Cell cycle and their different phases | Questions | Biology Notes

Cell Cycle and Cell Division Biology Notes

Introduction of Cell Cycle

Cell Cycle : All organisms start their life from a single cell and grow by the addition of new cells. The new cells arise by the division of pre-existing cells.
This idea was suggested by Rudolf Virchow in 1858 in a particular statement ‘Omnis Cellula e Cellula’, means every cell produces from a cell. This states that the continuity of life depends on cell reproduction or cell division.

Who introduced the Cell cycle?

Cell cycle was introduced by Howard and Pele in 1953. It is defined as the series of events by which a cell duplicates its genome and synthesizes other cell components and then divides into two daughter cells

Phases of cell cycle

Cell cycle occurs in the following two phases.

  1. Interphase (nondividing phase)
  2. M Phase or Mitosis Phase (dividing phase)


It represents the phase between two successive M phases. It constitutes or lasts for more than 95% of the whole duration of the cell cycle. Though it is called the resting phase, it is the time during which the newly formed cells prepare themselves for division i.e., to undergo both growth and DNA replication in an orderly manner.

M Phase

It is the phase of cell division in which already duplicated chromosomes get distributed into two daughter nuclei. It starts with the nuclear division (karyokinesis) and terminates after cytokinesis
Karyokinesis is the separation of daughter chromosomes and nucleus division and cytokinesis is the division of cytoplasm.
During this phase, all components of the cell reorganise for cell division. Since, the number of chromosomes remains same in both parent and progeny cells, it is also known as equational division.

Cell Division

It is a very important phenomenon in all living organisms. The concept of cell division was firstly propounded by a scientist Nageli and was observed by Flemming in 1882 in reptelean Triturus mascules and gave it a name mitosis. Its complete extensive and exclusive study and was done by Belar in 1920. This is also called cell production.

Modes of Cell divison

Cell division usually occurs in following three ways

  1. Amitosis
  2. Mitosis
  3. Meiosis

It is very rare and is not considered an exact mode of cell division. It occurs only in some specialised cells like mammalian cartilage, embryonic membrane of some vertebrates, old tissues, diseased tissues, etc.


It was first explained by Eduard Strasburger. It usually takes place in somatic cells of animals Thus, it is known as somatic division.
Mitosis occurs in gonads for the multiplication of undifferentiated germ cells. It is continuous process that gives rise to two identical cells but the number of chromosomes in them remains the same.
It occurs in various phases such as prophase, metaphase, anaphase, telophase and then cytokinesis.

Cell cycle cell division mitosis meiosis

The significance of Mitosis

It helps in the growth and development of multicellular organisms, in the healing and repair of wounds; in maintaining the chromosome number and nucleocytoplasmic ratio, etc.


The term meiosis was given by JB Farmer and Moore in 1905. Meiosis as division process is restricted to only reproductive cells due to which gametes (sex cells) are produced. It occurs at a particular time during which a diploid cell divides to give rise to four haploids cells. It basically produces gametes in animals, some lower plants, various protists, and fungi. Meiosis is asexually reproducing organisms form asexual reproductive bodies like spores. As meiosis results in the reduction of the number of chromosome in the daughter cells by half, so it is also known as reduction division.

It consists of two stages of division that occur successively in an organism with one time chromosome replication.

First Meiotic Division (Meiosis I)

Second Meiotic Division (Meiosis II)

Meiosis I

In this phase of division, parental chromosomes replicate to produce identical sister chromatids and the number of chromosomes reduces from diploid (2n) to haploid (n) and hence, this type of division is called heterotypic division. Like mitosis, it also involves the four phases of division as described below.
Prophase I:- The prophase I of meiosis is more longer thatn the prophase of mitosis and it takes more than 90% of time required for meiosis.
Prophase I is further divided into 5 sub-stages such as leptotene, zygotene, pachytene, diplotene and diakinesis

Metaphase I during the course of this phase, spindle shifts to the position formerly taken by the nucleus and the synapsed pair of chromosome (bivalent) get arranged around the equator of the spindle and are attacked by their centromere.

