Cell Cycle and Cell Division Class 11
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.
Cell Cycle
Cell Cycle was introduced by Howard and Pele in 1953. It is defined as the series of events hy which a cell duplicates its genome and synthesises other cell components and then divides into two daughter cells.
Phases of Cell Cycle
Cell cycle occurs in the following two phases
1.Interphase (undividing phase)
2. M Phase or Mitosis Phase (dividing phase)
1. Interphase
It represents the phase between two successive M phases. It constitutes or lasts for more than 95% of the whole duration of cell cycle. Though it is called the resting phase, but 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.
Interphase involves three phases as given below
G4 Phase
First gap phase, represents time between mitosis and initiation of DNA replication, cells are metabolically active, grow continuously, produce carbohydrates, lipids, proteins, etc, DNA content remains the same, replication do not occur.
S Phase
Synthesis phase, DNA replication occurs, DNA content doubles from 2C to 4c
G2 Phase
Second gap phase, preparatory phase for cell division, proteins and RNA synthesises, lead to cell division or mitosis.
2.M Phase (Dividing 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 remain 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. 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 was doneh Belar in 1920. This is also called cell production.
Modes of Cell Division
ell division usually occurs in following three ways
- Amitosis
- Mitosis
- Meiosis
1. Amitosis
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.
2. Mitosis
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 cell .It is a 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.
Different phase of Mitosis
Prophase
- First and the longest phase of cell division. It continues for about 50 minutes.
- Involves the condensation of chromosomal material.
- Nucleoli and nuclear envelope disappears.
- Duplicated chromosomes have two identical sister chromatids which are held together by a disc-lke structure (kinetochore) at centromere.
- Spindles push centriole pairs to the opposite ends (poles). In plants, development of spindle fibres occur without the involvement of centrioles.
Metaphase
- Short and simple phase that lasts for only 2-10 minutes. Metaphase is the most suitable time to determine the number of chromosome and to study their morphology.
- Chromosome move towards the equatorial plane of the spindle.
- Spindle fibres extends towards the chromosome and attaches to the kinetochore.
- At equator of the spindle, the chromosomes get arranged in the form of a plate called metaphase plate and the chromosome reaches to the equator.
Anaphase
- It continues for about 2-3 minutes only.
- As each chromosome moves away from the equatorial plate, the centromeres of each chromosome splits and the sister chromatids separates to move towards the opposite poles.
- The spindle fibres elongate and cell becomes longer and anaphase ends when all the chromatids reaches the opposite poles.
Telophase
- The chromosomes that have reached their respective poles start to decondense and lose their individuality.
- Reconstructuon of nuclear envelope occur in each group of chromosomes.
- The chromosomes gradually uncoil and become long and slender and are at opposite poles.
- Spindle fibres get disappear.
- Viscosity of cytoplasm decreases, which would generally increase in prophase.
- Cell organelles like nucleolus, golgi complex and ER are reformed again.
Cytokinesis
At the end of mitosis, division of cytoplasm and equal distribution of cell organelles in two daughter cells occurs by the process of cytokinesis. During this process, the original larger cell divides into two smaller identical cells, which individually takes food, grow, divide and the process continues.
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.
3. Meiosis
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 haploid cells.
It basically produce gametes in animals, some lower plants, various protists and fungi. Meiosis in asexually reproducing organisms forms asexual reproductive bodies like spores. As meiosis results in the reduction of number of chromosomes 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.
(i) First Meiotic Division (Meiosis I)
(ii) Second Meiotic Division (Meiosis II)
1.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 than 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 nucleus and the synapsed pair of chromosomes (bivalent) get arranged around the equator of the spindle and are attached by their centromere.
Anaphase I
During anaphase I, homologous chromosomes of each pair gets separated and half of the chromosomes move to each pole. Reduction of chromosomes occur and each chromosome at individual poles is still double and have two chromatids.
Telophase I
The arrival of homologous 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 for plant cell and produces two cells each with one nucleus.
2. Meiosis II
The meiotic division is 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 preceeds, 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 bring stability in the number of chromosomes 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.
Cell Cycle and Cell Division Class 11 | Cell Cycle and Cell Division Notes Class 11 Cell Cycle and Cell Division Class 11
Cell Cycle and Cell Division Class 11 | Cell Cycle and Cell Division Notes Class 11 Cell Cycle and Cell Division Class 11
Cell Cycle and Cell Division Class 11 | Cell Cycle and Cell Division Notes Class 11 Cell Cycle and Cell Division Class 11
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