CELL CYCLE

CELL CYCLE

Cell cycle is the series of growth and development stops a cell undergo between its birth formation by the division of a mother cell and reproductive division to make a new daughter cell.

Cell division or cell multiplication or cell reproduction is the process of formation of new or daughter cells from the pre- existing or parent cells.

Significance of cell division:

  •   It is essential for the continuity of life.
  •   It forms the basis of evolution.
  •   In unicellular organisms, cell division is a means of asexual reproduction which produces two or more new individual from the mother cell. The group of such identical individual is known as clone.
  •   In muticellular organisms, life starts from single celled zygote and transforms into an adult,
  •   Cell division is the basis of repair and regeneration of old and worn tissues.

Cell division is categorized into three types:

        I.            Amitosis

      II.            Mitosis

    III.            Meiosis

        I.            Amitosis:

In greek a= no, mitos= thread, osis=state

 

Introduction:

Ø  Remak (1955) discovered amitosis in RBCs of chick embryo, but the term coined by Flemming(1882).

Ø   This is followed by centripetal constriction of cytoplasm to form two daughter cells.

Ø  Amitosis occurs in mega-nucleus of paramecium, endosperm cells of seeds, cartilage cells and diseased cells.

Amitosis is characterized by:

Ø  Intact nuclear envelope is found throughout the division.

Ø  Chromatin doesn’t condense into definite chromosome.

Ø  Spindle fibers are not formed.

Ø  Chromatin distribution occurs unequally which causes abnormalities in metabolism and reproduction.

 

      II.            Mitosis:

In greek mitos = thread, osis= thread.

History:

·         Mitosis was first discovered in plant cell by strassburger (1875), Later W. Flemming discovered in animal cell.

·         Term mitosis was coined by Flemming in 1882.

Introduction:

  •   Mitosis is a type of cell division in which chromosomes are equally distributed resulting in two genetically identical daughter cells
  •   Mitosis is the process of forming identical daughter cells by replicating and dividing the original chromosomes. Mitosis deals only with the segregation of the chromosomes and organelles into daughter cells. In mitosis, nuclear DNA of the cell condenses into visible chromosomes and is pulled apart by the mitotic spindle, a structure eveloped by microtubules.
  •   Mitosis is the description of how somatic or non- reproductive cell divide. Somatic cell makes up most of our body tissues and organs including skin, muscles, lungs, gut and hair cells.
  •   In mitosis each daughter cells have same chromosomes and DNA as the parent cells. The daughter cells developed from mitosis are called diploid cells (have complete sets of chromosomes). Since the daughter cells have exact copies of their parent cell DNA, no genetic diversity is created through mitosis in normal healthy cells.
  •   Mitosis is found to occur in somatic cell. Mitosis is carried out in following stages:

1)      Interphase: Prefix” inter “means between, reflecting that interphase takes place between one mitotic division and next. It is divided into three stages:

Ø  G1 Phase (gap one): Cell grows physically larger, copy organelles and makes the molecules building blocks. Metabolic changes prepare the cell for division. At a certain point; restriction point(R point), the cell is committed to division and moves to S- phase.

Ø  S- Phase (Synthesis phase) : cell synthesize a complete copy of DNA in its nucleus. It also duplicates a microtubule organizing structure called the centrosome. Helps in separation of DNA during M-phase. DNA synthesis replicates the genetic material. Each chromosome now consists of two sister chromatids.

Ø  G2 Phase (gap two): Cell grows larger, synthesize proteins and organelles and begin to reorganize its contents in preparation for mitosis.G2 ends with the beginning of mitosis.

Ø  M-phase: It is divided into following phases:

a)      Prophase

b)      Metaphase

c)       Anaphase

d)      Telophase

 

a)      Prophase:

·         During prophase chromosomes gets condenses and become more visible.

·         Spindle fibers emerges out from the centromere .

·         Nuclear envelope breaks down.

·         Centriole divides and migrates to the opposite poles.

·         Kinetochores( point of attachment of spindle fibers) fibers forms.

 

Prometaphase:

·         Chromosomes are continued to condense.

·         Kinetochores appear at the centromere.

·         Microtubules in spindle fibres attach to the kinetochores.

 

b)      Metaphase:

·         Chromosomes are arranged at the equator of cell or lined up at the metaphase plate.

