· The ratio of the volume of the nucleus upon the volume of the cytoplasm is called as Nucleocytoplasmic ratio.
· Nucleocytoplasmic ratio = VN / VCyto = VN / VCELL – VN
· In a cell going to divide, nucleocytoplasmic ratio (karyoplasmic ratio) and surface area per unit volume decrease.
· The increase in nucleocytoplasmic ratio indicates the increasing activity of cell division.
· Smaller cells are metabolically more active than the larger cell due to the high nucleocytoplasmic ratio.
· The cell cycle is defined as the sequence of events in which the cell accumulates a large amount of nutrition and then divides into daughter cells.
· The process of forming daughter cells from parent cells is called cell division.
· A cell divides to maintain the balance between the bulk of cytoplasm and nucleus.


· The sequence of events that take place between two cell divisions is called the cell cycle.

The cell cycle consists of
A. Interphase (Resting pahse)
B. Mitotic phase

Fig: Showing cell cycle

A. Interphase
· Longest phase of the cell cycle.
· Nucleus is large-sized during interphase.
· Often called Resting stage (morphologically) or most Active stage (metabolically)
· Transcription (formation of mRNA from DNA) occurs through the interphase as well as m-phase excluding metaphase and anaphase.

Sub phases of Interphase
1. Gap one (G1) phase
· Enzymes and substrate are made ready
· Synthesis of structural and functional proteins
· Synthesis of Ribosomes mitochondria, chloroplast, lysosome, Golgi-complex etc.
· Nucleolus produces rRNA, tRNA and mRNAs

2. Synthesis (S) phase
· DNA synthesis
· DNA replication takes place.
· Synthesis of histone protein.

3. Gap two (G2) Phase
· Spindle and aster proteins are formed
· Centriole replicates.
· Energy storage takes place.
· Spindle fibre begins to form.

2. Mitotic phase
· Dividing phase.
· Mitosis consists of two phases
(a) Karyokinesis (Nuclear Division)
(b) Cytokinesis (Cytoplasmic Division)
    i) Cell plate method in plants
    ii) By constriction or furrowing method in Animals.

Go phase/Differentiation:
· Cell cycle is restricted in G1 phase
· Restricted cell undergoes Go phase or Differentiation.
· Differentiation is the process of becoming permanent.

Various Types of Cell Division

(i) Amitosis
· Discovered by Robert Remark.
· It is unequal cell division
· Division completes without forming spindle fibres.
· Budding and fission are examples of amitosis.
· It is common in prokaryotes and some lower organisms
· Mesosome helps in the distribution of DNA and other genetic materials as well as in cytokinesis in bacteria during amitosis cell division.

(ii) Mitosis
· Discovered by: Strasburger (plants) and Flemming (animals)
· It is an equational type of cell division.
· Also called somatic or indirect or equational or homotypic cell division
· Based on DNA Content, mitosis is reductional cell division
· It is common in somatic cells.
· Single-cell divides into 2 daughter cells.
· Helps in growth, replacement and repair of cells and is a method of asexual reproduction (unicellular organisms only)
· Mitosis takes place in haploid, diploid, and polyploids.
· Colchicine is also called mitotic poison.
About Colchicines:
· Is an alkaloid
· Extracted from the corm of colchicum plant.
· Inhibits the formation of spindle fibres.
· It disturbs the metaphase and anaphase stages of cell division.

Mitosis is of two types
(a) Astral: In animals
(b) Anastral: In plants
· Best material to observe mitosis is the root tip cell.
· Number of mitosis for the production of 'N' cells from single-cell is N-1
· E.g. If 8 daughter cells are produced from a single cell, then no. of mitosis is 8 – 1= 7.
· Number of cells after 'X' turns or generations of cell cycle= 2x X given number.
· E.g. How many times or generations a single cell divides to produce 64 cells?
Here, If 'x' is the number of generations then we get,
or, 2x = 64
or, 2x = 26
∴ x = 6
Hence, a cell should divide up to 6 generations to produce 64 cells.

(a) Karyokinesis:
1. Prophase
· Nuclear membrane breaks down.
· Nucleolus starts to disappear.
· Chromatin is changed into the chromosome.
· Spindle fibres start to form.
· Shortening and thickening of chromosomes.
· Longest phase of cell division.

Fig: Showing Mitotic prophase

2. Metaphase:
· Chromosomes move towards the equatorial plane forming equatorial or metaphase plates.
· Pairs of chromatids attached to the spindle at the centromere.

