CELL CYCLE AND CELL DIVISION | Chapter 10 | NCERT | NEET


Cell Cycle


  • The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of cells, and eventually divides into two daughter cells is called cell cycle.
  • DNA synthesis occurs in one specific stage of cell division but the distribution of chromosomes in cells occurs in complex series of events during cell division.

Phases of Cell Cycle

  • Human cell divides once in approximately 24 hours, which may vary in different organisms. In yeasts, it takes about 90 minutes to complete the cell division process.

Cell cycle is divided into two basic phases-

  1. Interphase– it is the phase between two successive M phases. Interphase lasts for 95% of a cell cycle. This phase is called as resting phase but during this period the cells prepare itself for nuclear division by cell growth.
  2. M Phase– when the actual cell division or mitosis occurs. It starts with karyokinesis (nuclear division) or duplication of chromosome and end with cytokinesis or division of cell-matrix (cytoplasm division).

The interphase is divided into three further phases:

1. G1 Phase (Gap 1)/ Generation 1/ Pre-DNA Synthesis Phase/ Post-Mitotic Phase:

  • G1 phase represents the interval between mitosis and initiation of DNA replication. 
  • During G1 Phase the cell is metabolically active and Continuously grows in size.
  • Production of cell organelles (like Lysosome, Golgi bodies, Vacuoles, E.R, Ribosomes, etc.) except Mitochondria and Chloroplast.
  • Synthesis of mRNA, rRNA, and tRNA
  • The decision of cell division takes place in this phase.
  • In late G1 phase, a restriction point occurs(R Point). If the cell cross this R Point, It must be divide and If cell fail to cross this R Point, It can’t divide.
  • R Point is a critical level of specific protein molecules, these protein molecules are known as Trigger protein or U-protein.
  • These protein helps to cell to cross R point, now these proteins are identifies as cyclins and cdk-kinase(cell cycles engine).
  • If cell can not cross the R point, It is said to be in G0 phase. It becomes a permanent cell and undergoes in differentiation.

2. S Phase (Synthesis Phase):

  • During Synthesis Phase, Replication or synthesis of DNA takes place and amount of DNA gets doubles per cell.
  • If the Initial amount of DNA is denoted as 2C then it increases to 4C. However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G1, even after S phase the number of chromosomes remains the same, i.e., 2n.
  • Histone Proteins are Synthesized in this phase.
  • The most active enzyme is- DNA Polymerase.
  • In Animal cells, Centrioles Duplicate.

3. G2 Phase (Gap 2)/ Generation 2/ Post-DNA Synthesis Phase/ Pre-Mitotic Phase:

  • Division of Mitochondria and Chloroplast.
  • Synthesis of Tubulin protein(Required for formation of Spindle Fibres)

⇒ In adult animals, some cells do not divide or may divide occasionally. These cells do not divide further and exits the G1 phase to enter an inactive stage called Quiescent Stage (GO) of cell cycle.

⇒ In animals mitotic division is present in only somatic diploid cells but in plants it is seen in both haploid and diploid cells.


Mitosis/ Somatic cell division/ Equational cell division


  • Mitosis cell division is also known as equational division because the numbers of chromosome remain same in parental and progeny cells.
  • Mitosis has been divided into four stages of nuclear division(Karyokinesis). 
  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase

1. Prophase:

  • Prophase is the first phase of mitosis followed by G2 phase. It involves following events-

i) Initiation of condensation of chromosomal materials.

ii) Movement of centrioles towards opposite poles of the cell.

iii) At the end of prophase, endoplasmic reticulum, nuclear membrane, Golgi complex disappears.

2. Metaphase:

  • Metaphase starts with complete disappearance of nuclear membrane. The most suitable stage for study of morphology of chromosomes. It involves

i) Condensation of chromosomal materials in to compact and distinct chromosomes made up of two sister chromatids attached with spindle fibres with kinetochores.

ii) Chromosomes are moved to spindle equator and get aligned along metaphase phase through spindle fibres to both poles.

iii) Chromosomes arrange at centre of cell called metaphase plate.

3. Anaphase:

  • Anaphase involves following steps:

i) Splitting of each chromosome at centromere into two sister chromatids.

ii)  Two chromatids start moving towards opposite poles, now chromatids referred to as daughter chromosomes of the future daughter nuclei.

4. Telophase:

  • Telophase is the last stage of mitosis which involves

i) Chromosomes reach at opposite poles and loose its identity as discrete unit. 

ii) Nuclear membrane reassembles around the chromosome clusters.

iii) Nucleolus, Golgi complex and ER reappear.

Cytokinesis:

  •  Cytokinesis is the division of cytoplasm of a cell after karyokinesis (division of chromosome) into two daughter cells. 
  • In animal cells, appearance of furrows in plasma membrane that deepens gradually and joins to divide cytoplasm into two parts.
  • In plant cells, wall formation starts at the centre and grows outwards to meet lateral walls. The formation of cell wall begins with formation of cell plate.


Significance of Mitosis:


i) Mitosis produces diploid daughter cells with identical genetic complement.

ii) It helps in repair of cells, especially in lining of gut and blood cells.

iii) Meristematic division in apical and lateral cambium results in continuous growth of plants.


Meiosis


  • The cell division that reduces the number of chromosome into half and results in the production of haploid daughter cells is called meiosis. It helps in production of haploid phase in the life cycle of sexually reproducing organism. It involves following events.

i) Two sequential cycles of nuclear and cell division called meiosis I and meiosis II but single cycle of DNA replication.

ii) It involves pairing of homologous chromosome and recombination of them.

iii) Four haploid cells are formed at the end of meiosis II.

