Cell Cycle and Cell Division Notes | Class 11 & NEET Free Notes

Cell Cycle and Cell Division Notes

In this article we will discuss about Cell Cycle and Cell Division Notes:- What is cell cycle, What is cell division, Amitosis, Mitosis and Meiosis

What is Cell Cycle?

  • Cell cycle is series of events involving cell growth and cell division
  • Cell growth, DNA replication and cell division constitute a cell cycle
  • Events happening in a cell cycle are genetically controlled
  • Duration of cell cycle varies in different organisms and different types of cells. The average duration of the human cell cycle is 24 hrs, whereas a yeast cell completes one cell cycle in 90 minutes
  • Discovered by Prevost and Dumas (1824)
  • Cell cycle is series of events that take place in cell, resulting in the duplication of DNA and division of cytoplasm and organelles to produce two daughter cells
  • Cell cycle is divided broadly into 2 phases
  1. Interphase
  2. M phase



  • Interphase is most active phase
  • It takes more than 95% time of the cell cycle
  • Series of metabolic changes occurs during interphase
  • These changes are not visible under microscope, so termed as resting phase
  • The Interphase is further divided into G1, S, and G2 phase

I) G1 Phase

  • Synthesis of enzymes, RNA, amino acids, ATP and nucleotides occur
  • Raw materials synthesis for S phase
  • Size of cell is increased

II) S Phase

  • DNA replication occurs
  • Synthesis of histone protein takes place
  • DNA doubles but chromosome number remains same
  • If animal cell centriole also duplicates

III) G2 Phase

  • Cell prepares itself for division
  • Synthesis of proteins and RNA takes place
  • ATP synthesis occurs


M Phase

  • Cell division occurs in M phase
  • This phase has a short duration
  • M phase is composed of two processes

i) Karyokinesis

  • Division of nucleus into two daughter nuclei
  • Four sub stages
  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase

ii) Cytokinesis

  • Division of cytoplasm resulting in two daughter cells

[ Read also: Cell The Unit of Life Complete Notes ]

Cell Cycle Checkpoint

  • Cell cycle checkpoints are control mechanisms in cell cycle which ensure its proper progression
  • Three most important checkpoints
  1. G1 Checkpoint
  2. G2 Checkpoint
  3. M checkpoint


I) G1 Checkpoint

  • This checkpoint checks internal and external conditions are right for division
  • Check cell size
  • Check Enzyme, Protein and ATP are synthesized
  • Check DNA damage or not
  • If a cell doesn’t get it may leave the cell cycle and enter a resting state called G0 phase

II) G2 Checkpoint

  • DNA replication completeness
  • Check DNA completely copied or not
  • Check Enzyme, Protein and ATP are synthesized
  • If errors or damage are detected
  • Cell will pause at G2 checkpoint to allow for repairs
  • If the damage is irreparable, the cell may undergo apoptosis

III) M Checkpoint

  • M checkpoint is also known as the spindle checkpoint
  • Chromosome attachment to spindle at metaphase plate
  • If chromosome is misplaced, the cell will pauses mitosis, allowing time for spindle to capture the stray chromosome



  • Production of daughter cell from parent cell is known as cell division
  • Cell division occurs as part of cell cycle

Types of cell division

  1. Amitosis
  2. Mitosis
  3. Meiosis



  • Remak discovered amitosis in RBCs of chick embryo
  • Term was coined by Flemming
  • Also known as direct cell division
  • Cell nucleus elongates, constricts in middle and divides directly into two daughter nuclei
  • This is followed by constriction of cytoplasm to form two daughter cells

Characteristics of Amitosis

  • Occurs in lower organisms like yeast, fungi, bacteria and in amoeba
  • Nuclear membrane does not disappear during cell division
  • Spindle fibers are not formed during cell division
  • Daughter cell does not have equal number of chromosomes



  • Mitosis is called as equational division
  • No change in number of chromosomes in parent cell & daughter cell
  • Two major processes occur during M Phase: Karyokinesis and Cytoinesis
  1. a) Karyokinesis
  • Nucleus division is occurs
  • Chromosomes are also separated
  • This phase consists of 4 stages
  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase


