Page 109 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

4.3 Mitosis: Cell Division That Preserves Chromosome Number 101

astral microtubules extend out from the centrosome toward The spindle fibers begin to disperse; a nuclear envelope
the cell’s periphery. forms around the group of chromatids at each pole; and one
Soon before the end of prometaphase, the kinetochore or more nucleoli reappear. The former chromatids now func-
of each chromosome’s previously unattached sister chro- tion as independent chromosomes, which decondense (un-
matid now associates with microtubules extending from coil) and dissolve into a tangled mass of chromatin. Mitosis,
the opposite centrosome. This event orients each chromo- the division of one nucleus into two identical nuclei, is over.
some such that one sister chromatid faces one pole of the
cell and the other faces the opposite pole. Experimental Cytokinesis: The cytoplasm divides (Fig. 4.10f)
manipulation has shown that if both kinetochores become
attached to microtubules from the same pole, the configu- In the final stage of cell division, the daughter nuclei emerging
ration is unstable; one of the kinetochores will detach repeat- at the end of telophase are packaged into two separate daugh-
edly from the spindle until it associates with microtubules ter cells. This final stage of division is called cytokinesis
from the other pole. The attachment of sister chromatids to (literally cell movement). During cytokinesis, the elongated
opposite spindle poles is the only stable arrangement. parent cell separates into two smaller independent daughter
cells with identical nuclei. Cytokinesis usually begins during
anaphase, but it is not completed until after telophase.
Metaphase: Chromosomes align The mechanism by which cells accomplish cytokinesis
at the cell’s equator (Fig. 4.10c) differs in animals and plants. In animal cells, cytoplasmic di-
During metaphase (middle stage), the connection of sister vision depends on a contractile ring that pinches the cell into
chromatids to opposite spindle poles sets in motion a series two approximately equal halves, similar to the way the pulling
of jostling movements that cause the chromosomes to move of a string closes the opening of a bag of marbles (Fig. 4.11a).
toward an imaginary equator halfway between the two Intriguingly, some types of molecules that form the contractile
poles. The imaginary midline is called the metaphase ring also participate in the mechanism responsible for muscle
plate. When the chromosomes are aligned along it, the contraction. In plants, whose cells are surrounded by a rigid
forces pulling sister chromatids toward opposite poles are cell wall, a membrane-enclosed disk, known as the cell plate,
in a balanced equilibrium maintained by tension across the forms inside the cell near the equator and then grows rapidly
chromosomes. Tension results from the fact that the sister outward, thereby dividing the cell in two (Fig. 4.11b).
chromatids are pulled in opposite directions while they are During cytokinesis, a large number of important orga-
still connected to each other by the tight cohesion of their nelles and other cellular components, including ribosomes,
centromeres. Tension compensates for any chance move- mitochondria, membranous structures such as Golgi
ment away from the metaphase plate by restoring the chro-
mosome to its position equidistant between the poles.
Figure 4.11 Cytokinesis: The cytoplasm divides, producing
two daughter cells. (a) In this dividing frog zygote, the contractile
Anaphase: Sister chromatids move ring at the cell’s periphery has contracted to form a cleavage furrow
to opposite spindle poles (Fig. 4.10d) that will eventually pinch the cell in two. (b) In this dividing onion root
cell, a cell plate that began forming near the equator of the cell
The nearly simultaneous severing of the centromeric con- expands to the periphery, separating the two daughter cells.
nections between the sister chromatids of all chromosomes a: © Don W. Fawcett/Science Source; b: © McGraw-Hill Education/Al Telser
indicates that anaphase (from the Greek ana- meaning up (a) Cytokinesis in an animal cell
as in up toward the poles) is underway. The separation of Contractile
sister chromatids allows each chromatid to be pulled to- ring
ward the spindle pole to which it is linked by kinetochore
microtubules; as the chromatid moves toward the pole, its
kinetochore microtubules shorten. Because the arms of the
chromatids lag behind the kinetochores, metacentric chro-
matids have a characteristic V shape during anaphase. The
attachment of sister chromatids to microtubules emanating 150 m Cleavage furrow
from opposite spindle poles means that the genetic infor-
mation migrating toward one pole is exactly the same as its
counterpart moving toward the opposite pole. (b) Cytokinesis in a plant cell

Telophase: Identical sets of chromosomes
are enclosed in two nuclei (Fig. 4.10e)
The final transformation of chromosomes and the nucleus
during mitosis happens at telophase (from the Greek telo- Cell
meaning end). Telophase is like a rewind of prophase. plate