·         Each sister chromatids is attached to a spindle fibre originating from opposite poles.

·         Chromosomes migrate to the equator of the spindle, where the spindles attach to the kinetochore fibres.

c)       Anaphase:

·         Centromere split in two.

·         Sister chromatids (now called chromosomes) are pulled towards opposite poles.

·         Anaphase begins with the separation of the centromeres and the pulling of chromosomes to the opposite poles of spindle.

·         Certain spindle fibres begin to elongate the cell.

d)      Telophase:

·         Chromosomes arrive at opposite poles and begin to decondense.

·         Nuclear envelope material surrounds each set of chromosomes.

·         Mitotic spindle fibres break down.

·         Spindle fibres continue to push spindle apart.

·         When chromosomes reach the poles of their respective spindles, the nuclear envelope reforms. Chromosomes uncoil into chromatin and the nucleolus reforms.

Cytokinesis:

It is the cytoplasmic division that starts during anaphase and completed by the end of telophase. It takes place by two methods:

         i.            Cell plate method: It occurs in plant cell. Spindle fibres persist at equatorial plane. Golgi vesicles fuse at the centre to form barrel shaped phragmoplast. Further addition of vesicles cause the phragmoplast to grow centrifugally till it meets with plasma membrane of the mother cell. Contents of phragmoplast solidify to become cell plate or future middle lamella which separates two daughter cells.

       ii.            Cleavage furrow method: It occurs in animal cell and pollen mother cells in some angiosperm. Here, a cleavage furrow appears at the middle, which gradually deepens and breaks the parent cell into two daughter cells. Special structure called mid body formed at the centre in a centripetal (towards centre) manner.

 

Significance of mitosis:

a)      Genetic stability: Mitosis maintains constant chromosome number  and genetic stability in all somatic and vegetative cells of the body.

b)      Growth: Mitosis increases cell number so that a zygote transforms into a multicellular adult.

c)       Surface- Volume ratio: As the size or volume increases of a cell increases, surface area increases accordingly.

d)      Nucleo-plasmic ratio:  When a cell grows in size, nucleocytoplasmic ratio decreases.

e)      Mitosis is a method of asexual reproduction and vegetative propagation.

f)       Mitosis provides new cells for repair, regeneration and wound healing.

g)      DNA content is reduced to half from parent cell to daughter cell.

 

Meiosis:

In greek meioum= to reduce, osis=state

History:

Vanzeneden (1883),  first reported meiosis.

Farmer and Moore(1905), coined the term meiosis.

Occurrence:

Cells undergo meiosis are known as meiocytes. In plants these meiocytes are known as microsporophytes of anther and megasporophyte of ovules.

In animals, these meiocytes are primary spermatophyte in testes and primary oocytes in ovaries.

Ø  It is also known as reductional division, during which exchange of genetic material between homologous chromosomes takes place and such division of the genetic material resulting in the formation of daughter cells.

Ø  It creates sex cells like female egg cells or male sperm cells. Each new contains a unique set of genetic information.

Ø  After meiosis, the sperm and egg cells can join to create new organism.

Ø  During meiosis, a small portion of each chromosome breaks off and reattaches to another chromosomes. This process is called crossing over or recombination.

Ø  Meiosis comes after G2 phase, when DNA replication is already completed so that the cells bear 2n and 4C at the beginning of meiosis.

Ø  During meiosis genetic information is exchanged between maternally and paternally inherited copies of pair of chromosomes in order to create new combination of genes. This process of genetic recombination helps to increase genetic variability within a species. It allows for the transmission of virtually limitless combination of genes from parents to offsprings.

Ø  Resulting gametes have 23 new chromosomes, one combination of each of the 23 pairs, representing unique combination of the original maternal and paternal copies.

Meiosis is categorized in two types:

a)      Meiosis I

b)      MeiosisII

 

a)      Meiosis I:

Ø  Meiosis I leads to the formation of haploid cells, containing single set of chromosomes i.e. 23 chromosomes in humans cells.( diploid cells contains 23 pairs or 46 chromosomes)

Ø  It halves the number of chromosomes.

Ø  Before meiosis I starts the cells goes through interphase. The parent cell use this time to prepare cell for cell division by gathering nutrients and energy and making copy of its DNA.

