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cell, for example, has about 2 m
of DNA—a length about 250,000 times greater than the cell’s diameter. Yet before the cell can divide to form genetically identical daughter cells, all of this DNA must be copied, or replicated,
and then the two copies must be separated so that each daughter cell ends up with a complete genome.
The replication and distribution of so much DNA is manageable because the DNA molecules are packaged into structures called chromosomes, so named because they take up
certain dyes used in microscopy (from the Greek chroma,
color, and soma, body) (Figure 12.3). Each eukaryotic chromosome consists of one very long, linear DNA molecule associated with many proteins (see Figure 6.9). The DNA molecule
carries several hundred to a few thousand genes, the units of
information that specify an organism’s inherited traits. The
associated proteins maintain the structure of the chromosome and help control the activity of the genes. Together, the
entire complex of DNA and proteins that is the building material of chromosomes is referred to as chromatin. As you
will soon see, the chromatin of a chromosome varies in its degree of condensation during the process of cell division.
Every eukaryotic species has a characteristic number of
chromosomes in each cell nucleus. For example, the nuclei of
human somatic cells (all body cells except the reproductive
cells) each contain 46 chromosomes, made up of two sets of
23, one set inherited from each parent. Reproductive cells, or
gametes—sperm and eggs—have half as many chromosomes
as somatic cells, or one set of 23 chromosomes in humans. The
Figure 12.4 A highly condensed, duplicated human
chromosome (SEM).
Circle one sister chromatid of the chromosome in this
micrograph.
DRAW IT
Sister
chromatids
Centromere
0.5μm
number of chromosomes in somatic cells varies widely among
species: 18 in cabbage plants, 48 in chimpanzees, 56 in elephants, 90 in hedgehogs, and 148 in one species of alga. We’ll
now consider how these chromosomes behave during cell
division.
Distribution of Chromosomes During
Eukaryotic Cell Division
When a cell is not dividing, and even as it replicates its DNA
in preparation for cell division, each chromosome is in the
form of a long, thin chromatin fiber. After DNA replication,
however, the chromosomes condense as a part of cell division: Each chromatin fiber becomes densely coiled and
folded, making the chromosomes much shorter and so thick
that we can see them with a light microscope.
Each duplicated chromosome has two sister chromatids,
which are joined copies of the original chromosome
(Figure 12.4). The two chromatids, each containing an identical DNA molecule, are initially attached all along their lengths
by protein complexes called cohesins; this attachment is known
as sister chromatid cohesion. Each sister chromatid has a
centromere, a region containing
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Jane B. Reece (Campbell Biology)