Cell Division

Cell Cycle

The entire changes or events taking place during the division of a mother cell into daughter cells are known as the cell cycle.

The cell cycle completes in the following phases:

1. Interphase
   1. G₁ phase
   2. S phase
   3. G₂ phase
2. M-phase or mitotic phase
3. Cytokinesis

Interphase

I) G₁ Phase / Gap One Phase / Growth Phase

• Active synthesis of RNA, carbohydrates and lipids occurs.
• Cell grows in size and vacuoles disappear, as written in the scan.
• Protein synthesis takes place.
• Necessary enzymes are synthesized.

II) S Phase / Synthetic Phase

• Replication of DNA molecule occurs.
• Histones (basic proteins) are synthesized.
• Actual duplication of chromosomal materials occurs.

III) G₂ Phase / Gap Two Phase / Second Growth Phase

• Synthesis of spindle proteins takes place.
• Synthesis of three types of RNA molecules by DNA occurs.
• Duplication of mitochondria, plastids and centrioles occurs.

M-Phase and Cytokinesis

M-Phase

M-phase is the division of nucleus. The supplied notes describe it as the mitotic phase for mitosis and meiotic phase for meiosis.

M-phase completes through:

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Cytokinesis

Cytokinesis is the last stage of the cell cycle. In this stage, division of cytoplasm takes place and finally a mother cell is divided into daughter cells.

Types of Cell Division

The cells may divide by any of the following three methods:

1. Amitosis cell division
2. Mitosis cell division
3. Meiosis cell division

1. Amitosis or Direct Cell Division

Amitosis is a direct cell division. It occurs among unicellular organisms such as some protozoans, bacteria, yeast, etc., as written in the source. At the end of amitosis two daughter cells are formed which are not necessarily identical to each other or to the mother cell.

2. Mitosis Cell Division

Mitosis is known as equational or indirect cell division. It occurs among vegetative or somatic cells. At the end of mitosis two daughter cells are produced which are qualitatively and quantitatively identical to each other and also to the mother cell. The number of chromosomes remains constant. The scan states that mitosis can occur in haploid or diploid cells.

Mitosis completes through:

1. Interphase
2. Karyokinesis
3. Cytokinesis

Stages of Mitosis

I) Interphase

It is the resting phase in which no visible changes occur in the nucleus, but the cell is metabolically active.

• Cell has a large nucleus with intact nuclear membrane and distinct nucleolus.
• Chromosomes occur as diffused, long, coiled and indistinct chromatin fibres.
• DNA/chromosome is duplicated into exact copies.
• RNA and proteins are also synthesized.
• Size of nucleus and cell enlarges.
• Duplication of mitochondria, plastids and centrioles occurs.

II) Karyokinesis

Karyokinesis is the division of nucleus and completes through prophase, metaphase, anaphase and telophase.

A) Prophase

• Chromatin fibres begin to condense and become chromosomes.
• Chromosomes begin shortening and thickening.
• Each chromosome appears to consist of two chromatids due to longitudinal splitting.
• The chromatids remain attached together at the centromere.
• Nuclear membrane and nucleolus begin to disorganize and disappear by the end of the phase.

B) Metaphase

• Nuclear membrane and nucleolus disappear.
• Spindle fibres appear at two opposite poles and attach to centromeres of chromosomes.
• All chromosomes come to lie at the equator of the cell in a single line.
• Chromosomes are distinctly visible and can be counted; their shape and size can be determined.

C) Anaphase

• Centromeres split longitudinally so that two sister chromatids separate.
• Each chromatid receives its own centromere and is known as a daughter chromosome.
• Daughter chromosomes migrate toward opposite poles.
• Each chromosome appears V-, J-, I- or U-shaped, as written in the source.
• At the end, two groups with equal chromosome number are formed at the opposite poles.

D) Telophase

• All daughter chromosomes at their respective poles become organized into nuclei.
• Each daughter nucleus has equal number of chromosomes as the mother cell.
• Spindle fibres partially or completely degenerate.
• Chromosomes uncoil and become an indistinguishable mass.
• Nuclear membrane appears around each group of chromosomes and nucleoli reappear.

III) Cytokinesis in Mitosis

Cytokinesis is the division of cytoplasm. It takes place by cell-furrow method in animal cells and by cell-plate method in plant cells.

Cell Plate Method

In plant cells, cytokinesis occurs by formation of a cell plate. Small granular bodies described in the scan as phragmoplast-related elements gather in the equatorial region of the cell to form a cell plate. The cell plate becomes the middle lamella. Finally, the mother cell is divided into two daughter cells having equal number of chromosomes.

Significance of Mitosis

• Mitosis maintains the fixed number of chromosomes in somatic cells.
• It helps in growth and development of new organs in multicellular organisms.
• It repairs old cells and regenerates damaged tissues and organs.
• It helps in asexual reproduction.

3. Meiosis Cell Division

Meiosis is known as reductional cell division. It occurs in reproductive cells of sexually reproducing organisms at the time of gamete formation. It takes place in diploid cells only, as stated in the source.

