Quick Revision Notes & Formula Sheet

Class 11 Biology - Cell Cycle and Cell Division

MP Board Education Board

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### 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).

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### 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.

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### 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.

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### 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.

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### 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.

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### Key Differences at a Glance

Feature	Mitosis (समसूत्री)	Meiosis (अर्धसूत्री)
Occurs in	Somatic cells (दैहिक कोशिकाएं)	Germ cells (जनन कोशिकाएं)
Ploidy	Diploid ($2n$) $\rightarrow$ Diploid ($2n$)	Diploid ($2n$) $\rightarrow$ Haploid ($n$)
Crossing Over	Absent	Present (in Pachytene I)
No. of Divisions	One division (2 cells formed)	Two successive divisions (4 cells formed)
Genetic Variation	No (Clones)	Yes (Variations produced)

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### 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.