📝 Chapter Notes & Revision

Cell Cycle and Cell Division

🏫 MP BoardClass 11Biology

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Class 11 Biology - Cell Cycle and Cell Division

MP Board Education Board

### Introduction to Cell Cycle

  • Cell Cycle (कोशिका चक्र): The series of events by which a cell grows, duplicates its genetic material (DNA), and divides into two daughter cells.
  • Duration: Varies from organism to organism and cell type to cell type (e.g., human cell cycle takes approximately 24 hours).

### Phases of the Cell Cycle

The cell cycle is broadly divided into two basic phases:

  1. Interphase (अंतర अवस्‍था): The resting/growth phase where the cell prepares for division. It occupies 95% of the total duration of the cell cycle.
  2. M Phase (Mitosis/Meiosis Phase): The actual period of division.

1. Interphase Sub-phases:

  • $G_1$ Phase (Gap 1 / Pre-DNA synthesis phase):
    • Cell grows continuously.
    • Metabolic activity is high.
    • RNA and proteins are synthesized.
  • S Phase (Synthesis phase):
    • DNA replication takes place.
    • Amount of DNA per cell doubles (if initial is $2C$, it becomes $4C$), but the chromosome number ($n$) remains the same.
    • Centriole duplicates in the cytoplasm (in animal cells).
  • $G_2$ Phase (Gap 2 / Post-DNA synthesis phase):
    • Proteins are synthesized in preparation for mitosis while cell growth continues.
    • Tubulin protein synthesis for spindle fibers occurs.
  • $G_0$ Phase (Quiescent stage / शांत अवस्‍था):
    • Cells that do not divide further exit $G_1$ phase and enter an inactive stage called $G_0$.
    • Cells remain metabolically active but do not proliferate unless called upon.

### M Phase (Mitosis - समसूत्री विभाजन)

Mitosis is equational division. It is divided into four stages of karyokinesis (nuclear division), followed by cytokinesis.

  1. Prophase (पूर्वावस्‍था):

    • Chromatin condenses into visible, compact chromosomes.
    • Nucleolus and nuclear membrane disappear.
    • Mitotic apparatus (spindle fibers) begins to form.

  2. Metaphase (मध्यावस्‍था):

    • Nuclear envelope completely disintegrates.
    • Chromosomes lie at the equator, forming the Metaphase plate.
    • Spindle fibers attach to the kinetochores of chromosomes.
    • Best stage to study the morphology of chromosomes.

  3. Anaphase (पश्चावस्‍था):

    • Centromere splits, and sister chromatids separate.
    • Chromatids move toward opposite poles (pulled by shortening of spindle fibers).
    • Shape of chromosomes appears as V, L, J, or I depending on centromere position.

  4. Telophase (अंत्यावस्‍था):

    • Chromosomes reach opposite poles and uncoil into chromatin network.
    • Nuclear envelope and nucleolus reappear.
    • Spindle fibers disappear.

### Cytokinesis (कोशिकाद्रव विभाजन)

Division of cytoplasm resulting in two daughter cells.

  • In Animal Cells: Brought about by the appearance of a furrow in the plasma membrane (cleavage method).
  • In Plant Cells: Brought about by cell plate formation (phragmoplast method) because of the rigid cell wall.

### Meiosis (अर्धसूत्री विभाजन)

Reductional division that occurs in germ cells to produce gametes. It maintains chromosome number constant across generations and generates genetic variations (crossing over).

Meiosis I (Reductional Division)

  • Prophase I: Longest and most complex phase, divided into 5 sub-stages:
    1. Leptotene: Chromosomes start condensing.
    2. Zygotene: Pairing of homologous chromosomes occurs (Synapsis). Formation of synaptonemal complex (bivalent/tetrad formed).
    3. Pachytene: Crossing over (exchange of genetic material between non-sister chromatids of homologous chromosomes) takes place, mediated by the enzyme recombinase.
    4. Diplotene: Dissolution of synaptonemal complex; X-shaped structures called Chiasmata appear.
    5. Diakinesis: Terminalization of chiasmata, nucleolus disappears, nuclear envelope breaks down.
  • Metaphase I: Bivalent chromosomes align on the equatorial plate.
  • Anaphase I: Homologous chromosomes separate, while sister chromatids remain attached at their centromeres.
  • Telophase I: Nuclear membrane and nucleolus reappear (may be skipped in some organisms).

Meiosis II (Equational Division)

  • Resembles mitosis.
  • Prophase II -> Metaphase II -> Anaphase II -> Telophase II
  • Centromere splits in Anaphase II, separating sister chromatids.
  • Result: Four haploid ($n$) daughter cells are formed from a single diploid ($2n$) parent cell.

### Key Differences at a Glance

FeatureMitosis (समसूत्री)Meiosis (अर्धसूत्री)
Occurs inSomatic cells (दैहिक कोशिकाएं)Germ cells (जनन कोशिकाएं)
PloidyDiploid ($2n$) $\rightarrow$ Diploid ($2n$)Diploid ($2n$) $\rightarrow$ Haploid ($n$)
Crossing OverAbsentPresent (in Pachytene I)
No. of DivisionsOne division (2 cells formed)Two successive divisions (4 cells formed)
Genetic VariationNo (Clones)Yes (Variations produced)

### Important Terms & Definitions

  • Bivalent (द्विसंयोजक): A pair of homologous chromosomes synapsed during the meiosis I.
  • Chiasmata (काइऐज्मेटा): X-shaped points of attachment between non-sister chromatids where crossing over occurred.
  • Kinetochore: Disc-shaped protein structures on centromere where spindle fibers attach.
  • Synapsis: The pairing of homologous chromosomes during zygotene.