Question

What needs to happen for nuclear import as cell exits mitosis?

Explain the basic proteins/process involved in cytokinesis?

Answer 1

When a cell exits mitosis, there are several factors or molecules which needs to be transferred in to or out of the nucleus. So the transport of these molecules across the nuclear membrane is known as nuclear transport. Generally the small molecules can pass across the nuclear membrane easily without any regulation, but the entry ( nuclear import) and exit (nuclear export) of macromolecules is controlled by a highly designed regulatory processes of the cell. The regulation is done especially by the nuclear pore complexes of the nuclear membrane. The macromolecules like RNA and Proteins needs to bind with nuclear transport receptors which are called as importin for nuclear import and exportin for nuclear export.

The importins associated first with their cargo present in the cytoplasm and then they can react with the nuclear pore complexes to perform nuclear import. Once the importin-cargo complex enters to the nucleus from cytoplasm they react with the Ran-GTP present in the nucleoplasm and than causes conformational change in the structure of the importins to get dissociated from their cargo. Then the importin- RanGTP complex moves to the cytoplasm and binds there with Ran Binding Protein where the importin- RanGTP complex dissociates from each other amd then the GTPase activating protein binds with the Ran GTP causing hydrolysis of it to convert the RanGTP to RanGDP. Now the RanGDP binds with the nuclear transport factor NUTF2 and get transported in to the nucleoplasm. In the nucleoplasm the Ran-GDP interacts with a guanine nucleotide exchange factor which again convert the RanGDP to RanGTP and the nuclear import process goes on continuously.

Process of Cytokinesis:

After the nuclear division the cytoplasm gets divided known as cytokinesis and the cell division gets completed. The cytokinesis takes place by cell furrow formation in animal cells , but it takes place in plant cells by cell plate formation.

In Animal Cell:

In the animal cell the actin and myosin -ii proteins are associated with the cytokinesis process mainly. The actin and myosin proteins which are responsible for muscle contraction gets associated with each other in the cells which has completed karokinesis (nuclear division) and form a contractile ring. Then the actin and myosin proteins asoociated closely with each other and the contraction of contractile rings takes place which also brings the plasma membrane of the cell to get contracted and cell furrows are formed at the plane of cell division. With the more and more contraction of contractile ring the plasma membrane also gets contracted continusouly and at a point the plasma membrane from the opposite side of the cell at the plane of cell division meets with one another causing the separation or divison of cytoplasm in to 2 parts and hence results in the process of cyokinesis and formation of two new daughter cells from a single parent cell.

In Plant Cell:

The microtubules (composed ot tubulin protein), microfilaments etc composed to form phragmoplast. This phragmoplast helps in the association of cell plate in the plane of division of the cell. After the asembly of cell plate cellulose secretion takes place and also for the formation of plasma membrane and cell plate subsequently and hence two new cells are formed from a single cell along with the process of cytokinesis.

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