NEET Biology Notes The Cell Division Cell Cycle
It is genetically controlled series of events occurring in newly formed cells by which it undergoes growth and division to form daughter cells.
Phases of Cell Cycle
Eukaryotic cell cycle comprises of two main basic phases
- Interphase This phase includes G v S and G2 phases. The period between
M and S-phase is called G j-phase and that between S and M-phase is called G2-phase.
Interphase is the most suitable period to carry out fish (fluorescence in situ hybridisation). Fish is used for detecting and locating gene mutation and chromosomal abnormalities.
- M-phase This phase includes karyokinesis and cytokinesis, i.e. the actual cell division. Karyokinesis is the process of nuclear division,’ which involves separation of chromatids and their redistribution as chromosomes into daughter cells. Cytokinesis is the process of division of the cytoplasm, it results in the formation of daughter cells.
Hence, the cell cycle consists of four phases, i.e. G1; S, G2 and M.
Sometimes, a cell leaves the cell cycle at G1 -phase and enters a stage designated as G 0-phase (G zero), often called quiescent stage. Though, G0-cells are quiescent (inactive), yet they are busy carrying out their functions in the organism, e.g. secretion, attacking pathogens.
Some G0-cells like neurons are terminally differentiated, i.e. they will never re-enter the cell cycle but instead carry out their function in the organism until they die.
For other cells, G0 can be followed by re-entry into the cell cycle, e.g. lymphocytes in human blood with proper stimulation such as encountering the appropriate antigen, can be stimulated to re-enter the cell cycle (at G j) and proceed onto new rounds of alternating S-phases and mitosis.
G0 represents not simply the absence of signals for mitosis but an active repression of the genes needed for mitosis. Cancer cells cannot enter G0 and are destined to repeat the cell cycle indefinitely.
Mitosis was first observed by Strasburger (1870) in plant cells. The term ‘mitosis’ was coined by Flemming (1882). Mitosis occurs in two stages, i.e. karyokinesis, the division of nucleus and cytokinesis, the division of cytoplasm. Just prior to karyokinesis, the cell is in interphase. Process of nuclear division in eukaryotic cells is conventionally divided into four stages, i.e. prophase, metaphase, anaphase and telophase.
The number of cells within an organism also increases by mitosis and this process is called hyperplasia. It forms the basis for growth. If mitotic division goes uncontrolled in any part of the body, it results in the formation of malignant cells. These cells continue to divide resulting in the formation of malignant tumours. This condition is called cancer.
Significance of Mitosis
- It is restricted to the diploid cells only. However, in some plants and social insects haploid cells also divide by mitosis.
- It results in the production of diploid daughter cells with identical genetic combination usually, resulting in genetic stability.
- The growth of multicellular organisms is due to mitosis and it also restore the nucleo-cytoplasmic ratio and surface volume ratio of cells.
- Mitosis is meristematic tissues like apical and lateral cambium, results in a continuous growth of plants throughout their life.
- Mitosis helps in cell repair and regeneration of injured and lost body parts.
- It is a method of multiplication in multicellular animals.
- It forms the basis of asexual reproduction in both plants and animals.
The term meiosis was coined by Farmer and Moore in 1905. It is a two stage process of cell division in sexually reproducing organisms that results in cell having half the chromosome number of the original cell, thus bringing about a reduction in the chromosome number from a diploid (2n) condition to a haploid (n) condition. Such a reduction becomes necessary for maintaining the chromosome number during sexual reproduction. Meiosis is necessary for the formation of gametes in animals and spores (microspores and megaspores) in plants.
Meiosis first is followed by second meiotic division, which is similar to mitotic division. The first meiotic division leads to the reduction of chromosome number to half and the second meiotic division segregate the replicated chromosomes. In anaphase-II of meiosis-II, the chromosome and centromere divide, while the sister chromatids separate and move towards opposite pole. Genetic recombination occurs due to meiosis (crossing over). Both mitosis and meiosis essentially follow the same sequence in all living organisms, which is an evidence of. the basic relationship between diverse groups of living organisms.
Zygotic or initial meiosis is a type of meiosis, which occurs immediately after fertilisation.
Significance of Meiosis
- It is the mechanism of conversion of specific chromosome number of each species in sexually reproducing organisms.
- It provides chance for the appearance of new gene combinations, owing to crossing over.
- It increases the genetic variability in the population of organisms from one generation to the next. Variations help in evolution.
Terms Related to Cell Division
- The term ‘karyokinesis’ was coined by Schneider (1887), while cytokinesis was coined by Whiteman (1887).
- Mitotic Poisons are substances that inhibit mitosis e.g. colchicine, chalones, cyanides and azides.
- Phragmoplast Persistent part of spindle apparatus with an interdigitated array of microtubules at the equator.
- Endomitosis (Endoduplication) It is replication of chromosomes without corresponding division of nucleus.
- Free Nuclear Division It is division of nucleus without being followed by cytokinesis, it gives rise to multinucleate condition.
- Intemuclear Spindle Formation In fungi, many algae, Amoeba, etc., the nuclear envelope does not degenerate. However, polar pores may appear. An internal spindle, called intranuclear spindle is formed which helps in equitable distribution of chromosomes.
- Dinomitosis Dinoflagellates possess condensed chromosomes even in interphase. Their nucleus is called mesokaryon. Nuclear envelope and nucleolus persist during mitosis. An intranuclear spindle in also not formed. Instead, cytoplasmic channels develop in the nucleus to help in the passage of replicated chromosomes to the two ends along the nuclear envelope.
Regulation of Cell Cycle
- It is a very complex process. It involves cyclins and cyclin dependent kinases (Cdks). The cell cycle is regulated by enzymes called cyclin dependent kinases (Cdks) (e.g. Cdc2, Cdk2). These phosphorylate serine and threonine residue on other protein or blocking activities related to cycle. All Cdks are active only when bound to certain cyclins (e.g. A, B, C, D and E) to form Cdk-cyclin complexes.
- Cdk (Cyclin dependent kinase) along with cyclins are major control switches for the cell cycle, causing the cell to move from G j to S or G2 to M.
- MPF (Maturation Promoting Factor) includes the Cdk and cyclins that triggers progression through the cell cycle.
- p53 is a protein that functions to block the cell cycle, when the DNA is damaged. Its levels increase in such conditions allowing time to repair DNA. If the damage is severe and irreparable then this protein can cause apoptosis (programmed cell death).
- p27 is a protein that binds to cyclin and cdk, blocking cell entry into S-phase. Recent research suggests that breast cancer prognosis is determined by p27 levels. Reduced levels of p27 predict a poor outcome for breast cancer patients.