Encyclopedia of Biology

mitochondrial matrix Each mitochondrion is sur-
rounded by a double membrane. The aqueous matrix is
bounded within the inner membrane and contains ribo-
somes and oxidative enzymes.

mitosis The cell division process in eukaryotic cells
that replicates chromosomes so that two daughter cells
get equally distributed genetic material from a parent
cell, making them identical to each other and the par-
ent. It is a four-step process that includes prophase
(prometaphase), metaphase, anaphase, and telophase.
Interphase is the time in the cell cycle when DNA is
replicated in the nucleus.
See alsoMEIOSIS.

mixed valency This is one of several names, such as
“mixed oxidation state” or “nonintegral oxidation
state,” used to describe COORDINATION compounds
and CLUSTERs in which a metal is present in more than

one level of oxidation. The importance in biology is

due to the often-complete delocalization of the valence

electrons over the cluster, allowing efficient electron-

transfer processes.

See alsoOXIDATION NUMBER.

mobbing The display of a flock of small birds

attacking a larger predatory bird to keep it away from

nests, a defensive posturing.

mobbing 223

Chromosomes appear

and shorten and thicken.

Nuclear membrane

disintegrates and

nucleolus disappears

Individual chromosomes,

each comprising two

chromatids, become aligned

along the equator of the

nuclear spindle

Chromosomes split at

the centromere and the

daughter chromatids

move to opposite poles

of the spindle

A nuclear membrane

forms around each

group of daughter

chromatids, or new

chromosomes

Prophase Metaphase

Anaphase Telophase

Mitosis is a four-step process that includes prophase
(prometaphase), metaphase, anaphase, and telophase.

Transmission electron micrograph of cytokinesis (division of a

cell’s cytoplasm) after mitosis of a human embryonic kidney cell.

This is a tissue culture cell. In this late phase of cell division

(telophase) the nucleus has divided into two (dark areas). Each

daughter nucleus contains genetic material identical to that of

the mother cell. Spindle microtubules can be seen in the cyto-

plasmic bridge between the two cells. The spindle is involved in

separating chromosomes during division of the nucleus. At

telophase, the cell membrane is drawn in to form a cleavage fur-

row, which will break to leave two daughter cells. Magnification:

×8500 at 8 ×10-inch size.(Courtesy © Dr. Gopal Murti/Photo

Researchers, Inc.)