• all those substances or chemicals which affect the cells
during mitosis or prevent them from entering into normal
mitotic divisions are called mitotic poisons. The various
mitotic poisons are:
(i) The enzyme ribonuclease acts as poison at prophase.
Azide and cyanide also inhibit prophase.
(ii) Mustard gas causes agglutination of chromosomes.
(iii) Chalones also inhibit mitosis. They are small peptides
or glycoproteins in the extracellular fluid.
(iv) The alkaloid colchicine inhibits the formation of
mitotic spindle (inhibits polymerisation of microtubules)
and holds the cells in metaphase. The chromosomes
and DNa undergo replication but remain within the
same cell. The nucleus does not divide. This increases
the number of chromosome sets per cell. This process
leads to endopolyploidy or endomitosis in which
nucleus contains multiple sets of chromosomes instead
of the normal two sets as found in a diploid cell. Such
cells are called polyploid cells.
(v) X-rays cause uncontrolled mitosis and induces
breakage of chromosomes.mitotic poisons
Significance of mitoSiS
• Growth and development : a
single celled zygote grows into
full grown baby by repeated
mitosis. Plants are able to grow
throughout their life due to
mitotic divisions in their apical
and lateral meristems.
• Mitosis is essential for growth and
development of a multicellular organism.
• Maintenance of cell size : an overgrown
somatic cell is induced to divide so that mitosis helps in
maintaining a proper surface area-volume ratio.
• Genetic stability : all the daughter cells of a multicellular
organism have the same number and type of chromosome
as parent cells due to equitable distribution of all the
chromosomes.
• Healing and regeneration : for healing of a wound, new
cells are produced by mitosis. Some organisms are able to
regenerate missing parts of body or also whole organism
through mitosis.
• Repairing : The mechanism for replacing old or worn out
cells is called repairing. In human body roughly 5 × 109 cells
are lost from skin surface, lining of alimentary canal, blood
cell etc. These are replaced by new cells formed through
mitosis.
• Evidence of basic relationship : The mechanism of
mitosis are similar in the majority of organisms, showing
basic similarity and relationship among them.meioSiS
• Meiosis was discovered and described for the first time
in 1876 by the distinguished German biologist Oscar
Hertwing (1849-1922) in sea urchin eggs.
• Edward Van Beneden (1846-1910) described it at the
level of chromosomes in Ascaris eggs in 1883.
• The english biologist Walther flemming in 1879, was the
first person to describe how chromosomes moved during
mitotic cell division.
• Most animals and plants reproduce sexually. Gametes of
opposite sex unite to form a cell that divides repeatedly by
mitosis and eventually gives rise to an adult body with some
100 trillion cells.
• The gametes that give rise to the initial cell are the products
of a special form of cell division called meiosis.
• Meiosis (Gk. meioum or meio-to lessen or to diminish) is a
double division which occurs in a diploid cell and gives
rise to four haploid cells. Meiosis ensures the production
of haploid phase in the life cycle of sexually
reproducing organisms whereas fertilisation
restores the diploid phase.
• Meiosis involves two successive
nuclear divisions, but no DNa
replication prior to second
division. Cells in meiosis are called
meiocytes, e.g., spermatocytes
and oocytes respectively.
• Depending upon the stage when
meiosis occurs, the meiosis is of three types- gametic, zygotic and sporic.
mechaniSm of meioSiS
- Meiosis is a continuous process but it is arbitrarily
divided into two sequential cycles of nuclear and cell
division called meiosis I and meiosis II but only a single
cycle of DNa replication happens. - Meiosis I is initiated after the parental chromosomes have
replicated to produce identical sister chromatids at the
S phase. - Meiosis involves pairing of homologous chromosomes
and recombination between them. - four haploid cells are formed at the end of meiosis II.
- Transition period between meiosis I and meiosis II is
called interkinesis or intrameiotic interphase. - Meiotic events can be grouped under the following
phases listed in the flow chart on the next page.