Fig: Metaphase

3. Anaphase
· Chromatids are separated due to the splitting of centromeres.
· Separated chromatids are now called chromosomes.
· Sister chromosomes move towards opposite poles.

Fig: Anaphase

4. Telophase
· Opposite to prophase.
· Chromosomes reach the poles of the cell and the nuclear membrane reappears.

Fig: Cytokinesis – cytoplasmic division.

Cytokinesis (Cytoplasmic Division) Occurs by
a. Cell plate formation in plants cells.
b. Cell furrow method on animal cells.
· Genetic stability within the population.
· Cell growth and replacement
· Regeneration and asexual reproduction

(iii) Meiosis
· Coined by Farmer and Moore
· Occurs in male and female germ cells
· It is the reduction division in which the number of chromosomes (genetic matter) is halved. i. e, diploid (2n) becomes haploid (n)
· Responsible for transmission of hereditary information from generation to generation.
· It is reductional cell division.
· Single-cell divides into 4 daughter cells.
· Meiosis occurs in diploid and polyploids but never on haploid.
· Meiosis is normal in even polyploids (2n, 4n, 6n,…) and abnormal in odd polyploids (n, 3n, 5n,…).
· Cell undergoing meiosis is called meiocyte cell
· Meiosis is never followed by meiosis (means Meiosis I is reductional and Meiosis II is equational cell division)
· If a cross is made between a diploid (2n) male and (4n) female, the resultant offspring will be triploid (3n).
· If a cross is made between diploid (2n) male and tetraploid (4n) female plant, the resultant endosperm will be pentaploid (5n).

Various Types of Meiosis
1. Zygotic Meiosis
· Meiosis occurs during germination of zygospore. 
E.g. Thallophytes algae and fungi

2. Sporic meiosis
· Occurs during spore formation. 
Eg. Bryophytes, pteridiphytes, gymnosperm, and angiosperm

3. Gametic Meiosis
· Meiosis occurs during gamete formation 
E.g. Animal

Meiosis cell division can be divided into TWO phases:

a) Meiosis – I
- Heterotypic division
- Actual reductional division
b) Meiosis – II
- It is homotopic cell division. 
i.e. equational cell division similar to mitosis

(a) Meiosis I includes
1. Prophase I
· Longest phase

Subphase of Prophase I
a. Leptotene
· Thread like chromosome with a beaded structure called chromomeres.

b. Zygotene
· Pairing of homologous chromosomes (maternal and paternal)
· Pairing process called synapsis occurs.
· Homologous chromosome forms bivalents.
· Cynaptomeal complex (protein) favours synapsis

c. Pachytene
· Chromosome of a homologous pair becomes twisted spirally around each other. They appear as tetrads.
· Crossing over (exchange of genetic materials between maternal and paternal chromosome) takes place.
· Crossing over always takes place between non-sister chromatids.
· Crossing over is favoured by two main enzymes, endonuclease and ligase.
· Endonuclease (helps in break down) whereas ligase (helps in joining)

d. Diplotene

· Bivalents repel each other.
· Chromosomes joined at several points along their length called chiasmata.
· Chiasmata are sites of crossing over
Note: Chiasmata formation takes place during pachytene but they are clearly observed in diplotene.

e. Diakinesis
· Terminalisation of chiasmata.

2. Metaphase – I
· Chromosomes or Bivalents arranged at equatorial plates in two lines.
· Bivalents are attached to the spindle at the centromere

3. Anaphase – I
· No splitting of centromere.
· Separation of homologous chromosomes, takes place, which is called a disjunction.
· Actual reduction takes place in this stage.

4. Telophase – I
· Spindle fibres disappear and the nuclear membrane reappears after the chromosomes reach their respective poles.

(b) Meiosis II includes
· Prophase II and Metaphase II which are similar to Prophase and metaphase of Mitosis.

Anaphase II.
· Centromere divided and chromatids pulled to opposite poles.
· Separated chromatids are now called chromosomes.

Significance of Meiosis
· Variation among offspring
· Restoration of the original number of chromosomes.
· Appearance of hidden characters.

1. How many Meiotic divisions are required to form?
a. 100 pollen grains
→ 1 cell forms 4 pollen grains
→ 25 cell forms 100 pollen grains

b. 100 megaspore.
→ Only one functional ovum or egg is produced from the reductional division of one megaspore
→ 100 megaspores are formed from 100 divisions.

c. 100 seeds.
→ 1 seeds = 1 megaspore + 1 microspore.
→ 100 seeds = 100 megaspore + 100 microspore.
= 100 division + 25 divisions
= 125 cell divisions.

Also, Read Notes of other Lessons in Botany:

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