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Revision Notes on Cell Cycle and Cell Division

  1. Introduction: It is the process by which a mature cell divides and forms two nearly equal daughter cells which resemble the parental cell in a number of characters.

  2. Discovery: Prevost and Dumas (1824) first to study cell division during the cleavage of zygote of frog.

  3. Nagelli (1846) was the first to propose that new cells are formed by the division of pre-existing cells.

  4. Rudolf virchow (1859) proposed “omnis cellula e cellula” and “cell lineage theory”.

  5. A cell divides when it has grown to a certain maximum size which disturb the karyoplasmic index (KI)/Nucleoplasmic ratio (NP)/Kernplasm connection.

  6. Two processes take place during cell reproduction.

  • Cell growth: (Period of synthesis and duplication of various components of cell).

  • Cell division: (Mature cell divides into two cells).

  1. Cell cycle: Howard and Pelc (1953) first time described it. The sequence of events which occur during cell growth and cell division are collectively called cell cycle. Cell cycle completes in two steps:

  • Interphase

  • M-phase/Dividing phase

(i) Interphase : It is the period between the end of one cell division to the beginning of next cell division. It is also called resting phase or not dividing phase. But, it is actually highly metabolic active phase, in which cell prepares itself for next cell division. In case of human beings it will take approx 25 hours. Interphase is completed in to three successive stages.

(a) G1 phase/Post mitotic/Pre-DNA synthetic phase/Gap Ist

(b) S-phase/Synthetic phase

(c) G2-phase/Pre mitotic/Post synthetic phase/gap-IInd

(ii) M-phase/Dividing phase/Mitotic phase

(a) Nuclear division i.e. karyokinesis occurs in 4 phases – prophase, metaphase, anaphase and telophase. It takes 5-10% (shortest phase) time of whole division.

(b) Cytokinesis : Division of cytoplasm into 2 equal parts. In animal cell, it takes place by cell furrow method and in plant cells by cell plate method.

  1. Duration of cell cycle: It depends on the type of cell and external factors such as temperature, food and oxygen. Time period for G1, S, G2 and M-phase is species specific under specific environmental conditions. e.g. 20 minutes for bacterial cell, 8-10 hours for intestional epithelial cell, and onion root tip cells may take 20 hours.

  2. Regulation of cell cycle: Stage of regulation of cell cycle is G1 phase during which a cell may follow one of the three options.

  • It may start a new cycle, enter the S-phase and finally divide.

  • It may be arrested at a specific point of G1 phase.

  • It may stop division and enter G0 quiscent stage. But when conditions change, cell in G0 phase can resume the growth and reenter the G1 phase.

  1. Cell division is of three types, Amitosis, Mitosis and Meiosis.

  2. Difference between cell Mitosis and Meiosis





I. General


Site of occurrence

Somatic cells and during the multiplicative phase of gametogenesis in germ cells.

Reproductive germ cells of gonads.


Period of occurrence

Throughout life.

During sexual reproduction.


Nature of cells

Haploid or diploid.

Always diploid.


Number of divisions

Parental cell divides once.

Parent cell divides twice.


Number of daughter cells




Nature of daughter cells

Genetically similar to parental cell. Amount of DNA and chromosome number is same as in parental cell.

Genetically different from parental cell. Amount of DNA and chromosome number is half to that of parent cell.

II. Prophase



Shorter (of a few hours) and simple.

Prophase-I is very long (may be in days or months or years) and complex.



Formed of 3 subphases : early-prophase, mid-prophase and late-prophase.

Prophase-I is formed of 5 subphases: leptotene, zygotene, pachytene, diplotene and diakinesis.


Bouquet stage


Present in leptotene stage.




Pairing of homologous chromosomes in zygotene stage.


Chiasma formation and crossing over.


Occurs during pachytene stage of prophase-I.


Disappearance of nucleolus and nuclear membrane

Comparatively in earlier part.

Comparatively in later part of prophase-I.


Nature of coiling



III. Metaphase


Metaphase plates

Only one equatorial plate

Two plates in metaphase-I but one plate in metaphase-II.


Position of centromeres

Lie at the equator. Arms are generally directed towards the poles.

Lie equidistant from equator and towards poles in metaphase-I while lie at the equator in metaphase-II.


Number of chromosomal fibres

Two chromosomal fibre join at centromere.

Single in metaphase-I while two in metaphase-II.

IV. Anaphase


Nature of separating chromosomes

Daughter chromosomes (chromatids with independent centromeres) separate.

Homologous chromosomes separate in anaphase-I while chromatids separate in anaphase in anaphase-II.


Splitting of centromeres and development of inter-zonal fibres

Occurs in anaphase.

No splitting of centromeres. Inter-zonal fibres are developed in metaphase-I.

V. Telophase



Always occurs

Telophase-I may be absent but telophase-II is always present.

VI. Cytokinesis



Always occurs

Cytokinesis-I may be absent but cytokinesis-II is always present.


Nature of daughter cells

2N amount of DNA than 4N amount of DNA in parental cell.

1 N amount of DNA than 4 N amount of DNA in parental cell.


Fate of daughter cells

Divide again after interphase.

Do not divide and act as gametes.

VII. Significance

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