Biology

1. Cell cycle:

It is an ordered set of events, which occurs between the formation of a cell and its division into two daughter cells.

The cell cycle is composed of 4 distinct phases i.e.

1.    Interphase:  interphase consists of following phases i.e.

G1 phase, S phase and G2 phase.

2.    M phase and

3.    C phase

G1, S, and G2 phases (the first three phases) together constitute the interphase and the M stage stands for mitosis and C phase for cytokinesis. In the simplest sense, a cell duplicates its contents and then divides in two. The cycle of duplication and division is known as the cell cycle.

Interphase + Nuclear division (mitosis) + Cytokinesis = Cell cycle

4.       Interphase:  During interphase, the cell is growing and preparing for mitosis (M phase) by accumulating nutrients and replicating DNA.

Interphase is the longest phase in cell cycle. Though this phase is sometimes called resting stage, but it is in fact the most active phase of the cell cycle.  Interphase consists of

i.        G1 phase (Gap 1):  G1 stage separates the end of mitosis and the start of the S phase. The timing and the relative lengths of the various stages of the cycle depends on the specific type of cell and on the local conditions. Cell cycle ranges 8 hours to 100 days or more. Differences in cell cycle times are mainly due to the variations in the length of G1 phase. Cells in a rapidly developing tissue have thus a short G-1 phase. Slowly dividing cells stay in G1 phase for days or more or in many organisms takes up most of the cell's life.

Events of G1 Phase:     

v  After cytokinesis, the newly formed daughter cells are quite small and low on ATP.

v  They acquire ATP and increase in size during the G1 phase.

v  In this phase cytoplasmic growth occurs and the cell is preparing its enzymatic machinery to be ready for the next stage S phase (synthesis).

v  The daughter cells become as large as the mother cell.

v  The chromosomes are thread-like and invisible (dispersed state).

v  No change in DNA amount or chromatin.

v  Each chromosome contains only a single molecule of DNA is called an unduplicated or unreplicated chromosome.

v  In G1 (first gap) intensive formation of biochemicals and cellular synthesis, production of mitochondria, plastids, endoplasmic reticulum, lysosomes, golgi apparatus, vacuoles takes place.

v  Nucleolus produces rRNA, Synthesis of rRNA, mRNA and ribosomes. Structural and function (enzymes) proteins, amino acids for histone formation, nucleotides and energy rich substance ATP - synthesized. Cell metabolic rate high, controlled by enzymes.

During G1 phase, a cell may follow one of the three options:

1.         Cell has reached the restriction point (R point). After a short rest it continues on the cycle and divide.

2.         The cell permanently stops division and enter G0 or quiescent stage. Often G₀ cells are terminally differentiated: they will never reenter the cell cycle but carry out their function in the organism until they die e.g. Secretion, attacking pathogens (lymphocytes).

3.         The cell cycle has been arrested at a specific point of G1 phase. The cell in the        

arrested condition is said to be in the G0 state. The cell in the G0 may be considered to be withdrawn from the cell cycle. When conditions change and growth is resumed the cell re-enters the G1.

Duration of G1 phase: G1 is typically the longest phase of the cell cycle. Cells usually remain in G1 for about 10 hours of the 24 total hours of the cell cycle.

ii.         S Phase(synthetic phase) Synthesis of DNA and histones phase:- It is bordered by both of the G1 and G2gap phases. During the S phase,

v  New DNA is synthesized by the cell resulting in each chromosome with two molecules of DNA.

v  The DNA content of the nucleus is doubled and proteins are synthesized.

v  During this stage every double-helical DNA molecule is duplicated, making two strands of DNA that are exactly identical (the DNA breaks apart at different points along the strands. New single strands join the two halves of original strands).

v  Two new DNA strands are formed, which are attached together by specific proteins, at a short sequence of DNA (which is found on each double helix) and called a centromere.

v  The two DNA copies that result from S phase are not visible through a light microscope because they have not yet condensed to form chromosomes (i.e., they remain chromatin).

v  Each chromosome has become two chromatids.

v  Once duplication is complete, histones proteins synthesized which cover each DNA strand (chromatin synthesis).

v  The number of chromosomes remain same as were present in the newly formed cell(1n or 2n) but each chromosome is changed from single stranded form to two stranded form.

