Module 4 : Molecular Cell Biology

Lecture 24 : Eukaryotic Cell (Part III)

 

The details of these stages are as follows:

Interphase: Interphase is a preparatory phase required to perform requisite steps. These are series of events in the nucleus as well as in cytosol of daughter cells to enable it to enter into the division phase. This phase has several phases, these are as follows:


G1 Phase: it is also known as growth phase. It start from the end of the mitosis and until the beginning of S Phase. During this phase, cellular proteins, enzymes are synthesized. Most of these enzymes are required for DNA synthesis in S Phase. Duration of G1 Phase depends on cell type within the organism. G1 Phase is under the control of p53 gene products. 


S Phase: Once cell grow and all factors, nucleotide is available, it starts DNA synthesis during S phase. At the end of this all chromosomes present in nuclei is replicated and DNA content doubled. No Change in ploidy. The synthesis of DNA occurs very fast to avoid exposure of newly synthesized DNA to mutagens.


G2 Phase: The growth phase between DNA synthesis and mitosis. During this phase, cell grow and synthesize the protein and cellular factors required for mitosis and cytokinesis.


G0 Phase: After G1 phase, quiescent, senescent and non-proliferative multicellular eukaryotic cell enter into the G0 phase. Cells remain in this phase for long period or indefinite period, as in the case of neuron. It is also common in fully differentiated cells. The fast growing cell never enters into G0phase and hence it is not an regular cell cycle phase and under specific condition cell enter into G0 phase.


Mitosis or M-Phase: After G2 phase, cells enter into the mitosis or M-phase to divide the DNA equally between two daughter cells. Each mitosis has 4 distinct phases to precisely divide DNA content of the cell (Figure 24.2).

These are as follows:
Prophase: During this phase, nuclear membrane is dissolved and chromatin condenses into chromosomes. The nucleolus in the nucleus disappear. In the beginning each cell has one centrosome, which replicate along with DNA to give rise a pair of centrosome to co-ordinate down-stream events. Each centrisome has microtubule to form spindle and assist in distribution of nuclear content during mitosis. Centrioles are considered to organize the microtubule assembly but they are not essential.