Lecture Notes: Cell Cycle and Cell Division

Introduction

• Welcome to Physics Voila.
• Today’s topic: Cell Cycle and Cell Division (Class 11).
• Inspirational Thought: "A winner is a dreamer who never gives up."
  • Success requires hard work alongside dreaming.
  • Importance of staying focused and dedicated towards goals.

Overview of Cell Cycle

• Cell Cycle: Sequence of events leading to cell division.
• Cells arise from pre-existing cells by division (Rudolf Virchow).
• Division is a controlled process, not random.

Definition of Cell Cycle

• Sequence of events:
  • Growth of cell
  • DNA replication
  • Organelle duplication
  • Energy synthesis
• Controlled by genetics; genes dictate when and where cell division occurs.

Phases of Cell Cycle

1. Interphase (longer phase where preparation for division occurs)
   • Cell growth in size (cytoplasm increases).
   • DNA replication occurs at a specific stage.
2. M Phase (Mitotic Phase) (actual division occurs)
   • Dramatic phase; takes about 5% of total time.
   • Divided into two steps: Karyokinesis (nuclear division) and Cytokinesis (cytoplasmic division).

Duration of Cell Cycle

• Example: Human cells take approximately 24 hours to complete the cycle.

Interphase Stages

1. G1 Phase (Gap 1)
   • Cell increases in size; organelle duplication begins.
   • Characterized by protein synthesis.
2. S Phase (Synthesis)
   • DNA replication occurs, resulting in doubled DNA content (4C).
   • Centrosome duplication and histone protein synthesis.
3. G2 Phase (Gap 2)
   • Further cell growth and organelle duplication; protein synthesis continues.
   • DNA amount is still 4C; chromosomes are not yet visible.

G0 Phase (Quiescent Phase)

• Cells that do not want to proliferate enter G0 phase.
• Can remain temporarily or permanently inactive.

M Phase (Mitotic Phase)

• Marked by nuclear division and cytoplasmic division.
• Karyokinesis stages:
  • Prophase: Chromatin condenses into visible chromosomes; nuclear membrane disassembles; spindle fibers form.
  • Metaphase: Chromosomes align at the metaphase plate; best stage for observing chromosomes.
  • Anaphase: Centromeres split, chromatids move toward opposite poles.
  • Telophase: Nuclear membranes reform; chromosomes de-condense.
• Cytokinesis: Division of cytoplasm.
  • Animal Cells: Plasma membrane constricts (centripetal movement).
  • Plant Cells: Cell plate forms (centrifugal movement).

Meiosis

• Meiosis: Reductional division producing four haploid daughter cells.
• Meiosis I: Reductional phase - results in two haploid cells.
• Meiosis II: Equational phase - similar to mitosis.
• Stages of Meiosis I include Prophase I, Metaphase I, Anaphase I, and Telophase I.
• Prophase I further divided into five stages (Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis).
• Key components:
  • Crossing over occurs during Pachytene, introducing genetic variation.
  • Homologous chromosomes pair up forming bivalents.

Significance of Mitosis and Meiosis

• Mitosis: Necessary for cell growth, repair, and regeneration.
  • Produces identical cells for healing and growth.
• Meiosis: Essential for sexual reproduction, introduces genetic variation.
  • Forms gametes (sperm and egg cells).

Conclusion

• Review key concepts through NCRT and solve relevant questions.
• Upcoming marathon session on July 8th for revision of Class 11 & 12 syllabus.
• Questions on the chapter will be discussed.
• Importance of understanding the processes for NEET examination preparation.