Duration of S phase: The length of S phase varies according to the total DNA that the particular cell contains; the rate of synthesis of DNA is fairly constant between cells and species. Usually, cells will take between 5 and 6 hours to complete S phase.

iii.           G2 phase (Gap2): Since the formation of new DNA(S phase) is an energy draining process, the cell undergoes a second growth and energy acquisition stage, the G2 phase. The energy acquired during G2 is used in cell division (in this case mitosis).During the pre-mitotic gap phase (G2),

v  Synthesis of RNA and protein continues, but DNA synthesis stops.

v  The mitotic spindle proteins are formed.

v  The mitotic spindle is structure that is involved with the movement of chromosomes during mitosis.

v  The multiplication of the chloroplasts and mitochondria (by binary fission!) and production of materials needed for mitosis (a nuclear! event) takes place.

v   During G2 phase chromatin begins to condense into the relatively compact structures called chromosomes (which, as a result of condensation, become visible through a light microscope).

v  These chromosomes remain attached through their centromeres.

v  The durations of the S phase, the G2 phase and mitosis is generally constant in most cell types.

Duration of G2 Phase: G2 is shorter, lasting only 3 to 4 hours in most cells. In sum, then, interphase generally takes between 18 and 20 hours. Mitosis, during which the cell makes preparations for and completes cell division only takes about 2 hours.

                                                            Cell Cycle

2.         M Phase(Mitosis): M phase consists of two coupled processes i.e.

i.              Mitosis: in which cells’ chromosomes are divided between the two daughter cells.

ii.             Cytokinesis: in which cells’ cytoplasm divides in two newly formed distinct cells. 

i.          Mitosis (Somatic Division):

Mitosis is the common method of nuclear division, followed by cytokinesis (cytoplasmic division). It usually occurs in vegetative or somatic cells therefore it is known as somatic division.

“In this division the mother cell produces two genetically identical daughter cells which resemble each other and also parent cell qualitatively and quantitatively.”

                                                            OR

“The separation of separate sister chromatids into two new cells with exactly the same number of chromosomes and half the amount of nuclear DNA i s known as mitosis.”

:

  Different Stages of Mitosis

Mitosis phase is the shortest phase in cell cycle. Mitosis is a continuous process. Mitosis has been subdivided into five phases

a.      Prophase (A stage of chromosome condensation):

v  At the beginning of prophase chromosomes appear as thin, filamentous uncoiled structures.

v  Chromosomes become coiled, shortened and more distinct in prophase, which is of much longer duration than other stages.

v  Nucleoli disappear.

v  Each chromosome longitudinally splits into two sister chromatids. Double structure of each chromosome is visible at late prophase.

v  The duplicated chromosome subunits (each one called chromotid) join together at the centromeres.

v  And two chromatids are attached to spindle tubules (Chromosomal fibers) with the help of protein plates called kinetochores specialised structures develop on either surface of centromere of each chromosome.

v  The kinetochore is the actual site of the insertion of the spindle threads and is a permanent part of the chromosome.

Prophase

                                                        

b.      Prometapase

v  The nuclear membrane disintegrates.

v  When the nuclear membrane dissolves, there is no differentiation between cytoplasm and nucleoplasm.

v  The chromosomes are attached to the spindles through their centromeres.

v  Such mitosis is called extra-nuclear mitosis or eumitosis.

v  The mitosis which takes place within the nuclear membrane and is called intranuclear mitosis or premitosis e.g.many protozoans and some animals cells

v  The chromosomes move freely and proceed towards the equator.

v  Microtubules a system of microtubules needed to move the chromosomes begins to form during prophase.

v  The microtubules, also called spindle fibers, form from an area of the cell called the centrosome.

v  During interphase, the cell has one centrosome but just before prophase, the centrosome duplicates, producing a second centrosome.

v  During prophase, microtubules radiate from each centrosome. Some of the    microtubules extend from one centrosome toward the other.

v  Spindle fibers extend from the poles to the equator.

v  The spindle apparatus forms.The spindle apparatus which moves the chromosomes consists of two proteins actin and tubulin

v  The entire complex of centrosomes and spindle fibers is called the spindle apparatus.

v  The two centrosomes move to the opposite poles of the nucleus.

v  At the end of prometaphase, the two centrosomes are at opposites poles of the cell and some of the spindle microtubules are attached to the chromosome at the knietochore.

v  The microtubule from one pole may attach to the knietochore first, and the chromosome and the chromosome begin to move toward the other pole aligning the chromosomes at the equatorial plane.

v  Microtubules can only remain attached to a kinetochore where there is a force exerted on the chromosome from the opposite end of the cell.

v  Kinetochore microtubules are spindle fibers that attach to the kinetochores and move the chromosomes to the center of the cell.

v  The next phase (Metaphase) begins when the chromosomes become aligned in the center of the cell.