Meiosis I Meiosis II
Prophase I Prophase II
Metaphase I Metaphase II
Anaphase I Anaphase II
Telophase I Telophase II

Meiosis I/ Heterotypic division/ Reductional division

  • Meiosis is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.

1. Prophase I:

  • During Leptotene, the chromosome becomes distinct and visible under microscope. Compaction of chromosome continues throughout the leptotene phase.
  •  During Zygotene stage, chromosomes start pairing together (synapsis). The paired chromosomes are called homologous chromosome. Synaptonemal complex formed by a pair of homologous chromosome is called bivalent or a tetrad.
  • During Pachytene stage, crossing over between non-sister chromatids of homologous chromosome occurs for exchange of genetic materials. The crossing over is enzyme – mediated process which involves enzyme recombinase.
  • Diplotene is recognized by dissolution of synaptonemal complex and tendency to separation of bivalent except at the site of crossing over. This forms an X like structure called chiasmata.
  •  Diakenesis is marked by terminalisation of chiasmata. The nuclear membrane breaks and nucleolus disappear.

2. Metaphase I:

  • the bivalent chromosome align at equatorial plate and microtubules from the opposite poles of the spindle get attached to the pair of homologous chromosomes.

3. Anaphase I:

  • The homologous chromosome separate but sister chromatids remain attached at centromere.

4. Telophase I:

  • During Telophase I, nuclear membrane and nucleolus reappears and cytokinesis follows. This is called as dyad of the cells.
  • The stage between two meiotic divisions is called interkinesis and it is generally short lived. There is no replication of DNA during Interkinesis.

Meiosis II/ Homotypic division/ Equational division

  • Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated. In contrast to meiosis I, meiosis II resembles a normal mitosis.

1. Prophase II:

  • In prophase II, nuclear membrane disappears and chromosome becomes compact.

2. Metaphase II:

  • At metaphase II stage, the chromosomes align at equator and microtubules attach with kinetochores of sister chromatids.

3. Anaphase II:

  • Anaphase II start with splitting of centromere of each chromosome to move towards opposite poles.

4. Telophase II:

  • Meiosis ends with Telophase II in which two groups of chromosomes get enclosed by nuclear membrane followed by cytokinesis to form tetrad of cells (four haploid daughter cells).


Significance of meiosis


  • Meiosis forms the gametes that are essential for sexual reproduction.
  • Crossing over introduces new recombination of traits.
  • Helps in maintenance of chromosome number of sexually reproducing organism. 
  •  Provides evidence of basic relationship of organisms.

Difference between Mitosis and meiosis-


Mitosis Meiosis
Takes place in the somatic cells. Takes place in reproductive cells.
It is a single division which produces two cells. It is a double division which produces four cells.
Haploid and diploid both kind of cells may undergo mitosis. Only diploid cells undergo meiosis cell division.
Crossing over absent. Crossing over takes place.
Pairing of chromosome does not occur. Pairing of homologous chromosome occurs.

Summary:

According to the cell theory, cells arise from preexisting cells. The process by which this occurs is called cell division. Any sexually reproducing organism starts its life cycle from a single-celled zygote. Cell division does not stop with the formation of the mature organism but continues throughout its life cycle. 

The stages through which a cell passes from one division to the next is called the cell cycle. Cell cycle is divided into two phases called (i) Interphase – a period of preparation for cell division, and (ii) Mitosis (M phase) – the actual period of cell division. Interphase is further subdivided into G1 , S and G2. G1 phase is the period when the cell grows and carries out normal metabolism. Most of the organelle duplication also occurs during this phase. S phase marks the phase of DNA replication and chromosome duplication. G2 phase is the period of cytoplasmic growth. Mitosis is also divided into four stages namely prophase, metaphase, anaphase and telophase. Chromosome condensation occurs during prophase. Simultaneously, the centrioles move to the opposite poles. The nuclear envelope and the nucleolus disappear and the spindle fibres start appearing. Metaphase is marked by the alignment of chromosomes at the equatorial plate. During anaphase the centromeres divide and the chromatids start moving towards the two opposite poles. Once the chromatids reach the two poles, the chromosomal elongation starts, nucleolus and the nuclear membrane reappear. This stage is called the telophase. Nuclear division is then followed by the cytoplasmic division and is called cytokinesis. Mitosis thus, is the equational division in which the chromosome number of the parent is conserved in the daughter cell.

In contrast to mitosis, meiosis occurs in the diploid cells, which are destined to form gametes. It is called the reduction division since it reduces the chromosome number by half while making the gametes. In sexual reproduction when the two gametes fuse the chromosome number is restored to the value in the parent. Meiosis is divided into two phases – meiosis I and meiosis II. In the first meiotic division the homologous chromosomes pair to form bivalents, and undergo crossing over. Meiosis I has a long prophase, which is divided further into five phases. These are leptotene, zygotene, pachytene, diplotene and diakinesis. During metaphase I the bivalents arrange on the equatorial plate. This is followed by anaphase I in which homologous chromosomes move to the opposite poles with both their chromatids. Each pole receives half the chromosome number of the parent cell. In telophase I, the nuclear membrane and nucleolus reappear. Meiosis II is similar to mitosis. During anaphase II the sister chromatids separate. Thus at the end of meiosis four haploid cells are formed.

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