Question

Howard and Pelc (1953) divided cell cycle into which four phase ? Explain about that four phase. ​

Answer 1

Answer:

Cell cycle (Howard and Pelc, 1953) is a series of changes that occur in a newly formed cell which involve its growth and division to form two daughter cells.

It consists of two states, a long non-dividing growing I-phase or interphase and a short dividing M-phase or mitotic phase. Interphase is a series of changes that take place in a newly formed cell and its nucleus before it becomes capable of division again. Therefore, it is also called inter-mitosis. Interphase cannot be listed as a stage of mitosis.

It is a critical time in terms of preparation for the division of the cell in that during this phase occurs duplication of chromosomes in mitotic cells and doubling of cell size. Interphase occupies the time between the end of telophase and the beginning of the next prophase. The duration of inter­phase varies from organism to organ­ism and it occupies 75-90% of the total generation time.

Periods of Cell Cycle:

The cell cycle is divided into four periods: G1, S, G2 and mitosis. On the basis of the synthetic activi­ties, interphase is divided into three sub-stages; Gj, S and G2 (G stands for growth and S for synthesis).

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1. G1 phase:

G of interphase varies in length of time occupying 25 to 50% of interphase time. G1 is the time “gap” between the end of mitosis and the start of DNA synthesis (S-phase). It is the most variable period; depending on the physiologic conditions of the cells, it may last days, months or years. Cells that stop proliferating become arrested at a specific point of G1 and remain withdrawn from the cell cycle in the G1 state.

The most important point in the regulation of cell proliferation occurs during G,, when the crucial decision of whether the cell undergoes a new division cycle or enters the G0 state is taken, but how this is achieved is not known. G: sub-stage is marked by a number of activities in preparation for S phase and it includes the synthesis and organisation of the substrates and enzymes necessary for DNA synthesis. Therefore G, is marked by the synthesis of RNA and protein.

2. S-phase:

It is the period of DNA synthesis. The chromosomes replicate find for this (heir DNA molecules function as template and form carbon copies. The DNA content doubles and duplicate set of genes are formed. Along-with replication of DNAs, new chromatin fibres are formed which, how­ever remain attached in pairs.

As chromatin fibres are elongated chromosomes, each chromosome comes to have two sister chromatids which remain attached at centromere. The S phase cells contain factors that induces DNA synthesis. Histones are synthesized during the S-phase, the period during which they become associated with the newly replicated DNA.

Explanation:

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