Question

describe the names primary molecular components and basic mechanisms of the two distinctscytoskeletal machines that are assembled to carry out mechanical processes of mitosis and cytokinesis

Answer 1

The two cytoskeletal machines that perform the mechanical processes of mitosis and cytokinesis are microtubules and microfilaments respectively. The formation of two genetically identical daughter cells is proceeded by separating the replicated chromosomes of the parent cell and allocating one copy to each daughter cell. This task is performed by a bipolar mitotic spindle composed of microtubules and various proteins that interact with them, including microtubule dependent motor protein during mitosis. Microfilaments form the contractile system of the cell which helps in the formation of cleavage furrow during cytokinesis. Microfilaments contains both actin and myosin filaments and form an extensive network in the cytoplasm of cells and may be associated with plasma membrane.

The microtubules in most cells extend outward from a microtubule organizing center (MOC), in which the minus end of microtubule are anchored. In animals, the major MOC is centrosome. During mitosis, microtubules extend outward from duplicated centrosomes to form the mitotic spindle, which is responsible for the segregation and distribution of chromosomes to daughter cells. The centrosome serves as the initiation site for the assemby of microtubules, which grow outward to the periphery from the centrosome. In mitosis the two centrosomes separate and move to the opposite side of the nucleus, forming the two poles of mitotic spindle. During this period the microtubule dynamics of of assembly and disassembly also changes. At first, the rate of microtubule disassembly increases followed by large number of microtubules emanating from the centrosome increases. This results in the disassembly of interphase microtubules and the outgrowth of a large number of microtubules from the centrosomes.

After the mitotic spindle has segregated the two sets of chromosomes to opposite poles of the cell, the residual central region of the spindle is required to maintain a functional contractile ring which is provided by microfilaments. In animal cells microtubules form the contractile ring whereas in plants it forms the phragmoplast. The contractile ring mainly composed of actin and myosin filaments and forms around the equator of the cell, just under the plasma membrane. As the ring contracts, it pulls the membrane inward, thereby dividing the cell into two. Plant cell cannot form contractile ring as they possess rigid cell wall, so cytokinesis in them occurs via cell plate method with the formation of phragmoplast.