𝗜𝗻𝘁𝗿𝗼𝗱𝘂𝗰𝘁𝗶𝗼𝗻
𝗚𝗿𝗼𝘄𝘁𝗵 is an irreversible increase in the size or mass of a living system. It occurs primarily in living organisms though non-living systems can show a chemical form of growth too and depends entirely on the activity of cells. Since every cell arises from a pre-existing mother cell through division, understanding the 𝗰𝗲𝗹𝗹 𝗰𝘆𝗰𝗹𝗲 is fundamental to understanding life itself.
A cell born after division doesn't divide immediately. It first grows, synthesizes the molecules it needs, and only then prepares for the next round of division.
𝗪𝗵𝗮𝘁 𝗜𝘀 𝘁𝗵𝗲 𝗖𝗲𝗹𝗹 𝗖𝘆𝗰𝗹𝗲?
- The 𝗰𝗲𝗹𝗹 𝗰𝘆𝗰𝗹𝗲 is the entire sequence of events by which a cell divides and prepares itself for the next generation.
- In simpler terms, it's the series of steps a cell follows to divide and give rise to future generations of cells. "Cell cycle terminology and phases were studied extensively by Howard and Pelc (1953)."and is broadly divided into 𝘁𝘄𝗼 𝗺𝗮𝗷𝗼𝗿 𝗽𝗵𝗮𝘀𝗲𝘀:
𝗜𝗻𝘁𝗲𝗿𝗽𝗵𝗮𝘀𝗲
𝗠-𝗣𝗵𝗮𝘀𝗲 (𝗠𝘂𝗹𝘁𝗶𝗽𝗹𝗶𝗰𝗮𝘁𝗶𝗼𝗻 𝗣𝗵𝗮𝘀𝗲)
𝗘𝘅𝗮𝗺 𝗧𝗶𝗽: Many students confuse "M Phase" with "Mitosis" alone remember M-Phase is the 𝘂𝗺𝗯𝗿𝗲𝗹𝗹𝗮 𝘁𝗲𝗿𝗺 covering both mitosis and meiosis.
𝗜𝗻𝘁𝗲𝗿𝗽𝗵𝗮𝘀𝗲: 𝗧𝗵𝗲 𝗣𝗿𝗲𝗽𝗮𝗿𝗮𝘁𝗼𝗿𝘆 𝗣𝗵𝗮𝘀𝗲
Interphase is subdivided into 𝘁𝗵𝗿𝗲𝗲 𝘀𝗲𝗾𝘂𝗲𝗻𝘁𝗶𝗮𝗹 𝗽𝗵𝗮𝘀𝗲𝘀, each with a distinct role in preparing the cell for division.
1. 𝗚₁ 𝗣𝗵𝗮𝘀𝗲 (𝗙𝗶𝗿𝘀𝘁 𝗚𝗮𝗽 / 𝗙𝗶𝗿𝘀𝘁 𝗚𝗿𝗼𝘄𝘁𝗵 𝗣𝗵𝗮𝘀𝗲)
- Occurs immediately after the M-phase, when daughter cells are formed.
- Also called the 𝗳𝗶𝗿𝘀𝘁 𝗿𝗲𝘀𝘁𝗶𝗻𝗴 𝗽𝗵𝗮𝘀𝗲.
- Involves 𝗥𝗡𝗔 𝘀𝘆𝗻𝘁𝗵𝗲𝘀𝗶𝘀 and growth of the 𝗻𝘂𝗰𝗹𝗲𝘂𝘀 𝗮𝗻𝗱 𝗰𝘆𝘁𝗼𝗽𝗹𝗮𝘀𝗺.
- No DNA replication occurs here.
2. 𝗦 𝗣𝗵𝗮𝘀𝗲 (𝗦𝘆𝗻𝘁𝗵𝗲𝘁𝗶𝗰 𝗣𝗵𝗮𝘀𝗲)
- The 𝗹𝗼𝗻𝗴𝗲𝘀𝘁 𝗽𝗵𝗮𝘀𝗲 of the entire cell cycle, occupying 𝟯𝟱–𝟰𝟱% of its duration.
