histone H1, which is thought to pull adjacent nucleosomes
together into a regular repeating array. This “linker” histone changes the
path the DNA takes as it exits the nucleosome core, allowing it to form a
more condensed chromatin fiber.
We saw earlier that, during mitosis, chromatin becomes so highly condensed
that individual chromosomes can be seen in the light microscope.
How is a chromatin fiber folded to produce mitotic chromosomes?
Although the answer is not yet known in detail, it is known that specialized
nonhistone chromosomal proteins fold the chromatin into a series
of loops (Figure 5−22). These loops are further condensed to produce the
interphase chromosome. Finally, this compact string of loops is thought
to undergo at least one more level of packing to form the mitotic chromosome
(Figure 5−23).
short region of
DNA double helix
“beads-on-a-string”
form of chromatin
ECB5 n5.201/5.24.5
2 nm
11 nm
QUESTION 5–2
Assuming that the histone
octamer (shown in Figure 5–20)
forms a cylinder 9 nm in diameter
and 5 nm in height and that the
human genome forms 32 million
nucleosomes, what volume of
the nucleus (6 μm in diameter) is
occupied by histone octamers?
(Volume of a cylinder is πr 2 h; volume
of a sphere is 4/3 πr 3 .) What fraction
of the total volume of the nucleus
do the histone octamers occupy?
How does this compare with the
volume of the nucleus occupied by
human DNA?
chromatin fiber
of packed
nucleosomes
30 nm
chromatin fiber
folded into loops
entire
mitotic
chromosome
centromere
NET RESULT: EACH DNA MOLECULE HAS BEEN
PACKAGED INTO A MITOTIC CHROMOSOME THAT
IS 10,000-FOLD SHORTER THAN ITS FULLY
EXTENDED LENGTH
700 nm
1400 nm
Figure 5−23 DNA packing occurs on
several levels in chromosomes. This
schematic drawing shows some of the levels
thought to give rise to the highly condensed
mitotic chromosome. Both histone H1 and a
set of specialized nonhistone chromosomal
proteins are known to help drive these
condensations, including the chromosome
loop-forming clamp proteins and the
abundant non-histone protein condensin
(see Figure 18–18). However, the actual
structures are still uncertain.