I) Prophase

  • First stage of mitosis
  • Chromatin material starts to condensation and take the shape of chromosomes
  • In each chromosome 2 chromatids are attached at centromere
  • Nuclear membrane disintegrates
  • Each centriole separate and migrates towards opposite pole of cell
  • Around centrole astral ray are formed
  • In addition longer spindle fibres are formed
  • Spindle fibres along with asters known as amphiaster or mitotic apparatus

II) Metaphase

  • Condensation of chromosomes is completed & observed clearly under microscope
  • Chromosomes lie at the equatorial plane
  • Spindle fibres extending from one pole to other are called continuous spindle fibres
  • Fibres extending from a pole to centromere of chromosome are called discontinuous spindle fibres
  • Metaphase is best stage in mitosis for analyzing chromosomes and to study their morphology

III) Anaphase

  • Chromatids of each chromosome are separated and form two chromosomes called daughter chromosomes
  • Daughter chromosomes move to opposite poles
  • Fibres which develop between the separating centromeres are called interzonal fibres
  • Migration of daughter chromosomes due to contraction of spindle fibres and stretching of interzonal fibres
  • The chromosomes appear as V, L, J and I shaped depending upon the position of centromere
  • At end of anaphase, two groups of chromosomes are formed, one at each pole
  • Number of chromosomes at each pole is equal to number of chromosome in the parent nucleus

IV) Telophase

  • Final stage of karyokinesis
  • Telophase is a reversal of prophase
  • Telophase chromosome became decondensed and uncoiled and form chromatin
  • Nucleolus, nuclear envelope and nucleoplasm is reformed
  • Spindle fibres break down and absorbed in cytoplasm

b) Cytokinesis

  • Division of cytoplasm into two daughter cells is called cytokinesis
  • Microtubules and microfilaments arrange on equator to form midbody
  • Periphery of equator contractile ring formed that is made up of actin and myosin
  • Due to actin and myosin contract, furrow forms from outside to inside in cell
  • Furrow deepens continuously and ultimately cell divides into two daughter cells

Significance of Mitosis

  • Mitosis ensure equal distribution of chromosomes and cytoplasm between daughter cells
  • Chromosomal number is maintained constant
  • Mitosis helps in growth and development of organisms
  • Helps in repair of tissues
  • Helps in asexual reproduction of organisms

Mitosis Plants vs. Animals

Animal Cell

  • Centrioles are present
  • Aster formation occurs
  • No cell plate is formed
  • Furrowing of cytoplasm occurs
  • Occurs in tissues throughout the body

Plant Cell

  • Centrioles are absent
  • No aster formation
  • Cell plate is formed
  • No furrowing of cytoplasm at cytokinesis
  • Occurs mainly in meristems

 Types of Mitosis

Anastral Mitosis

  • During mitosis these cells do not produce centrioles
  • Spindle fibres forms but no asters are observed
  • Commonly found in plants

Amphiastral Mitosis

  • During mitosis these cells produce two centrioles
  • Spindle fibres is associated with two asters
  • Commonly found in animals cell

Intranuclear Mitosis

  • Mitotic divisions take place inside the nucleus
  • Nuclear envelope remains intact
  • Spindle fibres forms inside the nucleus
  • Found in some protozoa and fungi


  • Chromosomes replicate without division of the cell nucleus
  • Resulting in polyploidy nucleus
  • Occurs in salivary glands of Drosophila and other flies

Free Nuclear Division

  • Karyokinesis is not followed by cytokinesis
  • It arises a multinucleated condition
  • Found in some fungi


  • Dinomitosis is type of mitotic division
  • Occurs in the class Dinophyceae
  • Nuclear membrane intact
  • No spindle fibres are formed
  • Chromosomes remain attached to nuclear membrane



  • Discovered in plant cells by Strasburger
  • The term was coined by Farmer and Moore
  • Meiosis also known as reductional division
  • Sexual reproduction in organisms takes place through the fusion of male & female gametes
  • They contain only half the number of chromosomes
  • Meiosis leads to formation of haploid cells
  • Meiosis is composed of two rounds of cell division: Meiosis I and Meiosis II
  • Each round of division contains a period of karyokinesis and cytokinesis

Meiosis I

  • Meiosis 1 separates the pair of homologous chromosomes
  • Reduces the diploid cell to haploid
  • The different stages of meiosis 1

