Biology today

flow chart : Stages of Meiosis

meioSiS i
• The heterotypic or reduction division is the first division
of meiosis. It starts after the interphase of the cell cycle.
During this division the number of chromosomes is reduced
to half.
• Like mitosis, it is studied under four stages — prophase,
metaphase, anaphase and telophase.

Prophase - i

•    It is more complicated and the longest phase of meiotic

division. It is even longer than the prophase of mitosis. It

is divided into five sub-phases — leptotene, zygotene,

pachytene, diplotene and diakinesis.

•    In prophase-I, the DNa coils tighter and individual

chromosomes first become visible under the light microscope

as a matrix of fine threads. Sex chromosomes (if present)

are often seen as heterochromatic (heteropycnotic)

bodies.

Leptotene

•    Leptotene or leptonema (Gr., leptos-thin, tainia-band, nema-

thread) is a period in which the nucleus has increased in size

and the chromosomes have become more apparent.

•    The leptotene chromosomes show bead-like thickenings

called chromomeres, which appear at irregular intervals

along their length.

•    Leptonemic chromosomes have a definite polarisation and

form loops whose ends are attached to the nuclear envelope

at points near the centriole. This peculiar arrangement is often

called the “bouquet.” In plant cells, the chromosomes may

sometime form a tangle of threads called synizetic knot on

one side of the nucleus. During leptotene the cytoplasm has

many polyribosomes, but endoplasmic vesicles are few. The

two centriole pairs begin to move to the opposite side. both

the centriole pairs or centrosomes develop astral rays from

the pericentriolar satellites.

Table : Characteristics of gametic, zygotic and sporic meiosis

Gametic meiosis Zygotic meiosis Sporic meiosis

1. Meiotic division occurs during gamete
   formation.

Meiotic division occurs in zygote. Meiotic division occurs during sporogen-

esis.

2. results in formation of haploid gametes. results in formation of haploid
   individuals.

results in formation of haploid spores.

3. Gametes fuse during fertilisation and form
   diploid zygote. It results in diplontic life
   cycle.
   E.g., gymnosperms and angiosperms.

Division of zygote results in

haploid organisms. It results in

haplontic life cycle.

E.g., In lower plants such as

algae.

Spores divide to form gametophytes

which form gametes. Gametic fusion

results in diploid sporophyte. i.e.,

having diplohaplontic life cycle. E.g., in

bryophytes.