Cell Division : Mitosis, Meiosis, Process and Differences

Cell Division is of two type’s viz. Mitosis and Meiosis. In mitosis chromosomes separates and form into two identical sets of daughter nuclei, and it is followed by Cytokinesis (division of cytoplasm). Meiosis is also called reductional cell division and the number of chromosomes is divided into half in this process. Meiosis is required to create the Gametes in animals and Spores in other organisms. Meiosis is a prerequisite for sexual reproduction in organisms with Eukaryotic cells.

Mitosis

Mitosis occurs exclusively in eukaryotic cells. In the prokaryotic cells, there is no Nucleus and cell division takes place by a process called binary fission. Further, in animals there is Open Mitosis which means that nuclear envelope breaks down before the chromosomes separate. In some kinds of fungi, such as Aspergillus, the process is “Close Mitosis”, in which the nucleus remains intact while the chromosomes divide.

W. Flemming (1882) was the first scientist to use the term mitosis. The cell cycle in mitosis is divided into interphase, Mitosis and Cytokinesis. Further, the interphase is divided into 3 stages viz. G1 stage, S stage and G2 stage.

Interphase

In the Interphase, DNA is duplicated and other essential substances are synthesized so that the cell grows and gets ready for the division. The first stage is G1-stage in which RNA and proteins are synthesized. When the cell reaches certain size, the S-stage starts. In the S-stage, The DNA molecule replicates and becomes two stranded as a consequence of replication. With the replicated DNA, the cell enters into G2 stage. In this stage some spindles called mitotic spindles start forming out of spindle microtubules, associated proteins, and any centrosomes or asters. The amount of DNA gets doubled.

Prophase

Each chromosome in the beginning of the Prophase is a two thread like structure. These threads are called Chromatids. The Chromatids remain coiled with each other. The point, where both Chromatids remain joined to each other is known as a centromere. Each Chromatid has small granule like structure. This granule like structure is called Chromomere. This process is followed by disappearing of Nucleolus and Nuclear membrane.

Metaphase

During the metaphase, the chromosomes lie at the equator and become thick and short. The Chromatids which were coiled to each other in the prophase, get uncoiled but lay side by side. In the metaphase stage, two kinds of spindle apparatus’ are formed. One type of fibers are called supporting fibers which are spread from pole to pole without any contact with the Chromosomes. The another kind of spindle is called tractile fibers, which are attached to the centromere on the one hand and pole to the other hand. During the last phase of the Metaphase, there is a division of the centromeres and the sister Chromatids get separated and independent from each other.

Anaphase

The independent Chromatids are now called Chromosomes. These now start moving towards the opposite poles of the spindle. The Anaphase stage lasts till the chromosomes assemble at each pole of the spindle.

Telophase

In the beginning of the Telophase, the nucleolus starts reappearing and the spindle apparatus starts disappearing. The nuclear membrane starts forming around each group of new Chromosomes.

Cytokinesis

A the end of the telophase, the division of the cytoplasm begins. In the plant cells, granular bodies formed from the Golgi apparatus and microtubules start collecting in the equatorial region of the cell. They together make the cell plate and the apparatus made for this particular purpose is called Phragmoplast. Phragmoplast is concerned with the formation of the cell plate. When the cell plate forms completely, it divides the cell into two daughter cells. This is called Plate method and is applicable to Plant Cells. In the animal cells, the division of the cytoplasm occurs by the furrowing method in which the cell membrane constricts parallel to the equator.

Mitosis: significance

• The number of the Chromosomes in Parent and daughter cells remains constant
• The parent and daughter cells are similar in all respects.
• The parent and daughter cells are genetically identical
• The purpose of Mitosis is growth by increasing number of cells.
• In most plants and animals the regeneration of the lost parts and vegetative propagation in some plant species takes place by Mitosis.

Meiosis

The cell division in the reproductive cells takes place by Meiosis. In meiosis the number of the chromosomes is reduced to half of that in the parent cells. Meiosis maintains the number of Chromosomes constant in all sexually reproducing organisms. The term “Meiosis” was proposed by Farmer and Moore in 1905.

