Title Med-RM_Bot_SP-1_Ch-2_Cell Cycle and Cell Division.pdf ✅

Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Chapter Contents Cell Cycle Phases of the Cell Cycle Interphase M-Phase Mitosis Significance of Mitosis Meiosis Amitosis or Direct Cell Division Cell division is defined as a process in which a single parent cell divides into two daughter cells. CELL CYCLE The sequence of events by which a cell duplicates its genome, synthesizes other constituents of the cell and eventually divides into two daughter cells is termed as cell cycle. The daughter cells then grow and again at maturity undergo cell division. It is genetically controlled series of changes. Growth is a continuous process which occurs throughout the life of a cell but duplication of organelles, DNA replication etc. occurs during a specific stage of the cell cycle. Cell growth results in disturbing the ratio between the nucleus and the cytoplasm. Therefore it becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio. The cell cycle is the period (time) between the formation of new daughter cells and their further division. PHASES OF THE CELL CYCLE Cell cycle basically consists of two phases namely : (i) Interphase (ii) M or Mitotic phase Interphase is a phase between two successive M phase, where the cell prepares itself for cell division. It constitutes more than 95% duration of the cell cycle. M phase or Mitosis is the phase where the actual cell division occurs. In this phase, the cell utilizes the constituents formed during interphase and divide into two daughter cells. Period of cell cycle varies from organism to organism and also from cell to cell, e.g., a yeast cell divides once in 90 minutes, a typical human cell divides approximately every 24 hours. Most of the cells lining the stomach are replaced after every three days due to high acidic environment. Chapter 2 Cell Cycle and Cell Division
44 Cell Cycle and Cell Division NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 INTERPHASE It is a long, non-dividing, growing phase of the cell cycle. It represents the most active stage of the cell cycle where both cell growth and DNA replication occur in an orderly manner. It is the period of intense growth and synthesis in which a cell stocks all the biomolecules required during cell division. It is called the resting phase because there is no apparent activity related to cell division rather, it deals with the changes that occur in a cell and the nucleus, before it enters into cell division. It is further divided into three stages: (i) G1 Phase G1 phase or Gap1 Phase is the longest phase of interphase present between mitosis and initiation of DNA replication. In this phase there is active synthesis of RNA and proteins. Cell also synthesizes nucleotides, ATP, amino acids etc. (required during S phase). Most of the organelles duplication occurs in this phase. If a cell divides frequently, then it has a shorter G1 phase and if a cell does not divide frequently, then it has a longer G1 phase. G0 Phase: The non-dividing cell enters the quiescent stage or G0 stage of the cell cycle. In this phase, the cell remains metabolically active but do not proliferate unless they are called on to do so. e.g. Nerve cell. (ii) S Phase In the S-phase or synthetic phase, DNA molecule of each chromosome replicates by the synthesis of a new DNA molecule on the template of the existing DNA. Each chromosome is now composed of two sister chromatids held together by a common centromere. The cell retains the original diploid (2n) chromosome number but now has duplicate set of genes. The cell has double the amount (4C) of DNA present in the original diploid cell (2n). Synthesis of histone proteins also occurs in this phase. In animal cell, during the S phase, DNA replication begins in the nucleus, and the centrioles duplicate in the cytoplasm. Centrioles are required during the M phase. The centrioles are absent in the higher plant cells. (iii) G2 Phase G2 phase or Gap2 phase is the second gap phase present between S and M phase. Tubulin protein synthesis take place. Mitochondria, Chloroplast and Golgi bodies duplicate in this phase. The DNA synthesis stops at this phase but cell synthesizes RNA, proteins etc. required during the next phase. In animals, mitotic division is only seen in a diploid somatic cells, whereas in plant cells, mitotic division is seen in both haploid and diploid cells. Note : Regulation of Cell Cycle (i) Decision of a cell to divide occurs in G1 phase. If a cell does not want to divide it will enter into G0 phase or Quiescent phase. When the conditions change, the cell can enter back into G1 phase. G1  S transition in the cell cycle is called as Restriction point or check point. This is the major check point. Once the cell crosses the restriction point, it is destined to complete cell cycle. Another minor check point is G2  M transition. (ii) Cell cycle is regulated by cyclin-dependent protein kinase. (iii) Cyclins are proteins that activate protein kinases to regulate eukaryotic cell cycle. (iv) G1 to S transition is carried out by G1 cyclin + cdc 2 kinase. (v) G2 to M transition is triggered by maturation promoting factor (MPF) formed by mitotic cyclin + cdc2 kinase. M-PHASE M phase or mitotic phase is the phase where actual cell division occurs. It is the most dramatic period of the cell cycle. It consists of two processes. I. Karyokinesis – where division of nucleus occurs (Karyon  Nucleus, Kinesis Movement) II. Cytokinesis – where division of cytoplasm occurs (Kytos  Cell/ hollow, Kinesis  Movement)
