D2.1.3 Equal and unequal cytokinesis Notes

Equal and Unequal Cytokinesis

1. It occurs after nuclear division (mitosis or meiosis), physically separating the cell into two distinct cells.
2. Te cytoplasm, organelles, and other cellular components are distributed between daughter cells.

Equal Cytokinesis: A Balanced Split

1. In most cases, cytokinesis divides the cytoplasm equally between daughter cells.
2. This ensures that both cells receive the resources they need to survive and function.
3. Key Features of Equal Cytokinesis
   1. Symmetrical Division: The cytoplasm is divided evenly, producing two daughter cells of similar size.
   2. Organelle Distribution: Each daughter cell receives at least one mitochondrion and other essential organelles.
   3. Purpose: Common in cells that need to grow, repair tissues, or maintain genetic uniformity, such as skin cells or root tip cells in plants.

Unequal Cytokinesis: Asymmetrical Division

1. In some cases, cytokinesis divides the cytoplasm unequally, producing daughter cells of different sizes.
2. This occurs for several reasons:
   1. One cell may be specialized to retain most resources (e.g., nutrients, organelles).
   2. The smaller cell may have a different function or may degenerate.
   3. This strategy is used when one daughter cell needs to be particularly large or well-resourced.

All Daughter Cells Must Receive Certain Organelles

Regardless of whether cytokinesis is equal or unequal, both daughter cells must receive at least one of certain organelles that cannot be synthesized from scratch.

1. Organelles That Must Be Inherited:
   1. Mitochondria:
      1. Cannot be made de novo (from scratch).
      2. Must be inherited from the parent cell.
      3. Each daughter cell must receive at least one mitochondrion to produce ATP.
   2. Chloroplasts (in plant cells):
      1. Like mitochondria, can only arise from pre-existing chloroplasts.
      2. Must be passed to daughter cells during division.
   3. Other membrane-bound organelles:
      1. Organelles like the ER and Golgi apparatus can form from pre-existing structures.
      2. Daughter cells typically inherit portions of these organelles during cytokinesis.

Examples of Unequal Cytokinesis

Two key examples demonstrate this:

Budding Yeast

1. Budding is a form of asexual reproduction in yeast where cytokinesis is highly unequal.
2. How budding works:
   1. A small bud (outgrowth) forms on the surface of the parent yeast cell.
   2. The nucleus divides, and one nucleus moves into the bud.
   3. The bud receives a small portion of cytoplasm and organelles (including at least one mitochondrion).
   4. The bud eventually detaches from the parent cell, becoming an independent daughter cell.
   5. A scar remains on the parent cell at the budding site.
3. Result:
   1. Parent cell: Large, retains most of the cytoplasm and organelles.
   2. Daughter cell (bud): Small, but still functional and capable of growth.

Oogenesis in Humans

Oogenesis is the production of egg cells (ova) in females. It involves two rounds of unequal cytokinesis during meiosis.

1. How oogenesis works:
   1. Stage 1: Formation of Primary Oocytes
      1. Oogenesis begins in the female fetus.
      2. Diploid germ cells (oogonia) undergo mitosis and then enter meiosis I.
      3. They become primary oocytes, which are arrested in prophase I until puberty.
   2. Stage 2: Meiosis I and First Unequal Cytokinesis
      1. After puberty, during each menstrual cycle, one primary oocyte completes meiosis I.
      2. Unequal cytokinesis produces:
         1. Secondary oocyte: Large cell that retains most of the cytoplasm, organelles, and nutrients.
         2. First polar body: Small cell with minimal cytoplasm that eventually degenerates.
   3. Stage 3: Meiosis II and Second Unequal Cytokinesis
      1. If fertilization occurs, the secondary oocyte completes meiosis II.
      2. Unequal cytokinesis again produces:
         1. Mature ovum (egg): Large cell with abundant cytoplasm, nutrients, and organelles to support early embryonic development.
         2. Second polar body: Small cell that degenerates.
   4. Result:
      1. One large, functional egg cell capable of supporting a developing embryo.
      2. Two or three small polar bodies that degenerate (they receive minimal cytoplasm but still contain at least one mitochondrion).

Comparing Equal and Unequal Cytokinesis