· 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.
CELL CYCLE
· 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
A. Interphase (Resting pahse)
B. Mitotic phase
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| 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
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
· 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.
· 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.
· 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.
· 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.
· 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.
· 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.
· 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.
· 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.
· 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.
Significances
· Genetic stability within the population.
· Cell growth and replacement
· Regeneration and asexual reproduction
· 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).
· 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.
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
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
· 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.
· 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.
· 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.
· 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.
· 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: