Cell Division            

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Question: What is Cell Division?

                

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Write a short note on the Cell Division 

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Stages of Cell Division 

Cell division

All living things are made of cells. A single cell divides many times to grow into a large organism.

Cell Division Definition: 

Cell Division is a fundamental and active biological process by which a cell produces its replica having similar structural and physiological properties and brings upon continuity of life.

There are three types of cell divisions amitosis, mitosis and meiosis. 

Amitosis and mitosis maintain the constant chromosome number of somatic cells. Meiosis reduces the chromosome number to half and keeps the chromosome number constant in sexually reproducing organisms.   

Amitosis ( Direct cell division) :

Amitosis Definition:

The cell division showing simple nuclear cleavage without the formation of spindle fibres is known as Amitosis or direct cell division.

Amitosis: A- Bacteria, B- Yeast, C- Ameoba.

Amitosis or direct cell division is the method of asexual reproduction in unicellular organisms like bacteria, fungi and protozoa and unicellular fungi like yeast. It is also the multiplication process in vertebrate foetal membranes.

• 1. Here the cell and its nucleus enlarge. 
• 2. Constriction appears in the middle of the cell membrane.
• 3. Nuclear constriction deepens and divides it into two daughter nuclei.
• 4. Cell invagination moves inwards, dividing parent cells into two equal-sized daughter cells.

Mitosis ( Eumitosis or Indirect cell division):

Mitosis: Overview 

Mitosis Definition:

The cell division in which the parent cell divides once giving rise to two identical daughter cells with the same chromosome number with similar form and characteristic features is called mitosis.

Mitosis is divided into karyokinesis and cytokinesis.

Karyokinesis:

Karyokinesis is the process of division of the nucleus which takes place in four distinctive phases. They are:

Prophase:

In prophase, condensation of chromosomes, spindle formation and disappearance of nucleolus and nuclear membrane take place.

Metaphase:

During metaphase, the chromosomes are arranged on the equatorial plane and the spindle fibres get attached to their centromeres.

Anaphase:

In Anaphase the centromere divides and also the chromosomes divide longitudinally. The separated chromosomes move to the opposite poles due to polymerization and de-polymerisation of the spindle fibres.

Telophase:

During telophase, the sister chromatids aggregate to the poles, nucleus and nuclear membrane reappear leading to the formation of two daughter nuclei in the two poles.

Cytokinesis:

It is the process of division of cytoplasm.

Telophase is followed by cytokinesis in plant cells by the formation of cell plates but in animal cells, it occurs by the formation of two lateral constrictions, which is simultaneous with nuclear division.

* Note: In eukaryotic cells, anastral mitosis ( spindle formation takes place from the nucleoplasm or nuclear membrane) is noted in plant cells but astral or amphiastral mitosis ( spindle fibres are formed from the centriolar astral rays) is noted in animal cells.

Meiosis:

Meiosis occurs in the gamete-producing cells of sexually reproducing organisms called meiocytes. 

Meiosis Definition:

Meiosis is a complex cell division process in which the diploid number of chromosomes of the mother cell is reduced to a haploid number of chromosomes in the four daughter cells.

Meiosis: Overview 

Meiosis not only reduces the chromosome number to half to keep the chromosome number constant in a particular species through subsequent generations but also produces variations to the recombination of genes by helping in evolution.

The process of meiotic cell division consists of two complete divisions of a diploid cell producing four haploid daughter cells.

It includes two separate divisions. The first one is reductional, i.e. Meiosis I, while the second one is equational (which is the same as mitosis) called Meiosis II.

Meiosis I

The first meiotic division starts in the interphase and is subdivided into the following divisional stages:

Prophase I:

The first meiotic prophase is a long and complicated phase. Thus it is further divided into Leptotene, zygotene, pachytene, diplotene and diakinesis.

Preleptotene is a preparatory phase when the nuclear reticulum uncoils to form the chromosomes.

During Leptotene ( or Leptonema) the chromosomes become distinct, they undergo synapsis with the formation of a synaptonemal complex in the zygotene. 

Zygotene (or Zygonema) is a stage of prophase I in which the pairing of homologous chromosomes and formation of the synaptonemal complex takes place which facilitates crossing over or recombination 

Pachytene (or  Pachynema) shows crossing over or recombination between non-sister chromatids of homologous chromosomes. 

During diplotene (diploma) the non-sister homologous chromosomes touch each other followed by their separation. The formation of chiasmata and terminalisation takes place. 

In diakinesis, the terminalisation continues and the number of chiasmata is reduced.

Metaphase I: 

After the long prophase I, in metaphase I, a spindle is formed between the centrioles ( 180° apart), the homologous chromosomes pair arranged on the equatorial plate, and centromere are directed towards opposite poles while chromatids remain at the metaphase plate.

Anaphase I:

 Anaphase I brings about the separation of homologous chromosomes but sister chromatids remain associated at their centromeres.

Telophase I:

During telophase I, the nuclear membrane and nucleolus reappear. The two daughter nuclei are formed on the opposite poles with a haploid number of chromosomes after the reduction of chromosome number

After telophase, cytokinesis ( just like mitosis) follows forming two daughter cells called diads. 

The two daughter cells quickly pass on to the second meiotic division after a short resting period called interkinesis ( 2nd interphase). Interkinesis is a short period between meiosis I and II. 

Meiosis II:

Meiosis II is initiated immediately after cytokinesis, mostly before the chromosomes have fully elongated.

During meiosis II the chromosomes divide like in mitosis with four substages, i.e., 

Prophase II:

  where the chromosomes are visible again because of dehydration. The centrosome divides forming the centriolar spindle. The nucleolus and nuclear membrane disappear. 

Metaphase II

The next phase of metaphase II is very brief where the chromosomes line at the equatorial line of the spindle fibres. The centromere attached to the spindle remains on the metaphase plate while the chromatids are extended towards the poles.

Anaphase II:

 The cell reaches the Anaphase II where the chromosomes divide longitudinally with each half having half of the centromere and single chromatid. They then move to the opposite poles.

Telophase II:

This phase is characterised by the reaching of daughter chromosomes to the opposite poles. The stage exhibits the formation of the nuclear membrane and nucleus. Four daughter nuclei with a haploid number of chromosomes are formed.

 With Cytokinesis II the second meiotic division results in the formation of four haploid cells having chromosomes composing both parental combination and new recombinant traits. 

They become the functional gametes in higher organisms and undergo sexual union to restore the original diploid chromosome status.