I) Karyokinesis I

  • Prophase I
  • Metaphase I
  • Anaphase I
  • Telophase I

II) Cytokinesis I


Prophase 1

  • Longest duration in Meiosis-I
  • It is divided into five sub-stages


  • Size of cell and nucleus increases
  • Chromosomes appear long, uncoiled thread like in structure bearing many bead like structures
  • Nuclear membrane and nucleolus remain as it is


  • Pairing of homologous chromosomes is called Synapsis
  • Paired homologous chromosomes are referred as bivalents or tetrad
  • Homologous chromosomes are held together by Synaptonemal complex


  • Longest phase because crossing over takes place
  • Chiasmata formation occurs
  • Small fragment of chromosome exchange between two non-sister chromatids


  • Bivalents begin to separate due to dissolution of synaptonemal complex
  • Non sister chromatids remain attached at the sites of crossovers.
  • Nuclear membrane and nucleolus begins to disappear


  • Movement of the chiasmata towards the end of chromosome is called terminalization
  • End of diakinesis the nuclear membrane and nuleolus get completely disappeared
  • Chromosomes are free in the cytoplasm
  • Spindle fibres begin to form

Metaphase I

  • Bivalents align at the equatorial plate
  • Spindle fibers are attached with the centromeres of the homologous chromosomes

Anaphase 1

  • Spindle fibres shorten
  • Homologous chromosomes of each bivalent segregate and move to opposite poles
  • Segregation of homologous chromosomes during anaphase 1 is called disjunction
  • Division of centromere is absent

Telophase I

  • Separated chromosomes form two haploid nuclei
  • Nuclear membrane and nucleolus reappears
  • The chromosomes get uncoiled into chromatin thread
  • The spindle fibres disappear totally

Cytokinesis I

  • In animals, cytokinesis occurs by the constriction of the cell membrane
  • Also known as furrowing method
  • In plants, it occurs through the formation of the cell plate
  • Resulting in the creation of two daughter cells


Interphase II or Interkinesis

  • Two daughter cells formed pass through a short stage called interphase-II
  • Resting phase between meiosis-I and meiosis-II
  • Synthesis of RNA and protein may take place
  • Centrosome undergo replication
  • No DNA synthesis occurs

Meiosis II

  • Daughter cell divides mitotically and results in four haploid cells
  • This division is also known as the homotypic division
  • This division does not include the exchange of the genetic material and the reduction of the chromosome number
  • The different stages of meiosis II

I) Karyokinesis II

  • Prophase II
  • Metaphase II
  • Anaphase II
  • Telophase II

I) Cytoinesis II


Prophase II

  • Centrioles move towards the opposite poles
  • Nuclear membrane and nucleolus disappear
  • Spindle fibre starts to form
  • Chromosome again become compact

Metaphase II

  • Chromosomes get arranged on the equator of the cell
  • Spindle fibres organize between poles and attaches to centromere of chromosome

Anaphase II

  • Centromere of each chromosome divides
  • Sister chromatids separates to form two daughter chromosome
  • Sister chromatids are pulled away towards the opposite poles
  • Daughter chromosome moves towards the opposite poles due to the shortening of Spindle fibres and the stretching of interzonal fibres

Telophase II

  • Meiosis ends with telephase II
  • Daughter chromosome enclosed by nuclear envelope
  • Chromosome elongates to form thin networks of chromatin
  • Nucleolus reappears due to the synthesis of ribosomal RNA

Cytokinesis II

  • The result of cytokinesis is four haploid daughter cells
  • Cytokinesis takes place by cell plate formation in plant cell
    • Successive methods: cytokinesis followed by each nuclear division resulting in 4 haploid cells. Eg. Monocot plants
    • Simultaneous methods: cytokinesis occurs only after meiosis-II to form 4 haploid cells. Eg. Dicot plants
  • In animal cells, cytokinesis occurs by furrow formation

Significance of Meiosis

  • Meiosis helps to maintain a constant number of chromosomes by reducing the chromosome number in the gametes
  • Essential for sexual reproduction in higher animals and plants
  • Meiosis helps in the formation haploid gametes and spores for sexual reproduction
  • Number of chromosome remain fixed in a species from generation to generation
  • Crossing over occurring brings genetic variations in offspring which helps in evolution of organisms
  • Failure disjunction in Meiosis leads mutation to the formation of polypoid forms

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