Cell Cycle and Cell Division
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1. PRØPHÅSË
It is the longest phase in mitosis.
Early Prophase:
The tangled chromatin fibres condense to chromosomes.
The nucleolus is seen attached to the chromosome at the nucleolar
organizer.
Late prophase:
Each chromosome splits into two chromatids attached together at the
centromere.
Condensation of chromosomes continues.
In animal cells, the centrioles move to opposite poles. They radiate out
astral rays (microtubular fibrils). Astral rays along with its centriole pair
is called aster. The 2 asters move to opposite poles and start spindle
formation.
The nuclear envelope, nucleolus, Golgi complexes and endoplasmic
reticulum disappear.
Spindle fibres originate from micro tubular proteins (tubulin).
2. MËTÅPHÅSË
The nuclear envelope completely disintegrates. Hence the chromosomes
spread through the cytoplasm of the cell.
Chromosome condensation is completed. At this stage morphology
of chromosomes can be best observed and studied easily under the
microscope. They will have two sister chromatids held together by the
centromere.
Chromosomes come to lie at the equator while plane of alignment of
the chromosomes at metaphase is called the metaphase plate.
The spindle fibres from both poles are connected to chromatids by their
kinetochores ( disc shaped structures ) on the surface of centromere.
(NEET-1 2016)
The chromosomes aligned at the equator such that one of the chromatid
at its kinetochores is connected to spindle fibers from one pole while the
other chromatid to the spindle fibres from the opposite pole.