- Key events include:
- 𝗥𝗲𝗽𝗹𝗶𝗰𝗮𝘁𝗶𝗼𝗻 𝗼𝗳 𝗗𝗡𝗔
- 𝗦𝘆𝗻𝘁𝗵𝗲𝘀𝗶𝘀 𝗼𝗳 𝗵𝗶𝘀𝘁𝗼𝗻𝗲 𝗽𝗿𝗼𝘁𝗲𝗶𝗻𝘀
- 𝗦𝘆𝗻𝘁𝗵𝗲𝘀𝗶𝘀 𝗼𝗳 𝗲𝗻𝘇𝘆𝗺𝗲𝘀 required for division
- This is the phase where the genetic material is literally doubled.
3. 𝗚₂ 𝗣𝗵𝗮𝘀𝗲 (𝗦𝗲𝗰𝗼𝗻𝗱 𝗚𝗮𝗽 𝗣𝗵𝗮𝘀𝗲)
- Also known as the 𝘀𝗲𝗰𝗼𝗻𝗱 𝗿𝗲𝘀𝘁𝗶𝗻𝗴 𝘀𝘁𝗮𝗴𝗲.
- Additional 𝗽𝗿𝗼𝘁𝗲𝗶𝗻 𝘀𝘆𝗻𝘁𝗵𝗲𝘀𝗶𝘀 occurs.
- The cell completes final preparations and readies itself to enter the M-phase.
𝗠-𝗣𝗵𝗮𝘀𝗲 (𝗠𝘂𝗹𝘁𝗶𝗽𝗹𝗶𝗰𝗮𝘁𝗶𝗼𝗻 𝗣𝗵𝗮𝘀𝗲): 𝗧𝘆𝗽𝗲𝘀 𝗼𝗳 𝗖𝗲𝗹𝗹 𝗗𝗶𝘃𝗶𝘀𝗶𝗼𝗻
The 𝗠-𝗽𝗵𝗮𝘀𝗲 is the actual division phase where the cell splits and completes the cell cycle. It occurs in two forms:
- 𝗠𝗶𝘁𝗼𝘀𝗶𝘀 — Equational division
- 𝗠𝗲𝗶𝗼𝘀𝗶𝘀 — Reductional division
𝗠𝗶𝘁𝗼𝘀𝗶𝘀: 𝗘𝗾𝘂𝗮𝘁𝗶𝗼𝗻𝗮𝗹 𝗖𝗲𝗹𝗹 𝗗𝗶𝘃𝗶𝘀𝗶𝗼𝗻
𝗠𝗶𝘁𝗼𝘀𝗶𝘀 is a type of cell division in which the 𝗰𝗵𝗿𝗼𝗺𝗼𝘀𝗼𝗺𝗲 𝗻𝘂𝗺𝗯𝗲𝗿 𝗶𝗻 𝗱𝗮𝘂𝗴𝗵𝘁𝗲𝗿 𝗰𝗲𝗹𝗹𝘀 𝗿𝗲𝗺𝗮𝗶𝗻𝘀 𝗲𝗾𝘂𝗮𝗹 to that of the mother cell. Key facts:
- Occurs 𝗼𝗻𝗹𝘆 𝗶𝗻 𝘀𝗼𝗺𝗮𝘁𝗶𝗰 𝗰𝗲𝗹𝗹𝘀
- Takes place in 𝗯𝗼𝘁𝗵 𝗽𝗹𝗮𝗻𝘁 𝗮𝗻𝗱 𝗮𝗻𝗶𝗺𝗮𝗹 𝗰𝗲𝗹𝗹𝘀
- 𝗗𝗶𝘀𝗰𝗼𝘃𝗲𝗿𝗲𝗱 𝗯𝘆 𝗪𝗮𝗹𝘁𝗲𝗿 𝗙𝗹𝗲𝗺𝗺𝗶𝗻𝗴
Mitosis is completed in 𝘁𝘄𝗼 𝘀𝘁𝗲𝗽𝘀: 𝗞𝗮𝗿𝘆𝗼𝗸𝗶𝗻𝗲𝘀𝗶𝘀 (nuclear division) and 𝗖𝘆𝘁𝗼𝗸𝗶𝗻𝗲𝘀𝗶𝘀 (cytoplasmic division).
