Cell division: Mitosis, Meiosis

Cell Division

The method through which a cell reproduces itself is through cell division. Growing and dividing new cells that are comparable to the injured ones can replace many damaged cells. Mitosis and meiosis are the two kinds of cell division. The majority of body cells contain 46 chromosomes and divide by a process called mitosis, which yields two brand-new, genetically identical daughter cells. The ova and spermatozoa, which are formed by meiosis, are the lone exception to this production of gamates (sex cells). Despite the fact that both kinds include cell reproduction, their strategies differ. The cell cycle, which is the time between two cell divisions, has two phases that can be seen under a microscope: interphase and mitotic phase.

Interphase: This is the longer phase and three separate stages are recognised:

• First Gap Phase (GI) - the cell grows in size and volume. This is usually the longest phase and most variable in length. Sometimes cells do not continue round the cell cycle but enter a resting phase instead (Go).
• Synthesis of DNA (S phase) - the chromosomes replicate forming two identical copies of DNA. Therefore, following the S phase, the cell now has 92 chromosomes, i.e. enough DNA for two cells and is nearly ready to divide by mitosis.
• Second Gap Phase (G2)- there is further growth and preparation for cell division.

Cell cycle

Mitotic (M phase): Mitotic, or somatic cell division or equational division is a process by which nuclei divide into two produce identical daughter cells. The number of each chromosome in each daughter nucleus is the same as the parent cells. In mitosis, one cell with the diploid number of chromosome (the usual number, 46 people) divides into two identical cells, each with the diploid number of chromosome. This production identical cell is necessary for the growth of the organ and for the repair of tissues.

Somatic cell division

• Prophase
• Metaphase
• Anaphase
• Telophase (and Cytokinesis)

Before mitosis can take places, a cell must have two complete set of chromosome because each new cell must have the diploid number. Mitosis cell division is completed in two cycles:

• Karyokinesis (nuclear division)
• Cytokinesis (cytoplasmic division)

 Karyokinesis

 There are four stages for the nuclear division.

• Prophase,
• Metaphase,
• Anaphase,
• Telophase.

Prophase: This is the first stage of mitosis. Events occurring in this stage are:

• The chromosomes coil up and become visible as short rods. Each attached at the centromere.
• The nucleolus and nuclear membrane disappear.
• The centreoles move towards opposite pole of the cell and organize the spindle extends across the equator of the cells.

Prophase

Metaphase: Events occurring in this stage are:

• The nuclear membrane and nucleolus are completely disappeared.
• The pairs of chromatids line up at the metaphase plate. The centromere of each pair is attached to a spindle fiber.
•  The centromeres now divide.

Metaphase

Anaphase: Events occurring in this stage are:

• Each chromatid is now considered a separate chromosome; there are two complete separate sets.
• The spindle fibers contract and pull the chromosomes, one set toward each pole of the cell.
• This phase completed within 10 minutes.

Anaphase

Telophase: Events occurring at this stage are:

• The sets of chromosomes reach the poles of the cell and become indistinct as their DNA uncoils to form chromatin.
•  A nuclear membrane and nucleoi reform around each set of chromosome.
•  Mitotic spindle disappears.

Telophase

Cytokinesis: The divisions of cytoplasm is known as cytokinesis. The division occurs in two methods:

• Cell plate method: Cell plate is formed at the equatorial region of the cell by the deposition of cell membrane dividing the cell into two new daughter cells, which are identical to parent cell.
•  Construction method: After telophase, the cell membrane is constricted at middle of the cell thus, dividing it into two identical cells.

Cytoknesis

Importance of Mitosis Cell Division

•  It helps in replacing the old and worn out cells.
• Injury or wound is healed by mitosis.
•  It is essential for growth and development of multicellular organisms

Meiosis

Meiosis is a more intricate kind of cell division that gives rise to egg and sperm cells. It is also known as the division of reproductive cells. A cell that has a diploid number of chromosomes splits twice during meiosis to produce four cells, each of which has a halploid number of chromosomes. Oogenesis is the process of meiosis occurring in the ovaries of females. Meiosis, also known as spermatogenesis in men, occurs in the testes. The reproductive system's oogenesis and spermatogenesis will be covered.

The haploid number of chromosomes in the egg and sperm cells formed during meiosis is 23 in humans. Meiosis is also referred to as reduction division since it reduces the amount of chromosomes in the egg or sperm. The 23 chromosomes of the sperm and the 23 chromosomes of the egg will then combine during fertilization to give the fertilized egg its original diploid number of 46. As a result, the new person's cells continue to contain the correct number of chromosomes.

Meiosis cell division can be split into two cycles, just like mitosis;

• Karyokinesis.
• Cytokinesis.

 Karyokinesis

 This cycle is again divided into two stages:

• Meiosis I (reductional/heterotypic division)
• Meiosis II (homotypic division)

• Meiosis I: there is no division of centromere. The nucleus undergoes the following complicated phases to reduce the chromosome number into half.

      Prophase I: It is a prolonged phase comprising of five sub-phases.

• Leptotene: at this stage, the chromosome are long and slender and definite     number of bead like granules called chromosomes are stayed along the length.
• Zygotene: Homologous chromosom align directly opposite each other and are held together at several points along their length. This phenomenon of pairing without fusion  is known as 'synapsis'.
• Pachytene: The chromosomes become short and thick. Each bivalent chromosome splitslongitudinally into two chromatids which remain attached to each other by a centromere. Exchange of genetic material between two homologous chromosomes takes place in this stage which is called as 'crossing over'.
• Diplotene: Repulsive force develops between the chromatids of homologous chromosomes separate from each other except one or two point of attachment where crossing over takes place is known as 'Chiasmata'.
• Diakinesis: The short and thick chromosomes appear like 'O' or 'X' due to the formation of chiasmata. Nuclear membrane and nucleolus disappear.

Prophase I

Metaphase I: All chromosomes come to at equatorial plane. The tractile fibers appear from opposite poles and get attached to the centromere of each chromosome.

Metaphase 1

Anaphase I: Chromosomes moves towards the opposite pole along the tractile fibres.

Anaphase I

Telophase I: Half number of chromosomes, each with two chromatids forms a compact grows up in each pole. Nuclear membrane and nucleolus reappear.

Telophase I

Meiosis II: It is also called mitosis cell division of meiosis. Thus, all stages are same but here we use prophase II, Metaphase II, Anaphase II and telophase II. Telophase II of meiosis produce 4 haploid daughter cells, which are separated by cytokinesis.

Meiosis II

Fig: Meiosis II cell division

• Cytokinesis

            In meiosis cell division, cytokinesis forms 4 daughter cells by two methods as of mitosis.

Cytokinesis of Meiosis

Fig: Cytokinesis of meiosis cell

 Importance of Meiosis Cell Division

•  It keeps the number of chromosomes constant in a species.
•  It is important for the continuity of species.
• Variation in living things is due to meiosis.
•  Variation helps the breeders in improving the races of useful animals.

Table: Difference between Mitosis and Meiosis Mitosis