Cell Reproduction and Binary Fission

Principles of Cell Reproduction

• German physician Rudolf Virchow (1821–1902) stated the third tenet of cell theory: "cells come from preexisting cell".
• Cell division is the process where a parent cell gives rise to two daughter cells.
• Daughter cells typically receive a complete set of identical hereditary materials and half of the cytoplasm from the parent cell.
• Purposes of cellular reproduction:
  • Reproduction and numerical multiplication in unicellular organisms.
  • Growth, development, and tissue formation in multicellular organisms (e.g., human embryo growth over nine months).
  • Replacement of worn-out and damaged cells during wound healing.
  • Production of new individuals in sexually and asexually reproducing organisms.

Examples of Asexual Reproduction

• Growth and development of stem cuttings in a malunggay (moringa oleifera) stem.
• Germination of runners in grasses.
• Sprouting of young fronds (leaves) in ferns.
• Formation of buds in Hydra.

Prokaryotic Division and Binary Fission

• Prokaryotic cells (such as Escherichia coli) lack a true nucleus and membrane-bound organelles.
• Prokaryotic DNA exists as a single, circular chromosome located in the nucleoid region.
• Binary fission steps:
  1. DNA duplicates during chromosome replication.
  2. Chromosomes segregate to opposite poles.
  3. A central cell wall (septum) grows and constricts to divide the parent cell into two identical daughter cells.

Eukaryotic Chromosomes

• Cell division in eukaryotic cells begins inside the nucleus.
• Chromatin consists of long, thin DNA threads wrapped around histone proteins.
• During preparation for cell division, chromatin fibers undergo coiling and compaction to form distinct, visible chromosomes.
• A normal human cell nucleus contains approximately 25,000 genes organized within 46 chromosomes.

Effects of Abnormal Chromosome Numbers

• Down Syndrome (Trisomy 21):
  • Cause: Extra copy of chromosome 21.
  • Characteristics: Intellectual disabilities, distinct facial features, potential heart defects, and increased risk of certain medical conditions.
• Turner Syndrome (Monosomy X):
  • Cause: Females with only one X chromosome (45, X).
  • Characteristics: Short stature, infertility, heart defects, and specific physical traits such as a webbed neck.
• Klinefelter Syndrome (XXY Syndrome):
  • Cause: Males with an extra X chromosome (47, XXY).
  • Characteristics: Reduced testosterone levels, infertility, breast development, and learning difficulties.