These notes for CBSE Class 11 Biology, Chapter 10, cover the Cell Cycle and Cell Division. The cell cycle is the sequence of events leading to cell duplication and division. It includes Interphase (G1, S, G2 phases) where the cell grows and synthesizes DNA and proteins, and M Phase (Mitosis) for nuclear and cytoplasmic division. A quiescent stage (G0) is also described for non-dividing cells. Mitosis is detailed through its stages: Prophase, Metaphase, Anaphase, and Telophase, followed by cytokinesis in animal and plant cells. The significance of mitosis for growth, repair, and reproduction is highlighted. Meiosis, a specialized division reducing chromosome number by half, is explained through Meiosis I (Prophase I, Metaphase I, Anaphase I, Telophase I) and Meiosis II (Prophase II, Metaphase II, Anaphase II, Telophase II), resulting in four haploid daughter cells. The notes conclude with the significance of meiosis in gamete formation and genetic variability. These notes are ideal for quick revision and understanding key concepts for exams.
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The sequence of events by which a cell duplicates its genome, synthesis the other constitutents of the cell and eventually divides into two daughter cells.
Interphase : •G Phase : Cell metabolically active and grows continuously.
• S Phase : DNA synthesis occurs, DNA content increases from 2C to 4C. 1 but the number of chromosomes remains some (2N). •G Phase : Proteins are synthesised in preparation for mitosis while cell
growth continues. 2 M Phase (Mitosis Phase) : Starts with nuclear division, corresponding to
separation of daughter chromosomes (karyokinesis) and usually ends with division
of cytoplasm (cytokinesis).
Quiescent stage (G 0 ) : Cells that do not divide and exit G
phase to enter an
inactive stage called G 0 1
. Cells at this stage remain metabolically active but
do not proliferate.
MITOSIS
Prophase :
(i) Replicated chromosomes, each consisting of 2 chromatids,
condense and become visible.
(ii) Microtubules are assembled into mitotic spindle.
(iii) Nucleolus and nuclear envelope disappear.
(iv) Centriole moves to opposite poles.
Metaphase :
(i) Spindle fibres attached to kinetochores (small disc-shaped
structures at the surface of centromers) of chromosomes.
(ii) Chromosomes line up at the equator of the spindle to form metaphase
plate.
Anaphase :
(i) Centromeres split and chromatids separate.
(ii) Chromatids move to opposite poles.
Telophase :
(i) Chromosomes cluster at opposite poles.
(ii) Nuclear envelope assembles around chromosome
cluster.
(iii) Nucleolus, golgi complex, ER reform.
Cytokinesis : Is the divison 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 centre dividing cell
cytoplasm into two.
Plant cytokinesis : Formation of new cell wall begins with the
formation of a simple precursor ? cell plate which
represents the middle lamella between the walls
of two adjacent cells.
Significance of Mitosis :
1. Growth ? addition of cells.
2. Maintenance of surface/volume ratio.
3. Maintenance of chromosome number.
4. Regeneration.
5. Reproduction in unicellular organism.
6. Repair and wound healing.
Meiosis :
• Specialised kind of cell division that reduces the chromosome num-
ber by half, resulting in formation of 4 haploid daughter cells.
• Occurs during gametogenesis in plants and animals.
• Involves two sequential cycles of nuclear and cell division called
Meiosis I and Meiosis II.
• Interphase occurs prior to meiosis which is similar to interphase of
mitosis except the S phase is prolonged.
• 4 haploid daughter cells are formed. Meiosis I
Prophase I : Subdivided into 5 phases.
Leptotene :
• Chromosomes make their as single stranded structures.
• Compaction of chromosomes continues.
Zygotene :
• Homologous chromosomes start pairing and this process of associa-
tion is called synapsis.
• Chromosomal synapsis is accompanied by formation of synaptone-
mal complex.
• Complex formed by a pair of synapsed homologous chromosomes is
called bivalent or tetrad.
Pachytene : Crossing over occurs between non-sister chromatids of ho-
mologous chromosomes.
Diplotene : Dissolution of synaptonemal complex occurs and the recom-
bined chromosomes separate from each other except at the sites of crossing
over. These X-shaped structures are called chaismata.
Diakinesis :
• Terminalisation of chaismata.
• Chromosomes are fully condensed and meiotic spindles assembled.
• Nucleolus disappear and nuclear envelope breaks down.
Metaphase I :
• Bivalent chromosomes align on the equatorial plate.
• Microtubules from opposite poles of the spindle attach to the pair of
homologous chromosomes.
Anaphase I : Homologous chromosomes separate while chromatids re-
main associated at their centromeres.
Telophase I :
• Nuclear membrane and nucleolus reappear.
• Cytokinesis follows (diad of cells).
Interkinesis : Stage between two meiotic divisions. (meiosis I and
meiosis II) Meiosis II
Prophase II
• Nuclear membrane disappears.
• Chromosomes become compact.
Metaphase II
• Chromosomes align at the equator.
• Microtubules from opposite poles of spindle get attached to kineto-
chores of sister chromatids.
Anaphase II
• Simultaneous splitting of the centromere of each chromosome, al-
lowing them to move towards opposite poles of the cell.
Telophase II
• Two groups of chromosomes get enclosed by a nuclear 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
3. Maintenance of chromosomal number : By reducing the chromosome
number in gametes. Chromosomal number is restored by fertilisation of gametes.
The cell cycle is the sequence of events by which a cell duplicates its genome, synthesizes other constituents, and eventually divides into two daughter cells.
The main phases are Interphase (G1, S, G2) and M Phase (Mitosis).
During the S phase, DNA synthesis occurs, increasing the DNA content from 2C to 4C, while the chromosome number remains the same.
Mitosis is a type of cell division that starts with nuclear division (karyokinesis) and usually ends with cytoplasmic division (cytokinesis), resulting in two daughter cells genetically identical to the parent cell.
Mitosis is significant for growth, maintenance of surface/volume ratio, repair, wound healing, and reproduction in unicellular organisms.
Meiosis is a specialized cell division that reduces the chromosome number by half, forming four haploid daughter cells. It occurs during gametogenesis in plants and animals.
In Prophase I of Meiosis, crossing over occurs between non-sister chromatids of homologous chromosomes, leading to genetic variability.
In animal cells, cytokinesis occurs by the formation of a furrow in the plasma membrane. In plant cells, a cell plate forms, which develops into a new cell wall.
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