                                                                                                                                                                                                                               PROMETAPHASE

c.          Metaphase:

v  The chromosomes line up in one plane to form the equatorial plate or metaphasic plate.

v  Chromosome lies in the middle of the spindle apparatus and is perpendicular to its axis.

v  Only the centromere lies on the equatorial plate, while the chromosome arms (chromatids) are directed away from the equator.

v  Smaller chromosomes are usually central in position whereas the larger ones are peripheral

                                                           

METAPHASE

                                               

v  At metaphase, the chromosomes are aligned on the cell’s midline. Approximately 15-35 mircotubules are attached to the kinetochore (by kinetochore microtubules).

v  There are also two types of nonkinetochore microtubules:-

v  Some microtubules radiate from the centrosome toward the metaphaseplate without attaching to chromosomes. Others are too short to reach the metaphase plate.

v  Still others extened across the plate and overlap with nonkinetochore microtubules from the opposite pole of the cell.

d.          Anaphase(centromere separation and chromatid migration):

v  Anaphase begins when the centromeres divide and the spindle apparatus starts pulling the kinetochores to the opposite poles (progressive shortening of the microtubular spindle fibers pulls the chromosomes in opposite directions toward the poles).

v  The daughter kinetochores move apart dragging the chromosomes (each now a single strand) to the poles.

v  Two cells begin to form.

v  In anaphse the centromeres divide and two sister chromatids separate and move to the opposite ends of the cell.

v  Microtubules pull a chromosome towards a pole by losing protein subunits are their centrosome and at the + end (attached to the kinetochore). The nonkintetochore microtubules are responsible for elongating the whole cell along the polar axis during anaphase.

v  When the chromosomes split in anaphase, the chromosome number is doubled.

v  Cytokinesis (division of the cytoplasm) begins in anaphase. Two cells will be produced as this process continues.                                                                                                                                                                                                                                                                                           

     ANAPHASE

e.  Telophase (recovery of nuclear envelope and decondensation).

Telophase begins when chromosomes reach the poles of the daughter cells.

Many of the events in telophase are the reverse of prophase, but there are now two nuclei instead of one.

v  Chromosomes decondense (uncoil).

v  Nuclear membrane reappears around daughter nuclei.

v  Spindle Fibers become disorganized. the spindle apparatus breaks down

v  The nucleolus reappears.

v  The cell pinches in the middle, beginning the formation of the two cells the new cell plate expanding centrifugally.

v  Each daughter cell gets the same complement of chromosomes and nucleoli as of the mother cell. During division, cell organelles like mitochondria, plastids, Golgi complex, lysosomes and the cytoplasmic matrix are distributed into the two daughter cells

   TELOPHASE

                                

3.      C Phase OR Cytoplasmic division (cytokinesis): The cell cycle culminates the division of cytoplasm by cytokinesis. Cytokinesis begins in anaphase and ends in telophase. Cytoplasm of the cell divides

i.   By Cell plate formation

ii.  By constriction

i.          By Cell Plate Formation:

v  Plant cells divide by formation of a new cell wall (cell plate ) between daughter nuclei.

v  The deposition of the new wall is regulated by a structure called the phragmoplast which contains microtubules, actin filaments and membranes.

v  Phragmoplast microtubules appear to direct the kinesin-driven transport of Golgi-derived vesicles containing cell wall components to the phragmoplast equator.

v  At the end of anaphase, Golgi-derived secretory vesicles carrying cell wall materials are transported to the equator of a dividing cell.

v  Fusion of these vesicles gives rise to a membrane-bound compartment, the cell plate.

v  The cell plate expands from the middle out (centrifugally) until it reaches the "zone of attachment" or division site on the mother cell wall.

v  Once this attachment has taken place, the cell plate undergoes a complex process of maturation (formation a new cell wall) during which callose is replaced by cellulose and pectin.

v  The places where vesicles of the cell plate fail to fuse, the cytoplasmic contact between the daughter cells are maintained. Such cytoplasmic channels are lined by plasma membrane to form plasmodesmata.

ii.         By Constriction:

Animal cell divides by constriction of cleavage. This furrow or cleavage deepens and spreads around the cell and completely divides into two cells.

Importance of mitosis Cell division is a fundamental part of a plant's existence and development

1.    Growth: The number of cells in a plant increases due to mitosis leading to growth.

2.    It results in the increase of size, shape and volume of plant parts.

3.    Asexual reproduction, regeneration and cell replacement: A number of plants propagate (vegetatively) by mitotic divisions of the cells. Regeneration of lost parts and cells replacement (tissue repair) occurs by mitosis in multicellular plants.

4.    As a result of mitosis, two nuclei are formed which have same number of chromosomes as the parent cell.

5.    Hereditary material is equally distributed between two daughter cells.

6.       Asexual reproduction, regeneration, healing of wounds and replacement of older cells are the gift of mitosis.

Conclusion:

1.    Cell division is very important process to continue organisms’ life cycle.

2.    Mitosis is a somatic cell division which causes growth of a living body.

3.    Pattern of mitosis is fundamentally same in all cells.

4.    By the process of mitosis not only chromosomes replicate but all necessary cytoplasm constituents and organelles are divided between two daughter cells.

