The following points highlight the top eight events of molecular mechanism in m-phase. The events are: 1. Molecular Mechanism of Chromosome Condensation 2. Molecular Mechanism of Nuclear Membrane Breakdown 3. Molecular Mechanism of Spindle Formation 4. Molecular Mechanism of Capturing Chromosomes by Microtubules 5. Molecular Mechanism of Separation of Chromatids and Others.
Events of Molecular Mechanism in M-Phase:
- Molecular Mechanism of Chromosome Condensation
- Molecular Mechanism of Nuclear Membrane Breakdown
- Molecular Mechanism of Spindle Formation
- Molecular Mechanism of Capturing Chromosomes by Microtubules
- Molecular Mechanism of Separation of Chromatids
- Molecular Mechanism of Chromosome Movement
- Molecular Mechanism of Nuclear Membrane Reassembly
- Molecular Mechanism of Cytokinesis
Event # 1. Molecular Mechanism of Chromosome Condensation:
Condensin and cohesin, two protein complexes are involved in this process (Fig. 5.11). Condensin, a dimeric protein molecule hinged at the centre with a globular domain and at each end with DNA and ATP binding domain. It forms a structural framework by forming intra-molecular crosslinking to coil DNA.
Activated M-Cdk (M-cyclin dependent kinase) phosphorylates condensin which uses energy of ATP hydrolysis to make loop. Cohesin, a multi-subunit protein, is deposited along the length of chromatids which help for cohesion of sister chromatids.
Nuclear membrane supported by nuclear lamina consists of lamin proteins which are phosphorylated by M-Cdk at specific serine residues. Phosphorylation of lamin dimers generates intermediate filaments of nuclear lamina. This causes de-polymerization of the lamin intermediate, and phosphorylated lamins are released into cytoplasm causing dis-integration of membrane (Fig. 5.12).
Event # 3. Molecular Mechanism of Spindle Formation:
During non-astral spindle formation in plant cells, microtubule nucleation and stabilization are dependent upon guanine-nucleotide exchange factor (GEF). GEF stimulates Ran, a GTP-ase, and activated Ran-GTP complex releases microtubule stabilizing protein complex which stimulates nucleated of microtubules around chromosomes.
Microtubule motor proteins, kinesin and dynein, participate in spindle formation (Fig. 5.13).
Microtubule assembly depends upon the balance of two proteins — microtubule associated protein (MAP) which increases microtubule assembly and catastrophin that destabilizes microtubules. M-Cdk causes phosphorylation of these two proteins.
Event # 4. Molecular Mechanism of Capturing Chromosomes by Microtubules:
Microtubules attachment with kinetochore, a plate like structure within centromere of chromosome, is mediated by centromeric binding proteins (CBF) (Fig. 5.14A). The kinetochore binds at the side of microtubules then slides to the (+) end (Fig. 5.14B). Until all the kinetochores are held by microtubules, the cell cycle is held in check.
The chromosomes exhibit saltatory behaviour, oscillating between, movement towards and then away from the pole or equator. The pushing and pulling forces, generated through polymerization and de-polymerization of (+) ends of microtubules, cooperate to move chromosomes to the metaphase plate (Fig. 5.14C).
Event # 5. Molecular Mechanism of Separation of Chromatids:
Disruption of cohesion between sister chromatids is initiated by a cascade of signalling event. Cdc-20 protein binds and activates anaphase promoting complex (APC) through phosphorylation which requires M-Cdk activity.
Activated APC cleaves and inactivates M-cyclin, thus M-Cdk is being inactivated. It also cleaves an inhibitory protein securin which inhibits the activity of a protease (separase). Destruction of securin releases separate which cleaves the sub-units of cohesion complex, thus unglues the sister chromatids (Fig. 5.15).
Event # 6. Molecular Mechanism of Chromosome Movement:
Pulling of chromosomes towards the poles is effected through the shortening of kinetochore microtubule by the loss of tubulin at (+) ends. Microtubule-motor proteins at the kinetochore use ATP to pull the chromosomes along the bound microtubule.
Event # 7. Molecular Mechanism of Nuclear Membrane Reassembly:
With the inactivation of M-Cdk by the degradation of M-cyclin, nuclear lamin is being dephosphorylated by phosphatase causing re-polymerization and reformation of nuclear lamina.
Vesicles produced by breakdown of ER and nuclear envelope in prophase, are being associated with the surface of the de-condensing chromosomes and fuse to form a continuous double membrane along with nuclear pore complexes (Fig. 5.16).
Event # 8. Molecular Mechanism of Cytokinesis:
A circumferential band of microtubules and actin filaments form a ring around the entire cell in pre-prophase which establish the divisional plane. Cell plate formation between two nuclei is guided by phragmoplast which is formed by the overlap of spindle microtubules.