Key Notes: Cell Cycle & Cell Division - NEET PDF Download

Cell Division

• Very important process in all living organisms. @ Continuity of life
  
  Cell division
• During the division of a cell, DNA replication and cell growth take place.
• Cell division, DNA replication and cell growth take place in a coordinated way to ensure correct division and formation of progeny cells containing intact genomes.

Cell Cycle

• The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides into two daughter cells.

• Cell growth (in terms of cytoplasmic increase) is a continuous process
• Cell division requirement → The replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of events. These events are under genetic control. (Cyclins & CDK kinase).
• A typical eukaryotic cell cycle (human cell) in culture, divide once in every 24 hours (23 Hours Interphase + 1 Hour M Phase).
• Duration of cell cycle vary from organism to organism and also from cell type to cell type.
• Yeast cell cycle in only about 90 minutes.

Phases of Cell Cycle

• Interphase
• M Phase (Mitosis phase)
  

  Interphase

• Represents the phase between two successive M phases.

• 95% of the duration of cell cycle.
• It is the resting phase à during which the cell is preparing for division.
• Cell growth and DNA replication takes place in orderly manner.
• Three phases → G1 Phase + S Phase + G2 Phase = Interphase
• G1 – Means Gap 1; S – Means Synthesis; G2 – Means Gap 2

The M Phase

• Actual cell division or mitosis occurs
• Starts with the nuclear division (karyokinesis)
• Ends with division of cytoplasm (cytokinesis).
• Most dramatic period of the cell cycle,
• Involving a major reorganisation of all components of the cell.

G₁ phase

• Interval between mitosis and initiation of DNA replication (S phase)
• Cell is metabolically active and continuously grows @ Protein & RNA Synthesis extensively

S or Synthesis Phase 

• DNA synthesis or replication takes place.
• Amount of DNA per cell doubles.
• 2C DNA becomes → 4C (But chromosome number remain same)
• Each chromosome have double amount of DNA
• 2n Cell of G1 phase is Still 2n is S phase
• DNA replication begins in the nucleus
• Centriole duplicates in the cytoplasm. (In animal cell)

G₂ phase

• Proteins are synthesised
• Preparation for mitosis (cell growth) continues. @ ATP storage takes place & Spindle fibre synthesized

G₀ Phase

• The cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0 ) of the cell cycle.
• Cells in this stage remain metabolically active
• Not proliferate unless required

Equational Division/Mitosis

Mitosis is divided into the following four stages

• Prophase
• Metaphase
• Anaphase
• Telophase

1. Prophase

• First Stage of Mitosis
• Initiation of condensation of chromosomal material.
• The centriole begins to move towards opposite poles of the cell.
• The completion of prophase marked by the following events:
  (i) Chromosomal material → Compact mitotic chromosomes.
  (ii) Chromosomes → Two chromatids attached at the centromere.
  (iii) Initiation of the assembly of mitotic spindle @ the microtubules
  (iv) In Late prophase → Golgi body, ER, nucleolus and the nuclear envelope disappears

2. Metaphase

• Begins when nuclear membrane completely disappeared
• Chromosomes are spread through the cytoplasm of the cell.
• Condensation of chromosomes is completed
  - @ Maximum condensation ; Visible under microscope
• Morphology of chromosomes is most easily studied.
• Metaphase chromosome → two sister chromatids & a centromere.
• Small disc-shaped structures at the surface of the centromeres are called kinetochores.
  - Sites of attachment of spindle fibres @ Protein Plates
• Metaphase is characterised by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole
• Metaphase plate
  - The plane of alignment of the chromosomes at metaphase
• The key features of metaphase are:
  Spindle fibres attach to kinetochores of chromosomes.
  Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles.

3. Anaphase

• At the onset of anaphase
  (i) Splitting of chromosome @ Splitting of Centromere
  (ii) Two daughter chromatids, now referred to as chromosomes of the future daughter nuclei,
  (iii) Daughter chromatid/New chromosome begin their migration towards the two opposite poles.
• As each chromosome moves away from the equatorial plate, the centromere of each chromosome is towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind.

4. Telophase

• Final stage of mitosis
• At the beginning of telophase
  (i) The chromosomes that have reached their respective poles
  (ii) They de-condense and lose their individuality.
• The individual chromosomes can no longer be seen and chromatin material tends to collect in a mass in the two poles
• Nuclear envelope assembles around the chromosome clusters.
• Nucleolus, golgi complex and ER reform. 