At the end of meiosis four daughter cells are formed with half the number of chromosomes of the mother cell. The four daughter cells are not identical to one another or to the mother cell. Meiosis consists of two nuclear divisions called Meiosis-I and Meiosis-II.

Meiosis-I

Meiosis-I is known as the reductional division. It completes in the following stages:

1. Prophase-I
2. Metaphase-I
3. Anaphase-I
4. Telophase-I

Prophase-I

Prophase-I completes through five stages:

1. Leptotene or leptonema
2. Zygotene or zygonema
3. Pachytene or pachynema
4. Diplotene or diplonema
5. Diakinesis

I) Leptotene

• Size and volume of nucleus increase.
• Chromosomes become visible due to condensation, shortening and thickening.
• Chromosomes have beaded structures along their entire length called chromomeres.
• Nuclear membrane and nucleolus remain intact.

II) Zygotene

• Chromosomes become more distinct by further shortening and thickening.
• Pairing of homologous chromosomes, one paternal and one maternal, occurs to form bivalents. The pairing process is called synapsis.
• A nucleoproteinous complex called synaptonemal complex is formed between homologous chromosomes.
• Nucleolus increases in size, as written in the source.

III) Pachytene

• Bivalents become shorter, thicker and more distinct.
• Each homologous chromosome splits longitudinally, except in the centromere region, into two sister chromatids; the bivalent appears as a tetrad.
• Exchange of chromatid segments between two non-sister chromatids of each tetrad takes place. This is called crossing over.
• Points at which crossing over takes place are known as chiasmata.

IV) Diplotene

• Homologous chromosomes of each bivalent begin to move away from each other throughout their length except at one or more points where non-sister chromatids remain attached.
• Homologous chromosomes tend to separate but remain attached at one or more chiasmata.
• Nuclear membrane remains intact while nucleolus tends to degenerate.

V) Diakinesis

• Chiasmata move toward the ends of chromosomes due to chromosome contraction. This movement is called terminalisation of chiasmata.
• Homologous chromosomes tend to separate but remain attached at one or more points along chromatids.
• Chromosomes become further shorter and thicker.
• Spindle fibres begin to form at the end of this stage.

Metaphase-I, Anaphase-I and Telophase-I

Metaphase-I

• Nuclear membrane and nucleolus disappear and spindle formation is completed.
• Bivalents arrange themselves into two parallel metaphase plates, as written in the scan.
• Bivalents are attached to spindle fibres by their centromeres.
• Centromeres of bivalents lie equidistant from equator and are directed toward poles, while arms generally lie horizontally in the equator.

Anaphase-I

• Centromere of each chromosome does not divide; two chromatids of each chromosome remain joined together by centromere.
• Homologous chromosomes migrate toward opposite poles by contraction of spindle fibres.
• Two groups of haploid chromosomes are formed, one at each pole, resulting in reduction of chromosome number.
• Homologous chromosomes at each pole are either paternal or maternal in origin.

Telophase-I

• Each pole receives one haploid set of chromosomes, each having two chromatids.
• Chromosomes uncoil and form chromatin threads due to decondensation.
• Spindle fibres disappear.
• Nuclear membrane reappears around the two groups of chromosomes at each pole and nucleolus also appears.

Meiosis-II

The notes describe Meiosis-II as meiotic mitosis. It is the second nuclear division and is similar to mitosis, but the two nuclei formed after Meiosis-I undergo division. It completes through four stages:

1. Prophase-II
2. Metaphase-II
3. Anaphase-II
4. Telophase-II

I) Prophase-II

• Chromatids become shorter, thicker and clearly visible.
• Each chromosome consists of two chromatids attached to a single centromere.
• Nuclear membrane and nucleolus begin to disappear and spindle fibres begin to appear.

II) Metaphase-II

• Nuclear membrane and nucleolus disappear.
• Chromosomes arrange on an equatorial plate.
• Spindle fibres organize between the two poles of the cell.
• Centromere of each chromosome divides longitudinally to form two daughter centromeres, each becoming the centromere of a chromatid.
• Centromere of each chromosome is attached to spindle fibres from both poles.

III) Anaphase-II

• Starts with repulsion/separation of centromeres of sister chromatids.
• Centromere of each chromosome separates.
• Sister chromatids move toward opposite poles due to contraction of spindle fibres.
• Separated chromatids are now called chromosomes.

IV) Telophase-II

• At each pole, chromosomes uncoil, elongate and form a network of chromatin fibres.
• Spindle fibres disappear around the poles.
• Nuclear membrane appears around each group of chromosomes and nucleolus reappears.

Cytokinesis

Cytokinesis occurs by cell-plate formation in plant cells and by cell-furrow formation in animal cells. Thus, by meiosis a diploid mother cell divides twice, forming four haploid gametes or sex cells, each having half the number of chromosomes of the mother cell.

Significance of Meiosis

• Maintains constant number of chromosomes in organisms across generations.
• Causes genetic variations among species.
• Forms haploid gametes or spores for reproduction.
• Helps in alternation of haploid and diploid generations of plants and animals.
• Maintains regularity of reproductive cycle.
• Helps in evolution.

Differences Between Mitosis and Meiosis