Anaphase I:- During anaphase, I, homologous chromosomes of each pair gets separated and half of the chromosome move to each pole. Reduction of chromosome occur and each chromosome at individual poles is still double and have two chromatids

Telophase I The arrival of homologus chromosome at opposite pole shows the end of meiosis I. During this phase, chromosomes uncoil and get elongated. Cytoplasm tends to get divided by cleavage (constriction) in an animal cell and by cell plate formation in a plant cell and produces two cells each with one nucleus.

Meiosis II

The meiotic division exactly the same in overall process as mitotic division. There is no reduction in the number of chromosomes and the haploid nuclei divide mitotically in order to produce four haploid daughter nuclei. Thus, each diploid nucleus which undergoes meiosis produces four haploid nuclei. The only difference between mitosis and meiosis II is that interphase do not proceed prophase in meiosis. It gets initiated immediately after cytokinesis, usually before the chromosomes have been fully elongated
After meiosis II, four daughter cells are formed from the original single parent cell and each one is haploid (n) in nature.

The phases involved in meiosis II are prophase Ii, metaphase II, anaphase II and telophase II.

Significance of Meiosis: – Meiosis is significantly proved to be the important mechanism in living organism because this process brings stability in the number of chromosome in an organism. It also increases genetic variability in the population of organism from one generation to next. As variations are important to the process of evolution, meiosis acts as a source of new genetic variation.

Questions related to Cell cycle and cell division which will helps to more deeply clear the topics

Where in the cell cycle does apoptosis occur?

In the normal pathway, DNA damage is an intracellular signal that is passed via 2 protein kinases, leading to activation of p53. Activated p53 promotes 4 transcription of the gene
for 5 a protein that inhibits the cell cycle. The resulting suppression of cell divisionensures that the damaged DNA is not replicated. If the DNA damage is irreparable, then the p53 signal leads to programmed cell death called apoptosis.

What controls the cell cycle?

The cell cycle control system has been compared to the control device of a washing
machine. Like the washer’s timing device, the cell cycle control system proceeds on its own, according to a built-in clock. However, just as a washer’s cycle is subject to both internal control (such as the sensor that detects when the tub is filled with water) and external adjustment (such as starting or stopping the machine), the cell cycle is regulated at certain checkpoints by both internal and external signals. A checkpoint in the cell cycle is a control point where stop and go-ahead signals can regulate the cycle. Three important checkpoints are found in the G 1 , G 2 , and M phases.

Which cell cycle phase takes the longest?

G1 is typically the longest phase of the cell cycle.

Which cell cycle phase is the shortest?


When during cell cycle are chromosomes visible?

Mostly chromosomes are not visible. The chromosomes are visible during the mitosis in the cell cycle

Why is Cell Division Important?

Cell division is necessary for the growth of organisms, for wound healing, and to replace cells that are lost regularly, such as those in your skin and in the lining of your gut.

What cell division is asexual?

The small organism like bacteria reproduced by the asexual method of reproduction. Thus all the growth in bacterial population is clonal, it’s division of existing cells to new one. No fertilization take place in this type of reproduction. The new daughter cell which is produced having the same genetic information as of their parents.

What cell division process is responsible for growth?

Mitosis cell division helps in the growth and development of multicellular organisms; in the healing and repair of wounds; in maintaining the chromosome number and nucleocytoplasmic ratio, etc.

How is cell cycle regulated?

The frequency of cell division varies with the type of cell. For example, human skin cells divide frequently throughout life, whereas liver cells maintain the ability to divide but keep it in reserve until an appropriate need arises say, to repair a wound. Some of the most specialized cells, such as fully formed nerve cells and muscle cells, do not divide at all in a mature human. These cell cycle differences result from regulation at the molecular level. The mechanisms of this regulation are of great interest, not only to understand the life cycles of normal cells but also to learn how cancer cells manage to escape the usual controls

How are mitosis and binary fission different?

Binary fission is taking place mainly in the prokaryotes and single cell organism like bacteria. Mitosis mainly occurs in multicellular Eukaryotes.

The term binary fission, meaning “division in half,” refers to this process and to the
asexual reproduction of single-celled eukaryotes, such as the amoeba. Mitosis consists of multiple phases: prophase, prometaphase, metaphase, anaphase, and telophase. Binary fission is not complex like the Mitosis.

Where is Mitosis in the cell cycle?

