Mitosis
Mitosis is a type of cell division in which a single cell divides to form two daughter cells, each containing the same number of chromosomes as the original parent cell. This process ensures that genetic material is accurately distributed to the new cells, maintaining consistency in the genetic information.
Mitosis occurs in somatic cells (all body cells except reproductive cells) of eukaryotic organisms (organisms whose cells contain a nucleus). In contrast, prokaryotic cells (bacteria and other simple organisms) divide differently through a process called binary fission, which does not involve mitosis.
Discovery of Mitosis
The process of mitosis was first observed and described by a German biologist, Walther Flemming, in the 1880s. He used microscopes to study cell division and noted the different stages in which the chromosomes change shape and position inside the cell.
Phases of Mitosis
Mitosis is divided into two major phases:
- Karyokinesis (division of the nucleus)
- Cytokinesis (division of the cytoplasm)
A. Karyokinesis (Nuclear Division)
Karyokinesis is the division of the nucleus, where the duplicated genetic material (DNA) is separated into two identical sets. It has four distinct stages:
1. Prophase
What happens during prophase?
- Condensation of Chromatin: Inside the nucleus, the thread-like chromatin (a combination of DNA and proteins) condenses to form chromosomes. Each chromosome consists of two sister chromatids, which are identical copies of DNA, joined together at a centromere (the central region of the chromosome).
- Disappearance of Nuclear Envelope: The nuclear envelope (a protective membrane around the nucleus) and nucleolus (a small structure inside the nucleus) break down, allowing the chromosomes to move freely.
- Duplication of Centrosome & Formation of Spindle Fibers:
- A structure called the centrosome (an organelle responsible for organizing cell division) duplicates itself.
- The two centrosomes move to opposite sides of the nucleus and start forming a network of spindle fibers made of microtubules. This structure is called the mitotic spindle and is responsible for moving chromosomes during division.
- Plant cells do not have centrosomes. Instead, their spindle fibers arise directly from the cytoplasm.
2. Metaphase
Key events in metaphase:
- Attachment of Spindle Fibers to the Kinetochore:
- The spindle fibers attach to the kinetochore, a specialized protein structure at the centromere of each chromosome.
- Each chromosome is linked to spindle fibers coming from opposite poles of the cell.
- Metaphase Plate Formation:
- The chromosomes align themselves in a straight line at the center of the cell, forming the metaphase plate (also called the equatorial plate).
- This ensures that the chromosomes are evenly split between the two new cells.
3. Anaphase
What happens in anaphase?
- The spindle fibers begin to pull the chromosomes toward opposite ends (poles) of the cell.
- The sister chromatids of each chromosome separate and move to opposite sides.
- Now, each chromatid is considered a separate chromosome.
This ensures that each new daughter cell will receive a complete set of identical chromosomes.
4. Telophase
Final stage of nuclear division:
- Formation of New Nuclear Envelopes: A new nuclear membrane forms around each set of separated chromosomes at both poles.
- Reformation of Chromatin: The chromosomes begin to unwind and return to their chromatin form, making them less visible under a microscope.
- Nucleolus Reappears: The nucleolus reforms inside each new nucleus.
At this point, the nucleus has successfully divided, and the cell prepares for cytokinesis.