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PinkMonkey Online Study Guide-Biology
All living organisms start their life with a single cell
(zygote). In unicellular animals, cell division means reproduction
which results in the formation of new organisms. In multicellular organisms,
however, this cell further divides and thus forms the basis of growth.
What initiates cell division is not exactly known but it is believed that
when the ratio between the volume of cytoplasm and nucleus, (called nucleoplasmic
index) exceeds its limit, the cell divides. Principally, there are
two types of cell divisions:
1. Mitosis which is characteristic division
of somatic or body cells.
2. Meiosis which is characteristic division in which
germ cells are formed in the reproductive organs.
Cell cycle and mitosis : The period from the beginning of one cell
division to the beginning of the next cell division or the period between
the end of one cell division to the end of next cell division, is known
as cell cycle. It includes two main phases, that is, interphase
and mitotic phase.
The interphase is divided into following three phases
as shown in (Figure 14.15).
1. First gap stage or G1 phase in which
cell grows.
2. S phase or synthetic phase in which DNA replicates.
3. Second gap phase or G2 phase in which
the protein synthesis increases. The completion of G2 phase
is marked by the beginning of mitosis.
The role of mitosis in cell cycle is to form two sets
on identical chromosomes and to distribute one complete set at each end
to form a daughter nucleus. Mitosis thus ensures that the new cell contains
the identical number and types of chromosomes present in original mother
cell, thus to continue the life-cycle.
In unicellular animals, reproduction is by mitotic division
but in higher organisms with evolution of sex, reproduction takes place
by the union of two gametes -- male and female (sperm and egg). Hence,
it is essential that at some stage (gamete formation) during life cycle
the number of chromosomes should be reduced to half, so that their union
will maintain the original number of chromosomes constant. This is done
by meiosis.
MEIOSIS
Meiosis is the characteristic division of the germ cells.
It is also called reduction division because in this the parent
cell produces four daughter cells (tetrads) each having half the number
of chromosomes present in the parent cell. In other words, meiosis is
a process of cell division in which diploid (2n) number of chromosomes
is reduced to haploid (n) number.
Meiosis is completed in two distinct nuclear divisions
which take place in quick succession, and are respectively called (1)
First meiotic or reductional division in which the number of chromosomes
is reduced to half and (2) Second meiotic division or equational division
in which the number of chromosomes is the same as at the end of first
division. Each division of meiosis like mitosis consists of four stages
prophase, metaphase, anaphase and telophase.
Cell division involves two stages, the nuclear division
called karyokinesis and the division of cytoplasm known as cytokinesis.
In any cell division the nucleus divides first followed immediately by
the division of cytoplasm.
It is the characteristic division of somatic or body cells resulting in the formation of daughter cells each receiving equal number of chromosomes like those of parent cells. Hence it is called equational division. It can be described into four orderly and consecutive phases such as prophase, metaphase, anaphase and telophase.
Figure 14.15 Cell division: Mitosis
Before the cell divides, it is said to be in the interphase which is sometimes inappropriately called ’resting phase’, though the cell is definitely not resting; in fact, all its metabolic or synthesizing activities are at its peak as the cell has to prepare itself for division. However, it is characterized by a large and distinct nucleus but indistinct chromosomes.
Prophase : It literally means preparatory phase.
It is initiated by the changes in the centrosphere. The centrosphere disappears
and the two centrioles that are set free begin to move away from each
other. Each of them radiates fibers, called an aster. The asters are absent
in plant cells. The long chromatin threads of DNA undergo condensation
to form a definite number of distinct chromosomes by dehydration and coiling.
Each chromosome is double, consisting of two identical components called
the chromatids connected by a common body called centromere. The nucleolus
and nuclear membrane begin to disappear and the fibers radiating from
pole to pole, called spindle fibers begin to appear. The spindle fibers,
along with asters at opposite ends, are referred to as a mitotic figure.
Metaphase : It literally means mid-way phase.
The formation of spindle is complete. The chromosomes get still shorter
and thicker and arrange themselves on the equatorial plane of the spindle.
The spindle shows two types of fibers, those running from pole to pole
(called interzonal fibers) and others connecting each pole to the centromere,
(called tractile fibers).
Anaphase : This is the separating phase. This
begins with the division of the centromere of each chromosome. Thus the
two sister chromatids of the same chromosome now completely separate and
start repelling. These sister chromatids now known as daughter chromosomes
take the form of a V or a U, and appear to be pulled towards the opposite
poles along the tractile fibers. Gradually they reach the opposite poles,
and that marks the end of anaphase.
Telophase : This is the reconstruction phase.
It includes the changes that are reversal of those in prophase. The chromosomes
aggregate at the opposite poles and once again get elongated, uncoil and
become indistinct. The spindle fibers simultaneously begin to disappear
and the two daughter nuclei are formed at opposite poles, each containing
an equal number of chromosomes as those of the parent nucleus.
Cytokinesis, or division of cytoplasm, usually
starts with the beginning of telophase. In animal cells it proceeds by
a cleavage furrow around the cell at the equatorial region. This furrow
increases and deepens until the cell is divided into two.
Mitosis in animal cell and plant cells are basically
similar except that in plant cells there are neither centrioles nor asters.
Moreover, cytokinesis in plant cell is not accomplished by furrowing but
by formation of a cell plate between the seperate sets of chromosomes.
The cell plate is impregnated by pectin to form middle lamella. Later
on cellulose is laid down on either side of the middle lamella to form
the cell wall, thus dividing the mother cell into twodaughter cells.
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