The main phases in Meiosis are similar to those in mitosis, but the outcome of the process is entirely different. Since, the chromosome number of the parent cell gets reduced in the Meiosis; four daughter cells are produced, each with one-half the chromosome number of the parent cell. Each daughter cell contains only one chromosome from each homologous pair.

Thus, Meiosis has been divided into two divisions called Meiosis I and Meiosis II. Meiosis I is also known as heterotypic division or reduction division and in this phase the Chromosome number is reduced to half. In meiosis II, the reduced number of Chromosomes is reproduced and total result is 4 cells.

Meiosis I

Prophase

Prophase in the meiosis is a long and complicated process. It has been divided into 5 sub phases called Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis. Each of them has been discussed here:

Leptotene: In Leptotene, the Chromosomes are long thread like structures with Chromomeres. Half of the Chromosomes are from male parent and half of the Chromosomes are female parent.

Zygotene: In the Zygotene sub phase, a process called “synapsis” takes place. In synapsis, the homologous chromosomes start paring and each pair is called bivalent. The Chromosomes become thicker and shorter.

Pachytene: There is further coiling in the Chromosomes and they become more thick. Each chromosome appears double and it is a 4 stranded stage. In this stage each chromosome are in “tetravalent” stage and each chromosome is called a “tetrad”. This is followed by exchange of pieces of homologous chromosomes which is called “Crossing Over”.

Diplotene: In this sub phase, the chromosomes move apart, however the homologous chromosomes don’t separate completely but remain joined at one or more point called Chiasmata. The Chromatids of the homologous chromosomes begin to separate from the centromere. Chiasmata move towards the telomeres and this is called terminalization.

Diakinesis: In this phase, the spirilization and condensation of chromosomes continues and the nucleolus & nuclear membranes disappear.

Metaphase-I

A nuclear spindle forms in this stage. The Chromosomes arrange themselves on the equator of the spindle and spindle fibers get attached to the centromeres of two homologous Chromosomes.

Anaphase-I

In this stage the homologous chromosomes start moving towards the opposite poles and as a sequel to this the chromosome number is reduced to half. Each chromosome at this stage consists of two Chromatids which are joined by a centromere.

Telophase-I

In this stage, the Chromosomes reach the opposite poles. Nuclear membrane is formed but nucleolus is NOT formed. The Chromosomes uncoil. The cell membrane at this stage may or may not be formed. The single nucleus is divided into two daughter nuclei and the number of chromosomes is halved.

Meiosis –II

The main purpose of making the number of Chromosomes halved is done in the Meiosis-I and now there is another division, which is just similar to the Mitosis process. It has the same phases viz. Prophase, metaphase, Anaphase, and Telophase. Each of them is suffixed by II so that study becomes easy.

Prophase-II

The nuclear membrane again gets disappeared. The two Chromatids of each chromosome remain separate from each other except at centromere. Nucleolus is also disappears at the end of Prophase II

Metaphase-II

The nuclear spindle is formed. Chromosomes arrange themselves on the equator of the spindle.

Anaphase –II

There is a division of centromere in this stage. The sister Chromatids which are now eligible to get themselves called by Chromosomes move to opposite poles.

Telophase –II

1. The sister Chromatids or chromosomes assemble at the poles. The nucleolus and nuclear membrane re appear.

At the end of telophase II, Cytokinesis again takes place and thus 4 daughter cells are produced. The result number of chromosomes in each cell is Haploid which is represented by n.

Meiosis : significance

The 4 cells formed as a result of meiosis contain the half number of Chromosomes.

During sexual reproduction, the doubling of the Chromosomes takes place. Meiosis keeps the number of Chromosomes constant from generation to generation.

By meiosis, new combinations of Chromosomes and genes are produced. This new combination occurs during crossing over and random distribution of paternal and maternal chromosomes occurs in the daughter cells.