𝗞𝗮𝗿𝘆𝗼𝗸𝗶𝗻𝗲𝘀𝗶𝘀: 𝗧𝗵𝗲 𝗙𝗼𝘂𝗿 𝗣𝗵𝗮𝘀𝗲𝘀 𝗼𝗳 𝗡𝘂𝗰𝗹𝗲𝗮𝗿 𝗗𝗶𝘃𝗶𝘀𝗶𝗼𝗻
1. 𝗣𝗿𝗼𝗽𝗵𝗮𝘀𝗲 (𝗟𝗼𝗻𝗴𝗲𝘀𝘁 𝗣𝗵𝗮𝘀𝗲 𝗼𝗳 𝗠𝗶𝘁𝗼𝘀𝗶𝘀)
Prophase unfolds in three stages:
- 𝗘𝗮𝗿𝗹𝘆 𝗣𝗿𝗼𝗽𝗵𝗮𝘀𝗲: Chromosomes coil into thin, thread-like structures; nuclear membrane and nucleolus are still intact.
- 𝗠𝗶𝗱 𝗣𝗿𝗼𝗽𝗵𝗮𝘀𝗲: Chromosomes uncoil slightly and become more visible; the nuclear membrane and nucleolus begin disappearing; centrioles (in animal cells) divide and move toward opposite poles.
- 𝗟𝗮𝘁𝗲 𝗣𝗿𝗼𝗽𝗵𝗮𝘀𝗲: Chromosomes develop clear arms attached to centromeres; the nuclear membrane and nucleolus disappear completely; centrioles reach the poles; chromosomes enter the cytoplasm.
2. 𝗠𝗲𝘁𝗮𝗽𝗵𝗮𝘀𝗲 (𝗕𝗲𝘀𝘁 𝗣𝗵𝗮𝘀𝗲 𝘁𝗼 𝗦𝘁𝘂𝗱𝘆 𝗖𝗵𝗿𝗼𝗺𝗼𝘀𝗼𝗺𝗲𝘀)
- Chromosomes become sharply visible and align on the 𝗲𝗾𝘂𝗮𝘁𝗼𝗿𝗶𝗮𝗹 𝗽𝗹𝗮𝘁𝗲.
- Larger chromosomes sit at the periphery; smaller ones cluster at the center.
- Centrioles form 𝗮𝘀𝘁𝗿𝗮𝗹 𝗿𝗮𝘆𝘀 and 𝘀𝗽𝗶𝗻𝗱𝗹𝗲 𝗳𝗶𝗯𝗲𝗿𝘀, of two types:
𝗗𝗶𝘀𝗰𝗼𝗻𝘁𝗶𝗻𝘂𝗼𝘂𝘀 (𝗰𝗵𝗿𝗼𝗺𝗼𝘀𝗼𝗺𝗮𝗹) 𝘀𝗽𝗶𝗻𝗱𝗹𝗲 𝗳𝗶𝗯𝗲𝗿𝘀 — extend from pole to centromere, driving the pushing and pulling forces
3. 𝗔𝗻𝗮𝗽𝗵𝗮𝘀𝗲 (𝗦𝗺𝗮𝗹𝗹𝗲𝘀𝘁 𝗣𝗵𝗮𝘀𝗲 𝗼𝗳 𝗠𝗶𝘁𝗼𝘀𝗶𝘀)
- Spindle fibers break, splitting chromosomes at the 𝗰𝗲𝗻𝘁𝗿𝗼𝗺𝗲𝗿𝗲.
- Centromeres face the poles; arms face the center.
- Chromosomes migrate to opposite poles.
4. 𝗧𝗲𝗹𝗼𝗽𝗵𝗮𝘀𝗲 (𝗥𝗲𝗴𝗲𝗻𝗲𝗿𝗮𝘁𝗶𝗼𝗻 𝗣𝗵𝗮𝘀𝗲)
- Chromosomes reach the opposite poles.
- The nuclear membrane and nucleolus 𝗿𝗲𝗮𝗽𝗽𝗲𝗮𝗿, marking the end of mitosis.
𝗖𝘆𝘁𝗼𝗸𝗶𝗻𝗲𝘀𝗶𝘀: 𝗗𝗶𝘃𝗶𝘀𝗶𝗼𝗻 𝗼𝗳 𝘁𝗵𝗲 𝗖𝘆𝘁𝗼𝗽𝗹𝗮𝘀𝗺
- 𝗔𝗻𝗶𝗺𝗮𝗹 𝗰𝗲𝗹𝗹𝘀: occurs via 𝗰𝗲𝗹𝗹 𝗰𝗼𝗻𝘀𝘁𝗿𝗶𝗰𝘁𝗶𝗼𝗻 (cleavage furrow).
