2: Prokaryotes

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The earliest cells on Earth were prokaryotic (pro- meaning before, karyo- referring to the nucleus). As is implied by the name, prokaryotic cells lack a nucleus, as well as any membrane bound organelles. Prokaryotes have been divided into two domains: Bacteria and Archaea. Alongside domain Eukarya, these three domains compose the tree of life, as we currently understand it.

Cell Structure

In their simplest form, these unicellular organisms possess a cell wall, cell membrane, cytosol, ribosomes, and DNA. As the vast diversity of life on Earth is prokaryotic, you can imagine that there is a great diversity of forms and many exceptions to these general guidelines.

The DNA in prokaryotic cells is a single chromosome in the form of a loop, often bundled into a condensed region called a nucleoid. There may be small loops of DNA present called plasmids. These small loops of DNA can be exchanged between prokaryotes and the environment and are frequently the location of antibiotic resistance genes.

On the exterior, prokaryotes have a cell wall. They may also have flagella (sing. flagellum) for movement and/or pili (sing. pilus) for interacting with other organisms.

The components of a generalized prokaryotic cell — Figure \(\PageIndex{1}\): The diagram above shows the structures present in a prokaryotic cells. There is a single, double-stranded circle of DNA and a smaller circle of double-stranded DNA that represents a plasmid. There is a cluster of ribosomes floating in the cytosol (the jelly-like matrix that fills the cell), but there are no membrane-bound organelles. The cell is surrounded by a cell wall with the cell membrane just inside of the cell wall. There is a single, long flagellum, as well as many shorter pili. Artwork by Nikki Harris CC BY-NC with added labels by Maria Morrow.

Reproduction

There is no nucleus in a prokaryotic cell, so they cannot undergo mitosis or meiosis. Instead, prokaryotes replicate by a process called binary fission (bi- meaning two, fission meaning to split apart).

Binary fission, a prokaryotic cell divides into two identical cells — Figure \(\PageIndex{2}\): The diagram above shows how one prokaryotic cell divides into two identical cells via binary fission. In the first step, the DNA is replicated. In step two, a copy of the DNA is segregated to each different side of the cell as it elongates. In step 3, a septum (division) begins to form in the center of the elongated cell, forming a new cell wall and dividing the original cell in two. The result is two identical cells, each with an identical DNA chromosome. Artwork by Nikki Harris CC BY-NC with added labels by Maria Morrow.

Endosymbiosis

Endosymbiosis (endo- meaning inside, symbiosis meaning shared life) is a process by which several organelles were acquired by lineages of eukaryotic organisms, including mitochondria and plastids.

Primary Endosymbiosis

Both mitochondria and the chloroplasts in plants were acquired through a process called primary endosymbiosis. In the primary endosymbiosis that resulted in chloroplasts, a photosynthetic prokaryote similar to modern day cyanobacteria was engulfed by a heterotrophic eukaryote (a method of eating called phagocytosis). Something went wrong during this process and the prokaryotic cell was not digested. Instead, it was trapped within the eukaryotic cell. It continued to photosynthesize, producing sugars that resulted in increased fitness for its host. Over a long period of time, genes were eventually transferred between the prokaryote and the eukaryotic nucleus and an organelle was formed: the chloroplast.

Secondary Endosymbiosis

Many unrelated groups of photosynthetic organisms obtained the ability to photosynthesize by a process called secondary endosymbiosis. This is similar to primary endosymbiosis, except the organism that gets engulfed is a photosynthetic eukaryote (whose chloroplasts were the result of primary endosymbiosis somewhere in their evolutionary history). For example, brown algae and diatoms are both the result of a secondary endosymbiotic event in which a heterotrophic heterokont (see Chapter 5.2 Water Molds) engulfed a red alga.

2.1: Cyanobacteria
Cyanobacteria are a group within Domain Bacteria that perform oxygenic photosynthesis are related to the chloroplasts in plants. They are often found in mutualistic relationships fixing nitrogen.
2.2: Root Nodules
Root nodules are a form of mutualism between bacteria and plant roots