Cell Cycle and Cell Division Revision Notes, Key Concepts & Important Questions
Cell Cycle and Cell Division is an important chapter in Class 11 Biology that explains how cells grow, replicate their genetic material, and divide to form new cells. The chapter covers the stages of the cell cycle, including interphase (G₁, S, and G₂ phases) and the M phase. It also explains the processes of mitosis and meiosis, their stages, and their significance in growth, repair, and reproduction. In addition, it discusses key events such as DNA replication, chromosome behavior, and cytokinesis.
These revision notes on Cell Cycle and Cell Division, prepared by ALLEN’s expert faculty, provide a clear overview of the chapter’s key concepts, stages, and important mechanisms involved in cell division. Designed for quick and effective revision, they help students strengthen conceptual understanding, remember important processes and stages, and prepare efficiently for board and competitive exams.
1.0Concept-Wise Important Revision Notes
1. What is cell division and why is it important?
Growth and reproduction are fundamental characteristics of cells and all living organisms. Cells reproduce by dividing into two, where each parental cell forms two daughter cells.
These daughter cells can grow and divide repeatedly, forming a large population of cells from a single parental cell. Thus, cycles of growth and division allow a single cell to develop into structures containing millions of cells.
The term mitosis was proposed by Flemming. Mitosis produces genetically identical daughter cells that are similar to the mother cell.
2. What are the causes of mitosis?
Two main explanations are given for the occurrence of mitosis.
a. Kern-Plasm Theory (Hertwig)
According to this theory, mitosis occurs due to disturbance in the karyoplasmic index (KI) or nucleocytoplasmic ratio.
Karyoplasmic Index:
KI = Vn / (Vc − Vn)
Where
- Vn = Volume of nucleus
- Vc = Volume of cell
- Vc − Vn = Volume of cytoplasm
Key idea:
- Small cells have high KI and the nucleus can efficiently control cytoplasmic activities.
- In large cells, the nucleus cannot effectively control the cytoplasm.
- The cell divides to restore the nucleocytoplasmic balance.
b. Surface-Volume Ratio
As a cell grows:
- Volume increases faster than surface area
- The surface becomes insufficient to absorb materials from the surroundings.
To increase the surface-volume ratio, the cell divides.
3. What is the cell cycle?
The cell cycle is the sequence of events through which a cell passes from one division to the next.
It includes:
- Cell growth
- DNA replication
- Cell division
These processes occur in a coordinated manner to ensure correct division and formation of daughter cells containing complete genomes.
Although cytoplasmic growth is continuous, DNA synthesis occurs only during one specific stage of the cycle.
Also Read: Cell Cycle and Cell Division
4. What are the phases of the cell cycle?
A typical eukaryotic cell divides once in about 24 hours, while yeast may complete the cycle in 90 minutes.
The cell cycle has two major phases.
Interphase
Interphase occurs between two successive M phases and consists of three stages:
- G1 phase (Gap 1) – cell growth
- S phase (Synthesis) – DNA replication
- G2 phase – preparation for cell division
M Phase (Mitotic Phase)
The M phase lasts about one hour in the human cell cycle.
Key events:
- Karyokinesis – nuclear division
- Cytokinesis – cytoplasmic division
Because chromosome number remains unchanged in daughter cells, mitosis is called equational division.
5. What is mitosis and what are its stages?
Mitosis is an equational division in which daughter cells receive the same chromosome number as the parent cell.
It occurs mainly in diploid somatic cells in animals, while in plants it may occur in both haploid and diploid cells.
Mitosis consists of four stages of nuclear division (karyokinesis):
- Prophase
- Metaphase
- Anaphase
- Telophase
Cytokinesis follows nuclear division.
6. Explain the stages of mitosis.
Prophase
- Chromatin condenses to form visible chromosomes
- Each chromosome has two chromatids joined at the centromere
- Centrosomes move to opposite poles
- Spindle fibres and asters form the mitotic apparatus
- Golgi complex, ER, nucleolus and nuclear envelope disappear
Metaphase
- Nuclear envelope completely disappears
- Chromosomes become fully condensed
- Kinetochores form at centromeres
- Spindle fibres attach to kinetochores
- Chromosomes align at the equatorial plane (metaphase plate)
This stage is ideal for studying chromosome morphology.
