TYPES OF CELL DIVISION
Cell cycle
- Mitosis takes place when new cells are added to multicellular organisms as they grow and when tissues are repaired or replaced. Root tips (root meristem) of onion (Allium cepa, 2n = 16) are the best plant material for the study of mitosis in labs. Root tips of Vicia faba (broad bean) are also used. In animals, cells at the base of the nails, bone marrow cells, and skin cells (stratum germinativum) are taken to study mitosis.
- Meiosis occurs in the production of gametes by organisms that reproduce sexually. The best material to study meiosis in the classroom is anthers from young unopened buds (buds before anthesis) of Tradescantia and onion and testes of Grasshopper.
- Aceocarmine is a nuclear basic stain used to study the cell division in plant material.
- Mitosis takes place when new cells are added to multicellular organisms as they grow and when tissues are repaired or replaced. Root tips (root meristem) of onion (Allium cepa, 2n = 16) are the best plant material for the study of mitosis in labs. Root tips of Vicia faba (broad bean) are also used. In animals, cells at the base of the nails, bone marrow cells, and skin cells (stratum germinativum) are taken to study mitosis.
- Meiosis occurs in the production of gametes by organisms that reproduce sexually. The best material to study meiosis in the classroom is anthers from young unopened buds (buds before anthesis) of Tradescantia and onion and testes of Grasshopper.
- Aceocarmine is a nuclear basic stain used to study the cell division in plant material.
Factors Controlling Cell Divison
- Cell Size When cells grow in size, their nucleo-cytoplasmic ratio, and surface area-volume ratio decrease. To maintain these ratios cells divide as smaller cells have high ratios, therefore more active.
- Mitogens are polypeptide growth factors that control cell proliferation. A common plant mitogen is the hormone cytokinin. Mitogens in human beings include lymphokines, Epidermal growth factor (EGF), or platelet-derived growth factors. (PDGF)
Generation time : Period between two successive divisions.
Cell Cycle: The entire sequence of events which takes place in a cell between one cell division and the next.
- Cell Size When cells grow in size, their nucleo-cytoplasmic ratio, and surface area-volume ratio decrease. To maintain these ratios cells divide as smaller cells have high ratios, therefore more active.
- Mitogens are polypeptide growth factors that control cell proliferation. A common plant mitogen is the hormone cytokinin. Mitogens in human beings include lymphokines, Epidermal growth factor (EGF), or platelet-derived growth factors. (PDGF)
Generation time : Period between two successive divisions.
Cell Cycle: The entire sequence of events which takes place in a cell between one cell division and the next.
Three major events of the cell cycle are
- Replication of DNA
- Karyokinesis (Nuclear division)
- Cytokinesis (Cytoplasmic division)
- Replication of DNA
- Karyokinesis (Nuclear division)
- Cytokinesis (Cytoplasmic division)
Cell Cycle consists of
- Long nondividing – I-phase
- Short dividing – M-phase
- Long nondividing – I-phase
- Short dividing – M-phase
I-PHASE/INTERPHASE
Cell cycle
No visible changes occur in the nucleus and it does not divide. Chromosomes remain as chromatin net work. Cell and nucleus grow and increase in size and prepare for division. It takes 70-95% of total generation time.
Metabolically very active and important phase - DNA, RNA, Proteins, Fats, Carbohydrates are synthesised, energy is stored and cell organelles doubled.
It is divided into three sub-stages – G1, S and G2
No visible changes occur in the nucleus and it does not divide. Chromosomes remain as chromatin net work. Cell and nucleus grow and increase in size and prepare for division. It takes 70-95% of total generation time.
Metabolically very active and important phase - DNA, RNA, Proteins, Fats, Carbohydrates are synthesised, energy is stored and cell organelles doubled.
It is divided into three sub-stages – G1, S and G2
(i) G1/Postmitoic/Gap-1 phase (42% of cell cycle)
There is large scale synthesis of RNA, proteins and other cytoplasmic materials, subcellular structures etc. required for the growth (protoplasm) of cell. Synthesis of the building blocks for nucleotides like purines and pyrimidines, ribose sugar etc. is also carried out in this phase.
The cells, which do not divide frequently have longer G1 phase. In this phase there are 3 options for the cell:
(a) Cell may continue on the cycle dividing at regular intervals.
(b) Cycle may be temporarily arrested at specific point in G1.
(c) Cell may permanently stop division by going out of the cycle e.g. the completely differentiated cell type like RBC, neurons, muscle cells. In this case cell enters G0 phase.
There is large scale synthesis of RNA, proteins and other cytoplasmic materials, subcellular structures etc. required for the growth (protoplasm) of cell. Synthesis of the building blocks for nucleotides like purines and pyrimidines, ribose sugar etc. is also carried out in this phase.
The cells, which do not divide frequently have longer G1 phase. In this phase there are 3 options for the cell:
(a) Cell may continue on the cycle dividing at regular intervals.
(b) Cycle may be temporarily arrested at specific point in G1.
(c) Cell may permanently stop division by going out of the cycle e.g. the completely differentiated cell type like RBC, neurons, muscle cells. In this case cell enters G0 phase.
THE REGULATION OF CELL CYCLE
takes place in G1 phase when it is decided that which above option the cell goes for. Hence there is a restriction point (R. point) in the G1 phase, after which the cell becomes committed to divide.
(ii) S - Phase : Synthetic phase(33% of cell cycle) phase of DNA synthesis or replication when the amount of DNA in doubled. Basic proteins (histones) are also synthesised. Once the cell enters S-Phase it is
bound to divide. DNA repair also takes place in this phase.
(iii) G2 / Pre-mitotic / Gap-2 phase (21% of cell cycle) After DNA doubling this shorter gap appears during which RNA synthesis and synthesis of some proteins continue. Cell size increases, Centrosome
becomes double.
Fig. Chromosome behaviour in cell cycle
takes place in G1 phase when it is decided that which above option the cell goes for. Hence there is a restriction point (R. point) in the G1 phase, after which the cell becomes committed to divide.
(ii) S - Phase : Synthetic phase(33% of cell cycle) phase of DNA synthesis or replication when the amount of DNA in doubled. Basic proteins (histones) are also synthesised. Once the cell enters S-Phase it is
bound to divide. DNA repair also takes place in this phase.
(iii) G2 / Pre-mitotic / Gap-2 phase (21% of cell cycle) After DNA doubling this shorter gap appears during which RNA synthesis and synthesis of some proteins continue. Cell size increases, Centrosome
becomes double.
Fig. Chromosome behaviour in cell cycle