Prokaryotic cells

It was once common practice to classify all living organisms as either animals or plants. With improved knowledge of living things, it has become apparent that there are two fundamentally different types of cells. The cells of animals, plants, fungi, and protists contain many membrane-bound organelles, one of which is the nucleus. These are referred to as eukaryotic cells (meaning ‘with a true nucleus’). Prokaryotes (meaning ‘before the nucleus’), which are mostly bacteria, are significantly smaller than eukaryotes, with a simpler internal organization.

Key Differences from Eukaryotic Cells

1. Cytoplasm Lacks Membrane-Bound Organelles: Unlike eukaryotes, prokaryotes do not possess organelles like mitochondria, endoplasmic reticulum, or Golgi apparatus. Cellular processes occur directly in the cytoplasm.

2. Smaller Ribosomes: Prokaryotes have 70S ribosomes, which are smaller than the 80S ribosomes found in eukaryotes. This difference is important for targeting antibiotics specifically to bacterial infections.

3. No Nucleus: Instead of a nucleus, prokaryotes have a single circular DNA molecule located in a region called the nucleoid. This DNA is not associated with histone proteins, unlike eukaryotic DNA.

4. Cell Wall Contains Murein: The cell wall of prokaryotes is composed of murein (peptidoglycan), a glycoprotein unique to bacteria. This provides structural support and protection against osmotic pressure.

In addition to the core differences, many prokaryotic cells also possess:

1. Plasmids: Small, circular DNA molecules separate from the main chromosome. Plasmids often carry genes for antibiotic resistance or other advantageous traits.

2. Capsule: A sticky layer surrounding the cell wall, providing protection and aiding in attachment to surfaces.

3. Flagella: Long, whip-like appendages used for locomotion.

Prokaryotic cells are about 100 to 10,000 times smaller than eukaryotic cells. Biologists believe eukaryotes evolved from prokaryotes and that the organelles were originally prokaryotic cells that formed a symbiotic relationship with the eukaryotic cell.

The structure of a prokaryotic cell.

Cell division

Prokaryotic cells do not have a nucleus, so they do not divide by mitosis. Instead, they reproduce through binary fission (a form of asexual reproduction), which involves:

• Replication of the large circular DNA and any plasmids they may have.

• Division of the cytoplasm to produce two daughter cells, each with a single copy of the circular DNA and a variable number of plasmid copies.

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