- 𝗣𝗹𝗮𝗻𝘁 𝗰𝗲𝗹𝗹𝘀: occurs via 𝗰𝗲𝗹𝗹 𝗽𝗹𝗮𝘁𝗲 𝗳𝗼𝗿𝗺𝗮𝘁𝗶𝗼𝗻.
𝗠𝗲𝗶𝗼𝘀𝗶𝘀: 𝗥𝗲𝗱𝘂𝗰𝘁𝗶𝗼𝗻𝗮𝗹 𝗖𝗲𝗹𝗹 𝗗𝗶𝘃𝗶𝘀𝗶𝗼𝗻
𝗠𝗲𝗶𝗼𝘀𝗶𝘀, 𝗱𝗶𝘀𝗰𝗼𝘃𝗲𝗿𝗲𝗱 𝗯𝘆 𝗙𝗮𝗿𝗺𝗲𝗿 𝗮𝗻𝗱 𝗠𝗼𝗼𝗿𝗲, is a reductional division occurring 𝗲𝘅𝗰𝗹𝘂𝘀𝗶𝘃𝗲𝗹𝘆 𝗶𝗻 𝗴𝗮𝗺𝗲𝘁𝗶𝗰 𝗰𝗲𝗹𝗹𝘀. One mother cell produces 𝗳𝗼𝘂𝗿 𝗱𝗮𝘂𝗴𝗵𝘁𝗲𝗿 𝗰𝗲𝗹𝗹𝘀, each with 𝗵𝗮𝗹𝗳 𝘁𝗵𝗲 𝗰𝗵𝗿𝗼𝗺𝗼𝘀𝗼𝗺𝗲 𝗻𝘂𝗺𝗯𝗲𝗿. It is completed in two stages: 𝗠𝗲𝗶𝗼𝘀𝗶𝘀-𝗜 and 𝗠𝗲𝗶𝗼𝘀𝗶𝘀-𝗜𝗜.
𝗠𝗲𝗶𝗼𝘀𝗶𝘀-𝗜
Meiosis-I includes Prophase-I, Metaphase-I, Anaphase-I, and Telophase-I.
𝗣𝗿𝗼𝗽𝗵𝗮𝘀𝗲-𝗜: 𝗧𝗵𝗲 𝗟𝗼𝗻𝗴𝗲𝘀𝘁 𝗣𝗵𝗮𝘀𝗲 𝗼𝗳 𝗠𝗲𝗶𝗼𝘀𝗶𝘀 (𝗙𝗶𝘃𝗲 𝗦𝘂𝗯-𝗦𝘁𝗮𝗴𝗲𝘀)
1. 𝗟𝗲𝗽𝘁𝗼𝘁𝗲𝗻𝗲:
- Chromosomes appear as long, thin, bead-like monovalent threads (both paternal and maternal).
- Attraction forces are low.
- Nuclear membrane and nucleolus remain intact.
2. 𝗭𝘆𝗴𝗼𝘁𝗲𝗻𝗲:
- Chromosomes become short and thick.
- Increased attraction causes homologous paternal.
- And maternal chromosomes to pair via 𝘀𝘆𝗻𝗮𝗽𝘀𝗶𝘀, stabilized by the 𝘀𝘆𝗻𝗮𝗽𝘁𝗼𝗻𝗲𝗺𝗮𝗹 𝗰𝗼𝗺𝗽𝗹𝗲𝘅 (a DNA-protein structure containing ubiquitin protein).
3. 𝗣𝗮𝗰𝗵𝘆𝘁𝗲𝗻𝗲:
- Chromosomes appear 𝘁𝗲𝘁𝗿𝗮𝘃𝗮𝗹𝗲𝗻𝘁 due to strong attraction, bringing sister and non-sister chromatids together for 𝗰𝗿𝗼𝘀𝘀𝗶𝗻𝗴 𝗼𝘃𝗲𝗿.
- The exchange of segments between non-sister chromatids at points called 𝗰𝗵𝗶𝗮𝘀𝗺𝗮𝘁𝗮.
4. 𝗗𝗶𝗽𝗹𝗼𝘁𝗲𝗻𝗲:
- Attraction forces weaken; the synaptonemal complex disappears.