NEET Cell Cycle and Cell Division 45 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Therefore, during M phase reorganisation and separation of all the components of the cell occur. Fig. : A diagrammatic view of cell cycle indicating formation of two cells from one cell MITOSIS (Equational Division) Mitosis was first observed by Strasburger in plant cell and by Walter Flemming in animal cell. The term mitosis was given by Walter Flemming in 1882. Definition : Mitosis is a process in which a parent cell divides into two identical daughter cells. These cells are identical to each other as well as to the parent cell. It is a type of division in which already duplicated chromosomes are distributed into two daughter cells equally, so that both the cells have the same number of chromosomes. Occurrence : It occurs in the somatic cells (body cells) of the animals, in the gonads for the multiplication of undifferentiated germ cells. In plants, it occurs in the dividing meristematic tissue and also in leaves, flowers and fruits during growth. Mechanism of Mitosis I. Karyokinesis : Karyokinesis is a continuous process in which a parent nucleus divides into two daughter nuclei. There is no pause during the process but for our convenience, mitosis has been divided into four stage of nuclear division. (1) Prophase : ‘Pro’ means first and ‘phase’ means stage. The following events occur during prophase : (i) The condensation of chromatin material takes place and during condensation, the DNA strands get untangled to form compact mitotic chromosomes. (ii) Each chromosome appears double and consists of two coiled sister chromatids joined by a centromere. Their ends are not visible in early prophase. Therefore, the chromosomes appear like a ball of wool. It is also called spireme stage. (iii) Centrosome which had undergone duplication during interphase, begins to move towards the opposite poles of the cell. (iv) Each centrosome radiates out microtubules called asters. The two asters together with spindle fibres form mitotic apparatus. (v) In animal cells, mitosis is called Amphiastral (spindle is associated with 2 asters). In plant cells, the mitosis is Anastral (no aster formation). (vi) Cells at the end of prophase, when viewed under the microscope, do not show golgi complex, ER, nuclear membrane and nucleolus. If the nuclear membrane disappears during the mitosis it is called Eumitosis or extranuclear and if mitosis is intranuclear means nuclear membrance remains intact then it is called Premitosis e.g., many protists and fungi.
46 Cell Cycle and Cell Division NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 (2) Metaphase : ‘Meta’ means second and ‘phase’ means stage. The following events occur during metaphase (i) The disintegration of the nuclear envelope marks the beginning of the metaphase. The nuclear envelope disappears, and chromosomes spread through the cytoplasm of the cell. (ii) The chromosomes contain two chromatids attached to each other through the centromere. The chromosomes during this stage are the thickest and shortest and hence, it is easy to study their morphology. (iii) Mitotic spindle formation is complete. The phenomenon of bringing the chromosomes on the equator of spindle is called congression. (iv) All the chromosomes align themselves at the equator. The plane of alignment of centromeres of all the chromosomes is referred as metaphasic plate or equatorial plate. (v) The centromere is surrounded by a small disc-shaped structure called kinetochore. The kinetochores forms the site of attachment of microtubules (Spindle fibres). Note : Colchicine inhibits mitosis by inhibiting formation of microtubules (Natural mitotic poison). Chromosome Centrioles Spindle fibre (Microtubular bundle) Kinetochore Kinetochore Chromatid Chromatid Aster Fig. : A metaphase chromosome with kinetochores joined by spindle microtubules (3) Anaphase : ‘Ana’ means up and ‘phase’ means stage. Following events occur during anaphase : (i) The centromere which holds the two chromatids together splits and separated daughter chromatids are now referred to as chromosomes of the future daughter nuclei. (ii) The spindle fibres attached to the kinetochore now shorten and daughter chromosomes begin to migrate towards the opposite poles. Formation of interzonal fibres occurs. (iii) During migration, chromosomes always move away from the equatorial plate. The centromere of the daughter chromosomes move towards the pole and the arms of chromosomes trail it. The anaphase ends when the chromosomes reach the poles. It is the best stage to study shape of the chromosomes (V, L, J, I). Chromosomal fibre Kinetochore Arms Chromatid (Single-stranded chromosome) Arm Kinetochore Interzonal fibre Fig. : Separation of chromatids during anaphase