identical DNA information
. How that DNA is
expressed (used to manufacture proteins) determines the
specialization of the cell, a point we return to at the chap-
ter’s end (p. 106).
Why is precise division of the genetic material during mitosis
Describe the events that occur during mitosis.
CONTROL OF CELL DIVISION
How often a cell divides is strictly controlled and varies with
cell type. Skin cells, blood-forming cells, and cells that line
the intestine, for example, divide often and continually. In
contrast, the immature cells that give rise to neurons divide
a speciF c number of times, and then cease—they become
specialized and remain alive, but they no longer divide.
Most types of human cells divide from forty to sixty times
when grown in the laboratory. Adherence to this limit can be
startling. A connective tissue cell from a human fetus divides
ve to sixty-three times, the average being about F fty
times. However, a similar cell from an adult divides only
fourteen to twenty-nine times, as if the cell “knows” how
many times it has already divided. In a body, however, sig-
nals from the immediate environment also inﬂ uence mitotic
A physical basis for this mitotic clock is the DNA at the
tips of chromosomes, called
where the same six-
nucleotide sequence repeats hundreds of times. Each mito-
sis removes up to 1,200 nucleotides. When the chromosome
tips wear down to a certain point, this signals the cell to
cease dividing. Studies show that severe psychological or
emotional stress can hasten telomere shortening. This may
be one way that stress can harm health.
Other external and internal factors inﬂ
uence the timing
and frequency of mitosis. Within cells, waxing and waning
levels of proteins called kinases and cyclins control the cell
Major Events in Mitosis
Chromatin condenses into chromosomes; centrioles move
to opposite sides of cytoplasm; nuclear membrane and
nucleolus disperse; microtubules assemble and associate
with centrioles and chromatids of chromosomes.
bers from the centrioles attach to the centromeres
of each chromosome; chromosomes align midway between
Centromeres separate, and chromatids of the chromosomes
separate; spindle F
bers shorten and pull these new
individual chromosomes toward centrioles.
Chromosomes elongate and form chromatin threads;
nuclear membranes form around each chromosome set;
nucleoli form; microtubules break down.
Cytoplasmic division is seen in these scanning
electron micrographs (
Electron Microscopy in Biology,
by R. G. Kessel and C. Y. Shih. © 1976