Cell Cycle: Introduction, Phase, Mechanism and Significance

The Cell Cycle


  • Cell cycle is series of events.
  • It occurs in cell which leads the cell to its maturation and subsequent division.
  • Discovered by Prevost and Dumas in 1824.
  • This entire process where a new cell population grows and develops with the help of one single parent cell is termed as the cell cycle.

cell cycle


  • It in the period of cell cycle in which animal cell prepared itself for mitosis.
  • Important events occurs during this period is DNA replication,
  • Synthesis of large amount of protein which in needed for division.
  • Interphase in divided into 3-subphases.

G1 phase

  • It is the period in which, animal cell prepared for DNA replication.
  • Rate of transcription is very high.
  • At the end of G1 phase, Go phase is present.
  • In case of animal, it is called restriction point
  • If growth factor / mitogens and nutrient condition is high then, animal cell exit from the Go phase and enter in S-phase.
  • In yeast, Go phase is called start point.
  • It is functionally similar to restriction point and controlled by mating factor and nutrient availability.

S-phase (Synthetic phase)

  • It is active phase of Interphase.
  • DNA replication occur in this phase.
  • Number of genetic quantity just doubled but character remain same.
  • Centrioles doubled in this phases.
  • S-phase is easily studied by measuring the amount of nucleotide incorporation by using radiolabeled / fluorescently labelled nucleotide and fluorescent microscope.
  • S-phase specific cln-cdk complex in synthesized in Late G1 phase but due to sik-protein, it is functionally inactive.
  • Late G1 phase specific cln-cdk complex phosphorylate the inhibitor protein and trigger its degradation by proteosomal complex.
  • S-phase specific cln-cdk complex phosphorylate the pre RC complex and triggered DNA replication.


  • It is the period between S and M-phase.
  • Animal cell prepare itself for mitosis.
  • Rate of transcription and translation in very high.
  • Cell size Increased.
  • It is the Last phase of interphase.

M-phase (mitotic phase)

  • It is the most active phase of the cell cycle.
  • It is divided into two sub-phases.
    • Karyokinesis.
    • Cytokinesis.
  • Division occur in M-phase.

Mechanism of Cell division

  • It is proposed by W. Fleming in 1882.
  • In mitosis, one round of DNA replication and only one round of chromosome segregation and generating two identical daughter cells.
  • It is a continuous process.

Stages of mitosis:

It start with Karyokinesis and end with cytokinesis.

  • Chromatin condensation.
  • Chromatin duplicated during S phase and consist two sister Chromatid and both sister chromatid, are held by centromere.
  • At late prophase, microtubules are polymerized and forms mitotic plate.
  • In thin phase nuclear membrane NPC, ER, Golgi body fragmented.
  • Two kinetochores at each chromosome, attached on opposite sides.
  • At metaphase chromosomes are properly aligned at equator site and form mitotic plate.
  • Microtubule show polarity.
  • Microtubule dependent motor protein play important role in arrangement of chromosome on equator line.
  • In this phase, Kinetochore microtubule pull the two chromosome at opposite pole.
  • Best phase for analysis of shape of chromosome.
  • During metaphase sister Chromatids are not separate because they are held by SMC and Cohesion.
  • During Anaphase APC causes-activation of separase and separase degrade SMC and cohesin. That’s why it allow separation of sister chromatid.
  • Anaphase in divided into two sub-phases


    • It allow microtubule depolymerization by using motor protein.
    • kinesis-13.

Anaphase B

    • It allow active polward movement by using Dynein motor protein.
    • During late anaphase, cellular organelles also move at polar site.
  • Chromatid move at opposite pole.
  • Kinetochores and microtubules are disappeared.
  • Chromatin decondensation.
  • Cellular organelle permanently move at polar region.
  • Nuclear membrane and NPC reappeared.
  • ER and GB reappeared.
  • It is completed by two different mechanism.
    • In animal cells it in completed by cleavage and furrow.
  • Cytokinesis is initiated by actin filament and form contractile rings.
    • Myosin and Rho-GTPase protein play important role in cleavage and furrow formation.
    • In plants due to presence of rigid cell wall, Ridge & furrows is not formed, that’s why cell separate with each other by cell plate formation.
    • At late anaphase golgi body forms fragmoplast which forms cell plate.
    • By fragmoplast it forms plasmodesmeta that connect cell electrically and mechanically.

Significance of mitosis

  • Maintain chromosome number.
  • Responsible for formation of somatic body.
  • It play important role in regeneration.
  • In unicellular organism, it is responsible for increase population size.


  • Proposed by Farmer and Moore.
  • One round of DNA replication followed by two round of chromosome segregation.
  • It is completed into two steps:


  • Homologous character are separate and called reductional division because number of chromosome is exactly half.
Meiosis- II
  • Sister chromatid are separate and actual haploidy is synthesized, that’s why it is called equational division.
Difference between Mitosis and Meiosis
Takes place in somatic cells Take place in germ cells
Nucleus divides only once Nucleus divides twice
Formation of two daughter cells Formation of four daughter cells.
Diploid daughter cells Haploid daughter cells
More frequent Less frequent
Formation of somatic organs from daughter cells Formation of germ cells.
Only one prophase, metaphase,  anaphase and telophase Two of each phase occurs and five sub-phases in prophase 1
Number of chromosome not changed Chromosome number reduced to half
No crossing over Crossing over occurs
Equational division Reductional division
mitosis and meiosis

References and Sources:

  • https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/meiosis
  • https://www.khanacademy.org/science/high-school-biology/hs-reproduction-and-celldivision/hs-the-cell-cycle-and-mitosis/a/hs-the-cell-cycle-and-mitosis-review
  • https://www.khanacademy.org/science/ap-biology/cell-communication-and-cell-cycle/cellcycle/a/cell-cycle-phases
  • https://quizlet.com/266870446/ch11-meiosis-mitosis-flash-cards/

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