 

Meiosis I is also divided in following phases:

        I.            Prophase I:

1)      Chromosomes condense and become visible.

2)      Cetrioles forms and move towards pole.

3)      Nuclear membrane begins to dissolve.

4)      Homologous chromosomes pair up and forming a tetrad (when homologous chromosomes line up in linear manner).

5)      Each tetrad is comprised of four chromatids.( two homologous chromosomes each with their sister chromatids)

6)      It is subdivided into following phases:

1)      Leptotene: In this stage chromosomes appear in thread like manner called chromatin. Condensation of chromatin occurs and chromosomes start to develop.

2)      Zygotene: Pairing of homologous chromosomes occurs at this stage and leads to the formation of synaptonemal complex. This process is known as synapsis. Here homologous chromosomes become closely associated to form pairs of chromosomes which is known as bivalent. It consists of four chromatids which are known as tetrad.

3)      Pachytene: Exchange of segments of non homologous chromosomes is known as crossing over. A point of exchange of chromosomes is known as chiasmata. It is also known as site of crossing over.

4)      Diplotene: Homologous chromosomes starts to separate but remain attach at certain points by chiasmata.

5)      Diakinesis: In this stage terminalization of chromosomes occurs. Homologous chromosomes continue to separate and chiasmata moves to the ends of chromosomes in zip like manner.

      II.            Metaphase I: Homologous pairs of chromosomes (bivalent) arranged as a double row along metaphase plate. Arrangement of joined chromosomes with respect to the poles of spindle apparatus is random along metaphase plate.

    III.            Anaphase I: Homologous chromosomes in each bivalent are separated and move to the opposite poles of the cell.

    IV.            Telophase I: Chromosomes become diffuse and the nuclear membrane reforms.

 

 

Meiosis II:

1)      Prophase II: Chromosomes begin to condense, nuclear membrane dissolves, spindle fibers form.

2)      Metaphase II: Spindle fibres attach to the kinetochores of chromosomes, Chromosomes line up in centre of cell.

3)      Anaphse II: Centromere divide and sister chromatids move to the opposite poles of cell and spindle fibres shorten.

4)      Telophase II: Chromosomes reach to the opposite poles of the cell and nuclear membrane reforms.

Cytokinesis: Cell division occurs here as like meiosis I.

Significance of meiosis:

         i.            Meiosis essentially maintains constancy in chromosomes from generation to generation.

       ii.            Crossing over and disjunction bring genetic variation within the species. The variations are important for evolution and in improvement of races.

      iii.            Meiosis causes conversion from sporophytic generation to gametophytic generation in plants.

     iv.            It leads to the formation of haploid gametes which is an essential process in sexually reproducing organisms. Fertilization restores the normal somatic (2n) chromosome number.

       v.            Meiosis generates genetic diversity through exchange of genetic material between homologous chromosomes during meiosis I, random arrangement of maternal and paternal chromosome in meiosis I, random alignment of sister chromatid at meiosis II.

     vi.            Meiosis reduces the chromosome number by half enabling sexual recombination.

    vii.            Meiosis of diploid cells produces haploid daughter cells which function as gametes.

  viii.            Gametes undergo fertilization restoring diploid number of chromosomes in zygote.

   ix.            Role of sexual reproduction in evolution.

   x.            The total number of possible outcomes is 2n.

 

 

Regulation of cell cycle:

Some cells divide rapidly (RBC must divide at a rate of 2.5 million per second). Such as nerve cells lost their capability to divide once they reach maturity. Cell cycle is controlled by cyclically operating set of reaction sequence that both triggers and coordinate.

Key events in cell cycle:

·         Cell cycle control system is driven by built in cluster that can be adjusted by external stimuli.

·         Check point, a critical control point in the cell cycle where stop and go ahead signals can regulate the cell cycle.

·         Three major check points are found in G1, G2, M Phases of G1 check point (restriction group) ensures that cell is large enough to divide and enough nutrients are available to support the resulting daughter cell. Most cells in human body are in G0 phase.

·         G2 check point ensures that DNA replication in S phase has been completed successfully.

·         Metaphase check point ensures that all of the chromosome are attached to the mitotic spindle by a kinetochore.

 

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