Cell Division: Mitosis

Mitosis is a critical process for all eukaryotic organisms, enabling growth, tissue repair, and asexual reproduction. It ensures that each new cell receives a complete and identical set of chromosomes from the parent cell. Before mitosis can begin, the cell must undergo a period of growth and DNA replication known as Interphase. Interphase is divided into three sub-phases:

• G1 Phase (First Gap): The cell grows, synthesizes proteins, and carries out its normal metabolic functions. Organelles are duplicated.
• S Phase (Synthesis): DNA replication occurs, resulting in each chromosome consisting of two identical sister chromatids joined at the centromere. The amount of DNA doubles, but the chromosome number remains the same.
• G2 Phase (Second Gap): The cell continues to grow, synthesizes proteins necessary for cell division, and checks for any errors in DNA replication. It prepares for the upcoming mitotic phase.

Interphase is often the longest phase of the cell cycle, ensuring the cell is adequately prepared for division.

Mitosis itself is a continuous process, but for ease of understanding, it is divided into four distinct stages:

• Prophase: This is the longest stage of mitosis.

  • Chromatin condenses into visible chromosomes. Each chromosome now consists of two identical sister chromatids joined at the centromere.
  • The nuclear envelope begins to break down into small vesicles.
  • The nucleolus disappears.
  • Centrosomes (in animal cells), which duplicated during interphase, move to opposite poles of the cell, and spindle fibers (microtubules) begin to form between them.

• Metaphase:

  • The condensed chromosomes align along the metaphase plate (equatorial plate), an imaginary plane equidistant from the two poles of the cell.
  • The spindle fibers attach to the kinetochores (protein structures located at the centromere) of each sister chromatid. This attachment ensures that each chromatid will be pulled to opposite poles.

The final two stages of mitosis complete the separation of genetic material:

• Anaphase: This is the shortest stage of mitosis, characterized by the separation of sister chromatids.

  • The centromeres divide, and the sister chromatids separate, becoming individual chromosomes.
  • Spindle fibers shorten, pulling the newly separated chromosomes towards opposite poles of the cell. Each pole receives an identical set of chromosomes.
  • The cell elongates as non-kinetochore spindle fibers lengthen.

• Telophase: This stage essentially reverses the events of prophase.

  • The chromosomes arrive at the poles and begin to decondense (uncoil) back into chromatin.
  • New nuclear envelopes form around the two sets of chromosomes at each pole.
  • The nucleoli reappear within the new nuclei.
  • The spindle fibers disassemble. At this point, the cell contains two identical nuclei, but the cytoplasm has not yet divided.