Kingdom Monera.

The Monera are the bacteria — the smallest, simplest and most ancient cells on Earth. They are prokaryotes: cells with no true nucleus, yet they live almost everywhere and run much of the planet's chemistry.

1 · Prokaryotic cells

Kingdom Monera contains all the prokaryotes — organisms whose cells have no membrane-bound nucleus and no membrane-bound organelles (no mitochondria, no ER). Their single, circular DNA lies free in the cytoplasm in a region called the nucleoid. They are usually single-celled and very small (~1–10 µm).

2 · Structure of a bacterium

• Cell wall — made of peptidoglycan (murein); gives shape and protection. (Gram staining of this wall divides bacteria into Gram-positive and Gram-negative.)
• Cell membrane — controls what enters and leaves.
• Cytoplasm with 70S ribosomes (for protein synthesis).
• Nucleoid — the single circular chromosome of DNA.
• Plasmids — small extra rings of DNA (often carry antibiotic-resistance genes).
• Capsule / slime layer — extra outer coat for protection and sticking.
• Flagella — whip-like threads for movement; pili — hair-like, for attachment and gene exchange.

3 · Shapes

Bacteria are classified partly by shape: cocci (spherical), bacilli (rod-shaped), spirilla (spiral) and vibrio (comma-shaped). They may occur singly or in pairs, chains (strepto-) and clusters (staphylo-).

4 · Nutrition & reproduction

Bacteria feed in every way: many are heterotrophic (saprotrophic decomposers or parasites), while others are autotrophic — photosynthetic (cyanobacteria, which also released much of Earth's early oxygen) or chemosynthetic.

They reproduce asexually by binary fission: the DNA is copied, the cell grows and then splits into two identical cells. In good conditions this can happen every 20 minutes, so numbers rise explosively.

5 · Importance

Helpful: decomposers that recycle nutrients; nitrogen-fixing bacteria (e.g. Rhizobium in root nodules) that enrich soil; gut bacteria; making yoghurt and cheese; producing antibiotics, insulin and vitamins in biotechnology. Harmful: they cause diseases such as tuberculosis, cholera, typhoid and tetanus, and spoil food.