The light micrographs show dividing lung cells from
a newt. The chromosomes appear blue and the
microtubules green. (The red fibers are intermediate
filaments.) The schematic drawings show details not
visible in the micrographs. For simplicity, only four
chromosomes are drawn.
Centrosomes (each
has 2 centrioles)
Early mitotic
spindle
Spindle pole
Kinetochore
Kinetochore
microtubule
Polar microtubule
Nucleolus
Centromere
Plasma
membrane
Fragments
of nuclear
envelope
Aster
Nuclear
envelope
Chromosome
consisting of two
sister chromatids
Chromatin
Interphase
Interphase
is the period of a cell’s life when it
carries out its normal metabolic activities and
grows. Interphase is not part of mitosis.
During interphase, the DNA-containing
material is in the form of chromatin. The
nuclear envelope and one or more nucleoli are
intact and visible.
There are three distinct periods of interphase:
G
1
: The centrioles begin replicating.
S:
DNA is replicated.
G
2
: Final preparations for mitosis are
completed and centrioles finish replicating.
Early Prophase
The chromatin condenses, forming barlike
chromosomes
.
• Each duplicat
ed chromosome consists of two
identical threads, called
sister chromatids
, held
together at the
centromere
. (Later when the
chromatids separate, each will be a new
chromosome.)
As the chromosomes appear, the nucleoli
disappear, and the two centrosomes separate
from one another.
• Micr
otubule arrays called
asters
(“stars”)
extend from the centrosome matrix.
The centrosomes act as focal points for
growth of a microtubule assembly called the
mitotic spindle
. As the microtubules lengthen,
they propel the centrosomes toward opposite
ends (poles) of the cell.
Late Prophase
The nuclear envelope breaks up, allowing the
spindle to interact with the chromosomes.
• Some of the gr
owing spindle microtubules
attach to
kinetochores
(ki-ne´ to-korz), special
protein structures at each chromosome’s
centromere. Such microtubules are called
kinetochore microtubules
.
The remaining spindle microtubules (not
attached to any chromosomes) are called
polar
microtubules
. The microtubules slide past each
other, forcing the poles apart.
The kinetochore microtubules pull on each
chromosome from both poles in a tug-of-war
that ultimately draws the chromosomes to the
center, or equator, of the cell.
Interphase
Interphase
Early Prophase
Prophase—first phase of mitosis
Late Prophase
Available at www.masteringaandp.com
Figure 3.33
Mitosis is the process of nuclear division in which the chromosomes
are distributed to two daughter nuclei. Together with cytokinesis, it produces
two identical daughter cells.
FOCUS
Mitosis
100
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