Cytokinesis 

• Division of Cytoplasm (Cell divides into two after nucleus division)
• In an animal cell → appearance of a furrow in the plasma membrane.
• Plant cells → Wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls.
• The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells.
• When karyokinesis is not followd bt cytokinesis then multinucleate condition (formation of syncytium)
• e.g., liquid endosperm in coconut

Significance of Mitosis

• Mitosis results in the production of genetically similar daughter cells
• 2n to 2n ; n to n (No change in chromosome number)
• The growth of multicellular organisms is due to mitosis.
• Cell growth → nucleo-cytoplasmic ratio disturbs → The cell to divide to restore the nucleo-cytoplasmic ratio.
• A very significant contribution of mitosis is cell repair.
• The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced.
• Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a continuous growth of plants throughout their life.

Meiosis

• Specialised kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells.
• Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing organisms
  - fertilisation restores the diploid phase.
• Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication.

Meiosis I

• Four phases
  (i) Prophase I (Largest Phase)
  (ii) Metaphase I
  (iii) Anaphase I
  (iv) Telophase I

Prophase I

• Typically longer and more complex
• Subdivided into the following five sub phases based on chromosomal behaviour, i.e., Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.
• During leptotene stage 
  - Chromosomes become gradually visible under the light microscope.
  - The compaction of chromosomes continues
• During zygotene stage
  (i) Chromosomes start pairing together (synapsis)
  (ii) Paired chromosomes are called homologous chromosomes.
  Synaptonemal complex → Formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad.
• During Pachytene stage
  (i) Bivalent chromosomes now clearly appears as tetrads.
  (ii) Characterised by the appearance of recombination nodules
  (iii) Recombination nodules are the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes.
  (iv) Crossing over is the exchange of genetic material between two homologous chromosomes.
  (v) Crossing over → An enzyme-mediated process and the enzyme involved is called recombinase.
  (vi) Crossing over leads to recombination of genetic material on the two chromosomes.
  (vii) At the end of pachytene, leaving the chromosomes linked at the sites of crossing over @ Diplotene begins
• During Diplotene stage
  (i) Dissolution of the synaptonemal complex
  (ii) Recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers. These X-shaped structures, are called chiasmata.
  (iii) Longest phase @ Can last months to years
• During Diakinesis stage
  (i) The final stage of meiotic prophase I
  (ii) Terminalisation of chiasmata.
  (iii) Chromosomes are fully condensed
  (iv) Meiotic spindle is assembled
  (v) Nucleolus disappears and the nuclear envelope also breaks down.
  (vi) Diakinesis represents transition to metaphase.

Metaphase I

• The bivalent chromosomes align on the equatorial plate
• The microtubules from the opposite poles of the spindle attach to the pair of homologous chromosomes.

Anaphase I

• The homologous chromosomes separate, while sister chromatids remain associated at their centromeres
  - Segregation of Mendelian characters

Telophase I

• The nuclear membrane and nucleolus reappear,
• Cytokinesis follows and this is called as dyad of cells

Interkinesis

• The stage between the two meiotic divisions I & II
• Generally short lived.
• Followed by prophase II
• Only G1 + G2 Phase ; No S Phase (DNA not duplicate)

Meiosis II

• Needed to normalise Chromosome (still have Double DNA because centromere not divided in Meiosis II so Chromosomes are Dyad)
• Have four phases
  (i) Prophase II
  (ii) Metaphase II
  (iii) Anaphase II
  (iv) Telophase II

Prophase II

• Resembles a normal mitosis prophase.
• The nuclear membrane disappears by the end of prophase II
• The chromosomes again become compact.

Metaphase II

• At this stage the chromosomes align at the equator
• Microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids. (Like Mitosis metaphase)

Anaphase II

• Splitting of the centromere of each chromosome (which was holding the sister chromatids together),
• Chromosome (separated sister chromatid) move toward opposite poles of the cell (Like Mitosis Anaphase)

Telophase II

• Meiosis ends with telophase II
• Nuclear envelope reappears
• Cytokinesis follows resulting in the formation of tetrad of cells i.e., four haploid daughter cells 

Significance of  Meiosis

• Maintain chromosome number generation after generation
  - @ Specific chromosome number of each species
• Increases the genetic variability
  - Crossing over brings variations
• Variations are very important for the process of evolution.