The cell cycle is divided into the five phases named, G1 (gap phase), S (synthesis), G2 (gap phase 2), Mitosis, Cytokinesis. as shown in the Fig.

Which cells undergo mitosis?

Mitosis occurs in almost every cell of eukaryotes. It occurs where the need of the number of cells is more. Whereas meiosis is occurred only in special cells, in animals, gametes, and in plants pollen.

Who discovered mitosis in animals?

When Flemming looked at the cells through what would now be a rather primitive
light microscope, he saw minute threads within their nuclei that appeared to be dividing lengthwise. Flemming called their division mitosis, based on the Greek word mitos, meaning “thread.” But Otto Bütschli might have claimed the discovery of the process presently known as “mitosis”

What mitosis stage is the longest?

Prophase is the longest phase of mitosis, but it occurs faster than interphase

What mitosis and meiosis have in common? or Similarities between mitosis and meiosis




I. General
(1) Site of occurrence Somatic cells and during the multiplicative phase of gametogenesis in germ cells. Reproductive germ cells of gonads.
(2) Period of occurrence Throughout life. During sexual reproduction.
(3) Nature of cells Haploid or diploid. Always diploid.
(4) Number of divisions Parental cell divides once. Parent cell divides twice.
(5) Number of daughter cells Two. Four.
(6) 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
(7) Duration Shorter (of a few hours) and simple. Prophase-I is very long (may be in days or months or years) and complex.
(8) Subphases Formed of 3 subphases : early-prophase, mid-prophase and late-prophase. Prophase-I is formed of 5 subphases: leptotene, zygotene, pachytene, diplotene and diakinesis.
(9) Bouquet stage Absent. Present in leptotene stage.
(10) Synapsis Absent. Pairing of homologous chromosomes in zygotene stage.
(11) Chiasma formation and crossing over. Absent. Occurs during pachytene stage of prophase-I.
(12) Disappearance of nucleolus and nuclear membrane Comparatively in earlier part. Comparatively in later part of prophase-I.
(13) Nature of coiling Plectonemic. Paranemic.
III. Metaphase
(14) Metaphase plates Only one equatorial plate Two plates in metaphase-I but one plate in metaphase-II.
(15) 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.
(16) Number of chromosomal fibres Two chromosomal fibre join at centromere. Single in metaphase-I while two in metaphase-II.
IV. Anaphase
(17) Nature of separating chromosomes Daughter chromosomes (chromatids with independent centromeres) separate. Homologous chromosomes separete in anaphase-I while chromatids separate in anaphase in anaphase-II.
(18) 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

(19) Occurrence Always occurs Telophase-I may be absent but telophase-II is always present.
VI. Cytokinesis
(20) Occurrence Always occurs Cytokinesis-I may be absent but cytokinesis-II is always present.
(21) 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.
(22) Fate of daughter cells Divide again after interphase. Do not divide and act as gametes.
VII. Significance
(23) Functions Helps in growth, healing, repair and multiplication of somatic cells.

Occurs in both asexually and sexually reproducing organisms.

Produces gametes which help in sexual reproduction.
(24) Variations Variations are not produced as it keeps quality and quantity of genes same. Produces variations due to crossing over and chance arrangement of bivalents at metaphase-I.
(25) In evolution No role in evolution. It plays an important role in speciation and evolution.


How mitosis relates to cancer?

Without mitosis, their is no cancer. In cancer cells grows or divided at abnormal speed and resulted in tumor formation. The cells are divided by the process named ‘mitosis’.

When mitosis occurs without cytokinesis?

Cytokinesis is the last phase of the mitosis, in which cell is divided into two parts. In mitosis, there is replication of chromosome and two number of the nucleus are formed for two different cell. If, the cytokinesis does not take place then the cell results with two nuclei.

Such a cell is called a multinucleated cell. This can be a normal process. For example, humans have certain multinucleated bone cells (osteoclasts) that are formed this way. Mitosis without cytokinesis is also observed in the early development of certain insects such as the fruit fly (Drosophila).

Why is mitosis known as equational division?

Mitosis is the process of cell division gives 2 number of daughter cells. The chromosome number in each daughter cell is equal to that in the parent cell, i.e., diploid. Hence, mitosis is known as equational division.

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