Page 42 - Biology - XII

Meiotic Cell Division
4 Meiotic Cell Division

EXPERIMENT - 8

Objective
To study stages of meiosis using permanent slides.

Principle/Theory
1. Meiosis (Gk. meioum - to diminish), takes place in ovary and anther in plants and in gonads (testis and
ovary) in animals
2. Meiosis is divided into two parts—
(a) Heterotypic division. Meiosis - I; the reductional division. This reduces the chromosome
number, (2n) to n. Prophase I is the longest phase of this division.
(b) Homotypic division. Meiosis - II; the equational division. This maintains the chromosome no.,
n as n. (Interphase stage is absent or if present, it is not distinctive).
3. In plants, microspore mother cells divide by two types:
(a) Successive type. The cell plate is formed in the centre and extends centrifugally on both sides.
This divides the cell into almost equal parts. This occurs in monocotyledons, e.g. onion.

The two nuclear divisions are followed by the cell wall formation. The tetrad produced are called
isobilateral.

(b) Simultaneous type. The division of cytoplasm starts centripetally dividing the cells with the
formation of furrows. Four cells are formed. The tetrad formed by this type are called tetrahedral
tetrad, e.g. dicotyledons. Other tetrads are decussate. T-shaped and linear.

Successive type Simultaneous type

Microspore
mother cell

Fig. 8.1: Successive type and simultaneous type tetrad formation

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