Introduction to Cell Division
This follows the page about the structure
of an animal cell.
Living cells divide to form new cells in order to repair worn-out
or damaged tissues throughout an organism, and (in the gametes
only) to enable the exchange of genetic material at the initial
stage
of the process of sexual reproduction.
(A gamete is a mature sex cell, specifically the ovum of the
female or the spermatozoon of the male.)
The two types of
cell division are generally called mitosis and
meiosis but, strictly, these terms refer to the stages of division
of the cell nucleus for somatic (non-reproductive) and reproductive
cells, respectively.
Definition of Mitosis
Mitosis is the type of cell division by which a
single cell divides in such a way as to produce two genetically
identical
"daughter cells". This is the method by which the body
produces new cells for both growth and repair of aging or damaged
tissues
throughout the body.
Mitosis is also referred to as "binary fission". Further
detail about the process of mitosis is included on the page
about mitosis, and also on the illustrations
of mitosis.
Definition of Meiosis
Meiosis, which is also referred to as "reduction
division",
is the form of cell division in which a cell divides into
four "daughter cells" each of which has half** of
the number of chromosomes
of the original cell. Meiosis occurs prior to the formation of
sperm (in males) and ova (in females). That is - meiosis only occurs
in the "gametes".
**The
cells return to having the normal (called "diploid")
number of chromosomes after fertilization of the ova by the sperm.
Meiosis consists of two successive divisions, each
of which is divided into four phases. The first meiotic division
is similar to mitosis (defined above) and
the second meiotic division is the "reduction" stage.
Meiosis enables the exchange of genetic material
between chromosomes.
Further detail about the process of meiosis is included on the
page about meiosis.
Definitions:
Before studying the pages about the processes
of mitosis and meiosis, it is useful to understand the following
terms:
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Chromosome |
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A chromosome is a thread-like structure
found in the nucleus
of cells.
Chromosomes are composed of
a long double filament of DNA (deoxyribonucleic acid)
coiled into a helix together with associated proteins. Genes
(the
most
basic
units of
genetic material) are arranged in a line along the length
of chromosomes.
The nucleus of each human somatic cell (i.e. those relating
to the nonreproductive parts of the body) contains
46 chromosomes - 23 of maternal origin (from the mother)
and 23 of paternal origin (from the father). |
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Diploid |
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The word "diploid" is an adjective
that may be used to describe cells, nuclei or organisms
in which each chromosome (except the Y sex chromosome)
is represented twice, a situation sometimes summarised
as 2n. This is better understood when
compared with the term "haploid":
The word "haploid" (and
also the word "monoploid")
is an adjective that may be used to describe cells, nuclei or organisms that
contain a single set of "n" unpaired chromosomes.
An example of use of these adjectives is: "In the case of the human
species, the gametes are haploid following meiosis."
But what is n ?
n is a number that varies according to species. In the human species, n=23, therefore
there are 46 chromosomes in all human body parts except for the gametes, which
contain only 23. |
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Chromatid |
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The simplest
complete definition of a chromatid is that it is one-half
of a replicated chromosome (About:
Biology).
The more detailed version is that a chromatid
is one of two identical strands of DNA that, together,
form a chromosome - each chromosome being composed
of
two
sister chromatids joined together at a centromere.
The
term "chromosome" applies provided that the
centromeres remain in contact. When the centromeres
separate (during anaphase of mitosis
and anaphase
2 of meiosis), the strands previously called "chromosomes"
are referred to as "daughter-chromosomes". |
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Centromere
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A kinetochore is
another term for a centromere.
A centromere (or "kinetochore")
is the part of a chromosome at which the two chromatids
(see above) are attached together. This (centromere) becomes
attached
to the spindle during mitosis and meiosis.
When chromosome division occurs the centromere divide longitudinally. |
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