Cell Division: Mitosis and Meiosis

The wounds in your body, the healing of the broken parts of plants, and the repairing of different damaged parts of animals occur due to the cell’s ability to divide. Cell division is when the parent cell multiplies into two or more daughter cells.

The cell division in eukaryotic organisms is usually of two types: mitosis and meiosis. Mitosis cell division is the process cells use to make exact replicas of themselves, so it occurs in somatic or non-reproductive cells. In contrast, meiosis does not produce identical daughter cells and occurs in reproductive cells.

The general steps of cell division are karyokinesis (a division of the nucleus) and cytokinesis (a division of cytoplasm). The karyokinesis stage of mitosis has two primary phases: interphase and M phase. The M phase is where cell division begins and has four steps: prophase, metaphase, anaphase, and telophase.

The karyokinesis of meiosis cell division has two meiotic divisions: meiotic I and meiotic II. Meiotic II follows meiotic I without intervening interphase. Meiotic I has four stages: prophase I, metaphase I, anaphase I, and telophase I without cytokinesis.

Mitosis Cell Division

Mitosis is a process of the cell cycle that occurs in plant and animal cells where the division of pre-existing cells produces two identical daughter cells. During mitosis, replicated chromosomes are separated into two daughter nuclei containing equal amounts of genetic information. On account of this, mitosis is also known as equational cell division.

Stages of Mitosis Cell Division

Howard and Pele (1953) have divided the cell cycle into G1, S, G2, and M phases. G1 phase, S phase, and G2 phase are combined in the interphase.

Interphase

It is the prolonged cell cycle phase where the daughter cell prepares before the mitosis phase begins. It is divided into three stages;

G1 phase

This phase is also known as the resting phase, first gap phase, or first growth phase.
DNA synthesis does not take place.
It involves the synthesis of RNA, protein, and membranes needed for the development of cytoplasm and nucleus of daughter cells.

S phase

Also known as S-phase or synthetic phase
It involves the synthesis of histone protein (needed for replication), and two DNA molecules are formed by replication.

G2 phase

Also known as the second gap or growth phase, or resting phase of interphase
The phase where DNA synthesis ends and the prophase stage initiates
Continuous synthesis of RNA and proteins that is required for cell growth takes place.

Mitotic phase or M-phase

It is the short period of chromosome condensation, separation, and cytoplasmic division. This phase initiates at the point of interphase (G2 phase). It is divided into the following phases;

Prophase (Pro= Before; Phasis= Appearance)

The appearance of a thin-thread-like condensing chromosome containing two chromatids held together by the centromere marks the first phase of mitosis, called prophase.
The cell begins the process of division.
The nuclear envelope disappears.
Formation of the spindle or mitotic apparatus in the cytoplasm takes place.

Prometaphase:

The nuclear envelope’s disappearance marks the prometaphase’s initiation and enables the mitotic spindle to interact with the chromosome.
The spindle appears to be aligning the chromosomes at the metaphase plate is the characteristic feature of the stage.
Microtubules attach to the kinetochore; balanced bipolar force holds the chromosomes on the metaphasic plate.

Metaphase

Chromosomes are the shortest and thickest.
Centromeres occupy the plane of the equator of the mitotic apparatus (equatorial or metaphasic plate)

Anaphase

It begins precipitously with the synchronous splitting of each chromosome into its sister chromatids called daughter chromosomes with one kinetochore.
After separation, each chromosome moves toward the opposite pole. As the microtubules of the mitotic spindle pull chromosomes, they appear V-shaped.

Telophase

The end of the polar migration of daughter chromosomes marks the beginning of the telophase.
Each separated daughter chromosome resumes their long, slender, extended
form as their coils relax
Nuclear envelopes reunify around each group of chromosomes to form daughter nuclei.
Mitotic apparatus except the centrioles disappears.
Telophase is followed by cytokinesis, constricting the cytoplasm into two separate cells.

Purpose/Significance of mitosis

To maintain proper size of the cell
To maintain equilibrium in quantity of RNA and DNA in the cell
To restore old or dead cells of the body
In some organisms, it is involved in asexual reproduction
Provides opportunity for the growth or development of organs and the body of individuals
Maintains equal distribution of chromosome to each daughter cell with pure genome as recombination or crossing over does not take place in mitosis.
Embryogenesis and blastogenesis both involves mitosis
Gonads and sex cells undergo mitosis in order to increase their number.

Meiosis Cell Division

Van Beneden first described meiosis cell division in 1883. Meiosis is defined as the process of cell division in which the original diploid cell divides twice to produce a total of four haploid cells. Thus, formed haploid cells consisting of half number of chromosome as the original diploid cell gives rise to gametes (sperm or eggs) that, on fertilization, supports sexual reproduction and a new generation of a diploid organism.

Stages of meiosis cell division

Meiosis has been divided into four stages; pre-meiotic interphase, meiosis I, intrameiotic interphase, and meiosis II.

Pre-meiotic interphase

Before entering into meiosis I, a cell undergoes a period of growth phase called interphase
DNA duplication occurs at the S-phase.
The nucleus and nucleolus become visible.

