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  The Cell Cycle, Biology tutorial

Introduction:

The method of cell division in eukaryotic cells is cautiously controlled. The cell cycle is the life-cycle of a cell, having cell division at the end of the cycle. Similar to a human lifecycle which is made up of different stages, similar to childhood, adolescence and adulthood, there are sequences of steps which lead to cell division.

These steps can be categorized into two main components: Interphase and mitosis.

Interphase: The phase if the cell generally performs its 'everyday' functions. For illustration, it is if a kidney cell does what a kidney cell is assumed to do.

Mitosis: The phase if the cell gets ready to become two cells.

1) Interphase: It is a long, metabolically active phase among the two successive mitotic cell divisions. It consists of three sub phases.

a) The first growth phase (G1): Throughout the G1 stage, the cell two times in size and doubles the number of organelles. The cell prepares for the DNA, RNA and protein synthesis.

b) The synthesis phase (S): The DNA is replicated throughout this phase. In another words, a similar copy of the entire cell's DNA is made. This makes sure that each new cell consists of a set of genetic material similar to that of the parental cell. The Replication or duplication of DNA and Centrioles occur.

c) The second growth phase (G2): The proteins are synthesized which will assist in the cell division. At the end of Interphase, the cell is prepared to enter the mitosis. The synthesis of proteins needed for the synthesis of spindle fibers occur.

2) Mitosis: All through mitosis, the nucleus splits or divides. Mitosis is followed by cytokinesis, when the cytoplasm splits or divides resultant in two cells. After cytokinesis, the cell division is complete. Scientists state that one parent cell or the dividing cell, makes two genetically similar daughter cells, or the cells which divide from the parent cell. The word 'genetically similar' signifies that each cell consists of a similar set of DNA, and this DNA is as well similar to that of the parent cell. When the cell cycle is not carefully controlled, it can cause a disease termed as cancer that causes cell division to occur too fast. A tumor can outcome from this type of growth.

The Four Phases of Mitosis:

Throughout mitosis, the two sister chromatids should be split apart. Each resultant chromosome is build up of 1/2 of the "X". However this procedure, each daughter cell receives one copy of each and every chromosome. Mitosis is classified into four phases:

A) Prophase:

i) It is the longest stage. Throughout this stage the chromatin is organized into different chromosomes by spiralization or coiling.

ii) The Centrioles build up into asters and move in the direction of the opposite poles of the cell to set up the plane of cell division.

iii) Spindle apparatus start to appear.

iv) Nucleolus and Nuclear membrane break up and disappear.

v) The chromosomes are set free in the cytoplasm.

B) Metaphase:

i) Spindle fibers are totally formed.

ii) The chromosome become small and thick having two dissimilar chromatids each.

iii) All the chromosomes move in the direction of the centre of the cell and arrange in the equatorial plane, right angles to the position of asters to make metaphase plate.

iv) Chromosomes are joined to spindle fibers at their centromeres.

C) Anaphase:

i) The centromere of the entire chromosomes experience longitudinal splitting and the chromatids of each chromosome separate to build up daughter chromosomes.

ii) The daughter chromosomes move in the direction of the opposite poles from the equator by the activity of the spindle fibers.

D) Telophase:

All through this, the events of prophase will be reversed.

i) The daughter chromosomes arrive at the opposite poles.

ii) The chromosomes experience despiralization to make long, thin thread like structures termed as chromatin.

iii) Nucleolus and nuclear membrane reappears.

iv) The spindle fibers vanish.

Significance of Mitosis:

a) It maintains the genetic stability within population of cells derived from the similar parental cell.

b) It assists the growth and repair of tissue.

c) It assists in the substitution of dead and worn out cells.

d) It signifies of reproduction in the lower organisms.

Meiosis:

The production of offspring by means of sexual reproduction comprises the fusion of two gametes, each having a complete haploid set of chromosomes. Gametes are made from specialized diploid cells. This specialized type of cell division which decreases the chromosome number by half, outcomes in the production of haploid daughter cells. This type of division is termed as meiosis. Meiosis makes sure the production of haploid stage in the life cycle of sexually reproducing organisms while fertilization restores the diploid phase. We come across meiosis all through gametogenesis in animals and plants. This leads to the formation of haploid gametes. The main features of meiosis are as shown:

1) Meiosis comprises two sequential cycles of nuclear and cell division termed as meiosis I and meiosis II however only a single cycle of DNA replication.

2) Meiosis I is started after the parental chromosomes have replicated to generate similar sister chromatids at the S-phase.

3) Meiosis comprises pairing of homologous chromosomes and recombination among them.

4) Four haploid cells are made at the end of meiosis II.

The events of Meiotic can be classified under the given phases:

Meiosis I

Meiosis II

Prophase I

Prophase II

Metaphase I

Metaphase II

Anaphase I

Anaphase II

Telophase I

Telophase II

Significance of Meiosis:

1) It assists to restore diploidy and maintain the constant number of chromosomes for a species.

2) Meiosis generates new combination of chromosomes and genes through crossing over and by an arbitrary distribution of maternal and paternal chromosomes to daughter cells. Such two events result in variations that are the food for the speciation.

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