Anaphase
- Centromeres split
- Sister chromatids separate
- Chromatids become daughter chromosomes
- Chromosomes move toward opposite poles
- Chromosome number temporarily appears doubled in the cell
Telophase
- Chromosomes reach opposite poles
- Chromosomes decondense into chromatin
- Nuclear envelope reforms
- Nucleolus, Golgi complex and ER reappear
- Two daughter nuclei are formed
Telophase is often called reverse prophase.
7. What is cytokinesis and how does it occur in animal and plant cells?
Cytokinesis is the division of cytoplasm following nuclear division.
Cytokinesis in Animal Cells
Steps:
- A contractile ring of actin and myosin forms at the equator
- Plasma membrane forms a cleavage furrow
- The furrow deepens inward
- Cytoplasm divides into two daughter cells
This occurs in centripetal order (outside to inside).
Cytokinesis in Plant Cells
Plant cells have a rigid cell wall, so cytokinesis occurs differently.
Steps:
- Golgi vesicles accumulate at the equator
- Vesicles fuse to form a cell plate
- Vesicles secrete calcium and magnesium pectate
- Cell plate develops into the middle lamella
- New cell wall forms from centre toward periphery
This occurs in centrifugal order.
Also Read: Plant cell and animal cell
8. What is a special case in mitosis?
In some organisms, karyokinesis occurs without cytokinesis, producing a multinucleate cell called syncytium.
Example: Liquid endosperm in coconut.
9. What is the significance of mitosis?
Mitosis plays several important roles:
- Produces genetically identical diploid daughter cells
- Responsible for growth of multicellular organisms
- Restores the nucleocytoplasmic ratio
- Helps in cell repair and replacement
- Enables continuous plant growth through meristematic tissues
- Development of an organism from a single zygote occurs through repeated mitosis.
2.0MEIOSIS
10. What is meiosis and what are its key features?
Meiosis is a special type of cell division that reduces the chromosome number by half and produces haploid daughter cells.
Key features:
- Occurs during gametogenesis
- Involves two successive divisions: Meiosis I and Meiosis II
- Only one cycle of DNA replication
- Produces four haploid daughter cells
- Daughter cells are genetically different
Meiosis maintains the haploid phase, while fertilisation restores the diploid phase.
11. Explain the stages of Meiosis I.
Meiosis I is called reduction division because chromosome number is reduced.
Prophase I
- Longer and more complex than mitotic prophase
- Homologous chromosomes pair
- Recombination occurs between non-sister chromatids
Metaphase I
- Bivalents align at the equator
- Spindle fibres attach to homologous chromosomes.
Anaphase I
- Homologous chromosomes separate
- Sister chromatids remain attached.
Telophase I
- Nuclear membrane and nucleolus reappear
- Cytokinesis produces two haploid cells (dyad).
12. What is interkinesis?
Interkinesis is the short interval between Meiosis I and Meiosis II.
Characteristics:
- It is similar to interphase
- Here, the DNA replication does not occur
- This phase prepares the cell for Meiosis II.
13. Explain the stages of Meiosis II.
Meiosis II resembles mitosis and is called equational division.
Prophase II
- Chromosomes condense again.
Metaphase II
- Chromosomes align at the equator
- Spindle fibres attach to kinetochores.
Anaphase II
- Centromeres split
- Sister chromatids separate and move to opposite poles.
Telophase II
- Nuclear envelope forms around chromosomes
- Cytokinesis produces four haploid daughter cells (tetrad).
14. What is the significance of meiosis?
Meiosis has two major roles:
- Maintains the chromosome number of a species across generations.
- Produces genetic variation, which is important for evolution.
3.0Memory Tricks
Stages of Mitosis : “PMAT = Please Make A Table”
P → Prophase
M → Metaphase
A → Anaphase
T → Telophase
Cell Cycle Phases: Trick: “Grow – Synthesize – Get Ready – Multiply”
G1 → Grow (cell increases in size)
S → Synthesize DNA
G2 → Get ready for division
M → Multiply (mitosis occurs)
4.0Previous Year Questions on Cell Cycle and Cell Division
Question: Which of the following stages of meiosis involves division of centromere?
- Metaphase I
- Metaphase II
- Anaphase II
- Telophase II
Answer: Anaphase II
Question: The process of appearance of recombination nodules occurs during which substage of prophase I?
- Pachytene
- Diplotene
- Diakinesis
- Zygotene
Answer: Pachytene
Question: Among eukaryotes, replication of DNA takes place in:
- S phase
- G₁ phase
- G₂ phase
- M phase
Answer: S phase