Meiosis I

It is also known as Reductive Division or Heterotypic Division. There is reduction of chromosome number occurs in two haploid daughter cells produced from the original diploid cell. It has been divided into five stages: Prophase I, Prometaphase, Metaphase I, Anaphase I, and Telophase I

Prophase I

It is the longest phase occupies up to 90%, with six sub-stages: proleptotene, leptotene, zygotene, pachytene, diplotene, and diakienesis.

Proleptotene or Proleptonema: Chromosomes are extremely thin, long, uncoiled, longitudinally single, and slender thread-like structures.
Leptotene or Leptonema: Chromosome becomes more uncoiled and long thread-like structure with a specific orientation inside the nucleus that looks like a ball of knitting wool. After duplication of centrioles, each pole of the cell possesses two centrioles. The process of homology search begins to initiate the pairing of homologs.
Zygotene or Zygonema: The pairing of homologous chromosomes that come from father (sperm) and mother (ova) takes place; such pairing is known as Synapsis (or Homologous Dyads). The pairing of homologous chromosomes is exact and specific (gene-for-gene). A protein-containing framework, Synaptonemal Complex (SC), joins the paired homologous chromosomes till crossing over completes.
Pachytene or Pachynema: Synapsed chromosomes become thick and short; each synaptonemal pair is called bivalent or dyads because of two chromosomes and four visible chromatids, respectively. The crucial genetic phenomenon “Crossing Over” takes place. It is the interchange of chromatin material between one non-sister chromatid of each homologous chromosome accompanied by chiasmata formation. During crossing over, recombination of genetic material occurs by mutual exchange of corresponding segments by breakage and reunion with enzymes recombinase and ligase, respectively.
Diplotene or Diplonema: Unpairing or desynapsis of homologous chromosomes initiates, and first, chiasmata appear. Synaptonemal complex disappears, leaving participating chromatids of the paired homologous chromosome, which physically joins at chiasmata. Chromatids of the paired homologous chromosome physically participate at one or more discrete points, known as Chiasmata, where crossing over occurs.
Diakinesis: The movement of chiasma from the centromere to the end terminal point of the chromosome occurs. This movement is terminalization. Chromatid remains connected by the terminal chiasmata and exists up to the metaphase.

Pro metaphase

The nuclear envelope disintegrates. The spindle assembles at the opposite pole of the cell. Chromosomes got coiled in a spiral manner and arranged on the spindle’s equator.

Metaphase I

Spindle fiber attached to the chromosome helps align the chromosome at the equator. This stage terminates as soon as the homologous chromosome starts to separate from each other.

Anaphase I

The homologous chromosome gets separated and moves towards the opposite pole. Actual reduction and disjunction occur at this stage. The number of chromosomes at each pole is precisely half (n) as each pole receives one homologous from each bivalent present in the cell.

Telophase I

The arrival of a half set of chromosomes at each pole defines the initiation of telophase. Nucleolus reappears. Chromosomes uncoil, and a nuclear envelope is formed around the chromosomes. After Karyokinesis, cytokinesis occurs through which two haploid cells are formed.

Intra Meiotic or Interkinesis

The short resting phase between telophase- I and Prophase-I. No DNA replication occurs in this stage.

Meiosis II

It is also known as the Equational Division or Homotypic Division. This stage includes dividing each haploid meiotic cell into two haploid cells. It consists of four steps;

Prophase II

Two pairs of centrioles are formed and move toward the opposite pole.
Chromosomes with two chromatids become short and thick.
The nuclear membrane and nucleolus disappear.

Metaphase II

Chromosomes get organized on the equator of the spindle.
Centromere divides into two. Thus, each chromosome produces two daughter chromosomes.

Anaphase II

Daughter chromosomes move towards the opposite pole due to the shortening of chromosomal microtubules and stretching of interzonal microtubules of the spindle.

Telophase II

Chromatids migrate to the opposite poles known as chromosomes.
A nuclear envelope is formed around the chromosome, and the nucleolus reappears.
After Karyokinesis, cytokinesis occurs in each haploid meiotic cell, resulting in four haploid cells.
Thus formed cells have different types of chromosomes due to the crossing over.

Purpose/Importance of Meiosis

Meiosis maintains a persistent number of chromosomes in the organisms.
By crossing over, genetic variations among the species can lead to evolution.
It also facilitates segregation and an independent assortment of genes.
Leads to the continuity of generations by producing gametes(sperm and ova) in sexually reproducing species.

References

Mitosis/ cell division/ Scitable. Scitable. https://www.nature.com/scitable/definition/mitosis-cell-division-47/
Meiosis. Scitable. https://www.nature.com/scitable/definition/meiosis-88/
Verma Ps and Agrawal VK (2008). cell biology, genetics, molecular biology, evolution and ecology. Ram nagar, New Delhi: S.Chand & Company Pvt.Ltd. https://books.google.com.pk/books?id=RXscEAAAQBAJ&printsec=frontcover#v=onepage&q&f=false
Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Meiosis. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26840/