Chapter 3
Cells: The Living Units
93
3
Histones provide a physical means for packing the very long
DNA molecules (some 2 meters’ worth per cell) in a compact,
orderly way, but they also play an important role in gene regula-
tion. In a nondividing cell, for example, the presence of methyl
groups on histone proteins shuts down the nearby DNA, and
attachment of a phosphate group to a particular histone protein
At various points, the nuclear envelope is punctuated by
nuclear pores
. An intricate complex of proteins, called a
nuclear pore complex
, lines each pore, forming an aqueous
transport channel and regulating entry and exit of molecules
(e.g., mRNAs) and large particles into and out of the nucleus
(Figure 3.29b, middle).
Like other cell membranes, the nuclear envelope is selec-
tively permeable, but here substances pass much more freely
than elsewhere. Small molecules pass through the relatively
large nuclear pore complexes unhindered. Protein molecules
imported from the cytoplasm and RNA molecules exported
from the nucleus are transported through the central channel of
the pores in an energy-dependent process by soluble transport
proteins (importins and others). Such large molecules must dis-
play specific signals to enter or exit the nucleus.
Te nuclear envelope encloses a jellylike fluid called
nucleo-
plasm
(nu
9
kle-o-plazm) in which other nuclear elements are
suspended. Like the cytosol, the nucleoplasm contains dissolved
salts, nutrients, and other essential solutes.
Nucleoli
Nucleoli
(nu-kle
9
o-li; “little nuclei”) are the dark-staining
spherical bodies found within the nucleus where ribosomal
subunits are assembled. Tey are not membrane bounded.
±ypically, there are one or two nucleoli per nucleus, but there
may be more. Nucleoli are usually large in growing cells that are
making large amounts of tissue proteins.
Nucleoli are associated with
nucleolar organizer regions
,
which contain the DNA that issues genetic instructions for syn-
thesizing ribosomal RNA (rRNA). As molecules of rRNA are
synthesized, they are combined with proteins to form the two
kinds of ribosomal subunits. (Te proteins are manufactured on
ribosomes in the cytoplasm and “imported” into the nucleus.)
Most of these subunits leave the nucleus through the nuclear
pores and enter the cytoplasm, where they join to form func-
tional ribosomes.
Chromatin
Seen through a light microscope,
chromatin
(kro
9
mah-tin) ap-
pears as a fine, unevenly stained network, but special techniques
reveal it as a system of bumpy threads weaving through the nu-
cleoplasm. Chromatin is composed of approximately
30%
DNA
, our genetic material
60% globular
histone proteins
(his
9
tōn), which package and
regulate the DNA
10% RNA chains, newly formed or forming
Te fundamental units of chromatin are
nucleosomes
(nu
9
kle-
o-sōmz; “nuclear bodies”), which consist of flattened disc-
shaped cores or clusters of eight histone proteins connected like
beads on a string by a DNA molecule. Te DNA winds (like a
ribbon of Velcro) twice around each nucleosome and continues
on to the next cluster via
linker
DNA segments (
Figure 3.30
1
and
2
).
Nucleosome (10-nm diameter;
eight histone proteins wrapped
by two winds of the DNA double
helix)
Linker DNA
Histones
(a)
(b)
1
DNA
double
helix (2-nm
diameter)
2
Chromatin
(“beads on a string”)
structure with
nucleosomes
3
Tight helical fiber
(30-nm diameter)
6
Metaphase
chromosome
(at midpoint
of cell division)
consists of two
sister
chromatids
5
Chromatid
(700-nm diameter)
4
Looped domain
structure (300-nm
diameter)
Figure 3.30
Chromatin and chromosome structure.
(a)
Electron micrograph of chromatin fiber (125,000
3
).
(b)
DNA packed in a chromosome. The levels of increasing
structural complexity (coiling) from the DNA helix to the metaphase
chromosome are indicated in order from the smallest (
1
DNA
double helix) to the largest and most complex (
6
chromosome).
(Text continues on p. 96)
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