CH-10 (CELL CYCLE AND CELL DIVISION) BIOLOGY CLASS -XI, CBSE NCERT EXAM NOTES

Chapter-10
Cell Cycle and Cell Division

Points to Remember

Cell Cycle : The sequence of events by which a cell duplicates its genome, synthesies the other constituents of the cell and eventually divides into two daughte cell.

Interphase : (Resting Phase)

• G ₁ Phase : Cell metabolically active and grows continuously but does not replicate DNA.
• S Phase : DNA synthesies occurs, DNA content increase from 2C, to 4C, but the number of chormosomes remains same i.e., 2n.
• G₁ Phase : Proteins are synthesised in preparation for mitosis while cell growth continues.

M Phase (Mitosis Phase) : Starts with nuclaer division, corresponding to separation of daughter chormosomes (karyokinesis) and usually ends with division of cytoplasm, (cytokinesis).

Quiescent stage (G ₀) : In adult animal cells that do not divide and exit G ₁ phase to enter an inactive stage called G ₀. Cells at this stage remain metabolically active but do not proliferate.

e.g., Heart Cells

Mitosis

Since the number of chormosomes in the parent and progeny cells is the same, it is called as equational division. Mitosis is divided into four sub stages.


1. Prophase :
(i) Replicated chormosomes, each consisting of 2 chromatids, condense and become visisble.
(ii) Microtubles are assembled inot mitotic spindle.
(iii) Nucelus and nuclear envelope disappear.
(iv) Centriole moves to opposite poles.


2. Metaphase :
(i) Spindle fibres attached to kinetochores (small disc-shaped structure at the surface of centromere) of chormosomes.
(ii) Chromosomes line up at the equator of the spindle to form mataphase plate.


3. Anaphase :
(i) Centromeres split and chromatids separate.
(ii) Chromatids move to opposite poles due to shortening of spindle fibres.


4. Telophase :
(i) Chromosomes cluster at opposite poles.
(ii) Nuclear envelope assembles around chromosomes clusters.
(iii) Nucelus, Golgi Complex, E.R. reforms.

Cytokinesis : The division of protoplast of a cell into two daughter cells after karyokinesis (nuclear division)

Animal Cytokinesis : Appearance of furrow in plasma membrane which deepens and joins in the center, dividing cell cytoplasm into two.

Plant Cytokinesis : Formation of new wall begins with the formation of a simple precursor - cell plate which represents the middle lamella between the walls of tow adjacent cells.

• When karyokinesis is not followed by cytokinesis, a multinucleated condition arises, This is called Syncytium.

Significance of Mitosis :

1. Growth-addition of cells.


2. Maintain of surface/ volume ratio. Maintain Nucelus-cytoplasmic ratio.


3. Maintenance of chromosomes number.


4. Regeneration.


5. Reproduction in unicellular organisms, lower plants and some insects.


6. Repair and wound healing.


7. Vegetative reproduction in plants takes place by mitosis.

Meiosis :

• Specialised kind of cell division that reduces the chromosomesnumber by half. Hence it is called reductional division.
• Occurs during gametogenesis in plants and animals.
• Involves two sequential cycles of nuclear and cell division called Meiosis I and Meiosis II.
• It results in 4 haploid daughter cells.
• Interphase occurs prior to meiosis which is similar to interphase of mitosis except the S phase is prolonged.

Meiosis I :

Prophase I : Subdivided into 5 phases.

(i) Leptotene :
• Chromosomes make their apperaance as single stranded structures.
• Compaction of chormosomes continues.


(ii) Zygotene :
• Homologous chromosomes start pairing and this process of association is called synapsis.
• Chromosomal synapsis is accompained by formation of Synaptonemal complex.
• Complex formed by a pari of synapsed homologous chormosomes is called bivalent or tetrad.


(iii) Pachytene : Crossing over occurs between non-sister chromatids of homologous chromosomes. The enzymes involved in the process is 'recombinase'. Recombination between homologous chromosomes is completed. Exchange of genetic material.


(iv) Diplotene : Dissolution of synaptonemal complex occurs and the recombined chromosomes separate from each other except at the sites of crossing over. These X-shaped structures are called Chiasmata. In oocytes of some vertebrates diplotene can last for month or years.


(v) Diakinesis : Terminalisation of Chiasmata.
• Chromosomes are fully condensed and meiotic spindles assembled.
• Nucleolus disapper and nuclaer envelope breaks down.


Metaphase I : Bivalent chromosomes align on the equational plate.
• Microtubles from opposite poles of the spindle attach to the pair of homologous chromosomes.
Anaphase I : Homologous chromosomes, separate while chromatids remain associated at their centromeres.

Telophase I :

• Nuclear membrane and nucleus reappear.
• Cytokinesis follows (dyad of cells).

Interkinesis : Stage between two meiotic division, (meiosis I and meiosis II) generally short lived, No replication of DNA during this.

Meiosis II : (It resembles the normal mitosis).

Prophase II :

• Nuclear membrane disappears.
• Chromosomes again become compact.

Metaphase II

• Chromosomes align at the equator.
• Microtubles form opposite poles of spindle get attached to kinetochores of sister chromatids.

Anaphase II

• Simultaneous splitting of the centromere of each chormosomes, allowing them to move towards opposite poles of the cell, by shortening of micortubles attached to kinetochores.

Telophase II

• Two gruop of chromosomes get enclosed by nuclaer envelope.
• Cytokinesis follows resulting in the formation of tetrad of cells i.e., 4 haploid cells.

Significance of Meiosis

1. Formation of gametes : In sexually reproducing organisms.


2. Genetic variability : Variations are very important for evolution.


3. Maintenance of chromosomal number : By reducing the chromosomes number in gametes. Chromosomal number is restored by fertilisation of gametes.