CELL CYCLE AND ITS PHASES

The total duration of cell cycle varies from organism to organism and also from cell type to cell type.

Yeast for example, can progress through the cell cycle in only about 90 minutes.

The cell cycle is divided into two basic phases :

1. Interphase

2. M-Phase (Mitosis phase)

1. Interphase :

It is also called as preparatory phase and a period of great metabolic activity.

It is the stage, between two successive cell divisions in which no division of chromosomes or cytoplasm occurs.

'In this stage the nucleus and cytoplasm remain metabolically and synthetically very active.

It generally covers over 95% of the total duration of cell cycle.

During this phase, replication of DNA, synthesis of nuclear histones, division of centrioles to form a new pair of centrioles, synthesis of energy rich compounds, RNA and proteins occur. Nuclear envelope remains intact.

Chromosomes occur in the form of long, coiled, indistinctly visible chromatin fibres.

The size of nucleolus is greatly increased due to accumulation of rRNA and ribosomal proteins.

Interphase is divided into three phases:

(a) G1 Phase (b) S or Synthetic Phase

(c) G2 Phase

(a) G1 Phase (Post-mitotic gap phase) : It corresponds to the interval between mitosis and initiation of DNA replication. Following biochemical changes occur during this sub-stage.

(i) The cell grows to its maximum size due to normal metabolic activity for the preparation of DNA replication, but no change occurs in the DNA contents of the cell.

(ii) It undergoes synthesis of new proteins and RNA. Transcription of rRNA, tRNA and mRNA occurs during this phase.

(iii) Nucleotides, amino acids and energy rich compounds (e.g., ATP) are formed.

(iv) It takes maximum time of all the stages. It is most variable in length, due to which time of cell division differs in cell to cell. G1 can be terminated by various stimuli, but once a cell has completed G1 and entered the 'S' phase to start replication of DNA, it cannot be terminated.

(v) Some cells in the adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. These cells do not divide further, exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle. Cells in this stage remain metabolically active, but no longer proliferate unless called on to do so, depending upon the requirement of the organism. Hence, this exit may be temporary or permanent.

Antephase is the end of G1 when the cell reaches a stage whereby, it will divide even under stress condition.

(b) S or Synthetic Phase

(i) In this phase, the synthesis or replication of DNA occurs on the template of existing DNA.

(ii) During this phase, the amount of DNA per cell doubles (means the organism will have duplicate set of genes). However, there is no increase in the chromosome number (ploidy level remains same). If the initial amount of DNA is denoted as 2C then, it increases to 4C, and if the cell had 2n number of chromosomes at G1, even after S phase the number remains same, i.e., 2n.

(iii) In animal cells, the replication occurs in nucleus, and the centriole duplicates in cytoplasm.

(iv) Histone proteins are synthesised in S-phase. S-phase is called invisible phase of cell cycle as replicated chromosomes are not visible at this stage.

(c) G2 Phase (pre-mitotic gap phase)

(i) In this phase, the cytoplasmic organelles such as mitochondria, chloroplast and golgi complex are doubled.

(ii) Synthesis of RNA and protein continues. Spindle protein (tubulin) synthesis and aster formation occurs.

(iii) A cell contains double the amount (4C) of DNA present in the original diploid (2N) cell.

(iv) The cell prepares itself to enter into "M" or Mitotic phase.

(v) It is also signified by the synthesis of some protein kinases for regulation of cell division.

2. M-Phase :

It represents the phase when the actual cell division or mitosis occurs.

It starts with the nuclear division, corresponding to the separation of daughter chromosomes (Karyokinesis) and usually ends with division of cytoplasm (Cytokinesis).

 

A diagrammatic view of cell cycle indicating formation of two cells from one cell

Regulation of Cell Cycle

Decision of a cell to divide occurs in G1 phase. If a cell is not to divide it will enter into G0 phase or Quiescent phase. When the conditions change, the cell can enter back into G1 phase. G1 S transition in the cell cycle is called as Restriction point or check point. This is the major check point. Once the cell crosses the restriction point rest of the cell cycle is completed. Another minor check point is G2 M transition.

Concept Builder

(i) Cell cycle is regulated by cyclin-dependent protein kinase.

(ii) Cyclins are proteins that activate protein kinases to regulate eukaryotic cell cycle.

(iii) G1 to S transition is carried out by G1 cyclin + cdc 2 kinase.

(iv) G2 to M transition is triggered by maturation promoting factor (MPF) formed by mitotic cyclin + cdc2 kinase, Nucleus attains the maximum size.

(v) The factors which determine whether a cell has to divided or not are

(a) Surface area: Volume ratio. A cell should have high surface area : volume ratio.

(b) Karyoplasmic index.

(vi) Onion root tips or other meristematic tissues are used to study mitosis.

(vii) Mitogens are substances which induce mitosis. e.g., Auxin, Cytokinin, Gibberellin, Insulin etc.

(viii) In animal cell, mitosis is called as Amphiastral (Spindle is associated with 2 asters).

(ix) In plant cells, the mitosis is called as Anastral (no aster, no centriole).

(x) If mitosis is extranuclear, it is Eumitosis.

(xi) If mitosis is intranuclear, it is called as Premitosis. If centrioles are present then it is called as centric.

The cell division is of three types

I. Mitosis II. Meiosis III. Amitosis

Chapter 10

Cell Cycle and cell division

Cell Cycle

Cells, and all living things for that matter, exhibit growth and reproduction. Each parental cell produces two daughter cells every time it divides, which is how all cells reproduce. A new cell population can be created by the growth and division of a single parental cell and its offspring, which is accomplished by the newly generated daughter cells. In other words, repeated cycles of growth and division enable the formation of structures made up of millions of cells from a single cell.

All living things go through the process of cell division. DNA replication and cell proliferation also happen when a cell divides. To ensure proper division and the production of offspring cells with complete genomes, processes like cell division, DNA replication, and cell development are coordinated. Cell cycle refers to the series of actions that a cell takes to reproduce its genome, synthesise the other components of the cell, and ultimately divide into two daughter cells. DNA synthesis only takes place during one particular stage of the cell cycle, despite the fact that cell growth (as measured by cytoplasmic expansion) is a constant process.During cell division, a complicated chain of processes transfers the replicated chromosomes (DNA) to the daughter nuclei. These occurrences are genetically determined.

Figure 1: The cell cycle

In order to form two genetically identical cells, cells go through a series of carefully timed and regulated steps of growth, DNA replication, and division. Interphase and the mitotic phase are the two main stages of the cell cycle. The cell develops and DNA replication occurs during interphase. The cell divides and the replicated DNA and cytoplasm are separated during the mitotic phase.

The replication and reproduction of cells, whether in eukaryotes or prokaryotes, happens during the cell cycle. Although it serves several purposes for organisms, it ultimately ensures their survival. Prokaryotes can continue to exist by dividing into two new daughter cells thanks to a process termed binary fission in the cell cycle.

Reproduction, growth, and gamete creation are the three primary purposes of cell division. For asexual reproduction, growth, repair, and regeneration, mitosis is necessary. The bodies must create new cells—and permit the death of old cells—in order to expand and develop. The process of healing an injury also requires cell division.If cells were unable to divide and produce new cells, living organisms would never be able to regenerate skin cells to treat rashes or regrow a fingernail.