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The contribution of Cytoplasm in nuclear division of Mitosis are as follows:
In nuclear division of mitosis, cytoplasm makes a contribution in several aspects:
[I] Spindle fibres:
These are achromatic fibres forming in cytoplasm of both animal and plant cells. Their origin is probably cytoplasmic rather than nuclear, although it is generally believed that nucleoplasm liberated after breakdown of nuclear membrane, forms the spindle fibres.
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But most compelling evidence for probable cytoplasmic origin is that, in animal cells, spindle apparatus develops before the breakdown of nuclear membrane.
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Spindle fibres are definite structures composed of protein chains linked by sulphur and hydrogen bonds. Protein components are mainly acidic being associated with RNA. Fibres consist of about 90% protein and 5% RNA. Protein-SH groups (sulph-hydryl group) of fibres are present in prophase but not in anaphase but disulphide (S-S) linkages maintain fibre integrity when fibres are formed.
Physically spindle fibre substance is like an elastic gel. Each fibre is filamentous or tubular in structure. A single fibre is 15µ in diameter and several filaments become arranged compactly in bundle forming a fibre.
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Spindle fibres are of three types:
1. Continuous fibres:
These fibres are continuous from one pole to another pole.
2. Chromosomal fibres:
These extend from Centromere of chromosome to either pole.
3. Inter-zonal fibres:
These are present between centromeres of separating chromatids.
Movement of chromosomes is dependent upon the behaviour of spindle fibres. It is important to remember that the axis of orientation of chromosomes during metaphase is predetermined by the orientation of spindle fibres axis. Besides these, there may be cytasters, like relic coils, being retentions of previous mitosis.
[II] Centrioles:
There are two small bodies present in the Centrosome which lie just outside the nuclear membrane. During interphase, they lie at one side of the nuclear membrane. In prophase, one Centriole begins to migrate around the periphery of nucleus while other remains at the same place. In final prophase, they become arranged opposite to each other.
Besides, fibres radiate from each Centriole, called astral rays. During metaphase and anaphase, centriolar structure remains relatively unchanged. In telophase, each Centriole duplicates itself.
Thus, in summary, mitotic cycle lasts from 30 minutes to 3 hours on the average. In it, each of the chromosomes undergoes a longitudinal doubling; the halves of all the chromosomes (chromatids) then separate into similar groups with the help of spindle fibres which reconstitute two new daughter nuclei maintaining equal number of chromosomes as were in the parent cell.
