Question

11 what has to happen for a cell to enter prophase and set up for metaphase? Give basic explanation of 4 things caused by M-Cdk activation-focus on condensins, lamins and Golgi effect.

Answer 1

For entry into prophase, the cell needs to condense the chromosomes, each of which consists of two sister chromatids.The sister chromatids are held together at the centromere. At the end of the prophase the nuclear envelope is broken down.

At metaphase stage, spindle fibres( microtubules ) attach to the condensed chromosomes. The spindle fibres bring the chromosomes at the metaphase plate.

Entry into prophase,thus,mitosis involves dramatic changes in multiple cellular components, leading to a major reorganization of cell structure. These events are mediated by acyvation of M-Cdk (Cdk1/Cyclin B). Activation of M-Cdk leads to various events which are given below;

1. Condensation of chromosomes driven by protein complexes called as condensins which are members of SMC proteins. The M-Cdk causes phosphorylation of condensins leading to their activation.

2. Nuclear envelope breakdown due to depolymerization of nuclear lamina. This happens due to phosphorylation of lamin filaments by M-Cdk. M-Cdk also phosphorylates several proteins in the inner nuclear membrane and the nuclear pore complex, leading to their disassembly.

3. Golgi apparatus breakdown into vesicles mediated by phosphorylation of multiple golgi matrix proteins by M-Cdk.

4. Reorganization of cytoskeleton by phosphorylation of microtubule-associated proteins by M-Cdk.