- Crossing over becomes clearly visible; 𝘁𝗲𝗿𝗺𝗶𝗻𝗮𝗹𝗶𝘇𝗮𝘁𝗶𝗼𝗻 begins.
5. 𝗗𝗶𝗮𝗸𝗶𝗻𝗲𝘀𝗶𝘀:
- Attraction forces are minimal; chromosomes appear clear.
- The nuclear membrane and nucleolus disappear.
𝗠𝗲𝘁𝗮𝗽𝗵𝗮𝘀𝗲-𝗜
- Homologous chromosome pairs align on the equatorial plate.
- Spindle fibers (continuous and discontinuous) generate pushing/pulling forces that reduce attraction between homologs.
𝗔𝗻𝗮𝗽𝗵𝗮𝘀𝗲-𝗜
- Chromosomes separate at the central point.
- Centromeres face the center; chromatids face the poles as chromosomes migrate outward.
𝗧𝗲𝗹𝗼𝗽𝗵𝗮𝘀𝗲-𝗜
- Chromosomes reach opposite poles.
- Nuclear membrane and nucleolus reappear.
𝗠𝗲𝗶𝗼𝘀𝗶𝘀-𝗜𝗜
- Meiosis-II proceeds essentially like 𝗺𝗶𝘁𝗼𝘀𝗶𝘀, ensuring equal chromosome distribution between daughter cells.
- After cytokinesis, this results in 𝗳𝗼𝘂𝗿 𝗵𝗮𝗽𝗹𝗼𝗶𝗱 𝗱𝗮𝘂𝗴𝗵𝘁𝗲𝗿 𝗰𝗲𝗹𝗹𝘀.
𝗦𝗶𝗴𝗻𝗶𝗳𝗶𝗰𝗮𝗻𝗰𝗲 𝗼𝗳 𝗠𝗶𝘁𝗼𝘀𝗶𝘀
- Maintains proper 𝗰𝗲𝗹𝗹 𝘀𝗶𝘇𝗲.
- Preserves the correct amount of 𝗗𝗡𝗔 𝗮𝗻𝗱 𝗥𝗡𝗔.
- Supports 𝗴𝗿𝗼𝘄𝘁𝗵 𝗮𝗻𝗱 𝗱𝗲𝘃𝗲𝗹𝗼𝗽𝗺𝗲𝗻𝘁 of body organs.
- 𝗥𝗲𝗽𝗹𝗮𝗰𝗲𝘀 𝗼𝗹𝗱 𝗮𝗻𝗱 𝗱𝗲𝗮𝗱 𝗰𝗲𝗹𝗹𝘀.
- Enables 𝗮𝘀𝗲𝘅𝘂𝗮𝗹 𝗿𝗲𝗽𝗿𝗼𝗱𝘂𝗰𝘁𝗶𝗼𝗻 in certain organisms.
- Increases 𝗴𝗼𝗻𝗮𝗱 𝘀𝗶𝘇𝗲.
- Forms the foundation of 𝗱𝗲𝘃𝗲𝗹𝗼𝗽𝗺𝗲𝗻𝘁𝗮𝗹 𝗯𝗶𝗼𝗹𝗼𝗴𝘆
𝗖𝗼𝗻𝗰𝗹𝘂𝘀𝗶𝗼𝗻
The 𝗰𝗲𝗹𝗹 𝗰𝘆𝗰𝗹𝗲 𝗮𝗻𝗱 𝗰𝗲𝗹𝗹 𝗱𝗶𝘃𝗶𝘀𝗶𝗼𝗻 are fundamental concepts in biology that explain how cells grow, replicate, and produce new cells. 𝗠𝗶𝘁𝗼𝘀𝗶𝘀 ensures growth, tissue repair, and asexual reproduction by producing genetically identical daughter cells, whereas 𝗺𝗲𝗶𝗼𝘀𝗶𝘀 reduces the chromosome number and creates genetic variation essential for sexual reproduction.
Understanding these concepts is essential for BSc Zoology, MSc Zoology, NEET, and CUET PG preparation.
Reading Suggested
Referrence
- Molecular Biology of the Cell
- Cell Biology, Genetics, Molecular Biology, Evolution and Ecology – P.S. Verma & V.K. Agarwal.