5.    No change in chromosome number.

6.    Mitosis is observed in all type of cells i.e. haploid, diploid or polyploidy.

7.    Mitosis usually occurs in vegetative or somatic cells.

8.    Hereditary material is equally distributed between two daughter cells.

9.      Regeneration, healing of wounds and replacement of older cells take place by mitosis.

_Quiz:

1.   During which stage of mitosis does cytokinesis usually occur in animals?

§  prophase

§  meta phase

§  anaphase

§  telophase

1.    During which stage of mitosis do the centromeres split?

§  Prophase

§  Interphase

§  Anaphase

§  Telophase

§  Synthesis stage

2.    How long does a cell cycle last?

§  It depends on the cell type

§  It is about the same length for cells of the same type

§  It varies greatly among cells of the same type

§  It is always about 24 hours.

3.    The three phases in interphase are known as G1, S and G2. What happens in G1?

§  The cell performs functions

§  The cell rests

§  The cell grows rapidly

§  DNA is copied

4.    What is the sister chromatids locked together with?

§  Centrioles

§  Centromere

§  Spindle fiber

§  Chromatin

5.    Cellular synthesis i.e. production of mitochondria etc take place in

§  S phase

§  G2 phase

§  G1 phase

§  Interphase

6.    In which phase DNA molecule is doubled and protein is synthesized?

§  G1 phase

§  G2 phase

§  S phase

§  M phase

8.   The mitotic spindle are formed during

§  G1 phase

§  G2 phase

§  S phase

§  M phase

9.   Somatic division is also known as

§  Mitosis

§  Amitosis

§  Meiosis

§  None of the

1.      Chromatids are attached to spindle tubules with the help of protein plates called

§  Centromere

§  Chromomere

§  Kinetochore

§  Centrioles

11. The spindle apparatus which moves the chromosomes consists of

§  Actin

§  Tubulin

§  Actin and tubulin

§  None of them

12. The entire complex of centrosomes and spindle fibers is called

§  Chromatin work

§  Spindle fibers

§  Spindle apparatus

§  Asters

13. The chromosomes line up in one plane to form the equatorial plate during

§  Anaphase

§  Metaphase

§  Prophase

§  Telophase

14.   Two cells begin to form during

§  Prophase

§  Anaphase

§  Metaphase

§  Interphase

15.   Chromosome number is doubled during

§  Anaphase

§  Metaphase

§  Telophase

§  Porphase

16.   Chromosomes reach to the opposite pole during

§  Anaphase

§  Prophase

§  Telophase

§  Metaphase

17.   The deposition of the new wall is regulated by a structure called

§  Protoplast

§  Chromoplast

§  Phragmoplast

§  None of them 

18. Regeneration, healing of wounds and replacement of older cells take place by

§  Mitosis

§  Meiosis

§  Amitosis

§  All of these

19. Plant cells divide by

§  Cell plate formation

§  Constriction

§  By splitting

§  All these

20. In a cell division in which two nuclei having same number of chromosomes as    

      the parent cell is called

§  Meiosis

§  Mitosis

§  Both of these

§  None of these

Summary:

·         Cell undergoes series of changes, which involve period of growth, replication of DNA, followed by cell division. This sequence of change is called cell cycle.

·         Cell cycle comprises of two phases i.e. interphase which is a period of non-apparent division and M phase which is the period of division.

·         Interphase or resting phase is divided into G1-phase, S-phase, and G2-phase.

·         G1-phase is the period of extensive metabolic activity in which cell normally grows in size and synthesized enzymes.

·         In S-phase DNA is synthesized and chromosomes number is doubled.

·         G2-phase stores energy for chromosome movements, synthesized mitosis protein, RNA and microtubule sub units.

·         M-phase is the division phase which consists of prophase, metaphase, anaphase and telophase.

·         During prophase chromatin material gets condensed and appears as a thread like chromosomes. Each chromosome splits into two chromatids. Nuclear envelop and nucleolus disappear.

·         Chromosomes arrange themselves at the equatorial plane of the spindle and attached to the spindle fiber by kinetochore during metaphase.

·         During centromere divides and two chromatids get separated. By the contraction of spindle fibers chromatids migrate towards opposite poles.

·         Now the chromatids reached to the opposite poles in telophase. Nuclear membrane and nucleolus reappear.

·         Cytokinesis take place, in animal cell a constriction or depression appears in the cytoplasm from outside to inside. While in plant cell cytokinesis take place with the appearance of cell plate.