Prokaryotes vs Eukaryotes: What Are the Key Differences?

Every living organism falls into one of two groups: eukaryotes or prokaryotes, with cellular structure determining which group an organism belongs to. Prokaryotes are unicellular and lack a nucleus and membrane-bound organelles. They are generally smaller and simpler and include bacteria and archaea. Eukaryotes on the other hand are often multicellular and have a nucleus and membrane-bound organelles, which help to organize and compartmentalize cellular functions. They include animals, plants, fungi, algae and protozoans.

In this article, we will explain in further detail what prokaryotes and eukaryotes are and outline the differences between the two. 

Comparing prokaryotes and eukaryotes 

Prokaryotes were the first form of life, scientists believe that eukaryotes evolved from prokaryotes around 2.7 billion years ago.¹

Current doctrines on the origins of eukaryotes state that two prokaryotes formed a symbiotic relationship and merged in a process known as endosymbiosis.² Endosymbiotic events are believed to have led to the development of membrane-bound organelles such as mitochondria. Thanks to mitochondria, eukaryotic ancestors had enough energy to develop into the more complex eukaryotic cells known today.

However, research from the University of Jena, published in the journal mBio, has highlighted prokaryotic bacteria that can “eat” other cells.³ This contradicts previous beliefs that only eukaryotes could perform endocytosis (a cellular process in which substances are brought into the cell and form intracellular vesicles). These more recent findings suggest the need to re-evaluate theories regarding the origin of eukaryotes.

The primary distinction between these two types of organisms is that eukaryotic cells have a membrane-bound nucleus, and prokaryotic cells do not. The nucleus is where eukaryotes store their genetic information. In prokaryotes, DNA is bundled together in the nucleoid region, but it is not stored within a membrane-bound nucleus.

The nucleus is only one of many membrane-bound organelles in eukaryotes. Prokaryotes, on the other hand, have no membrane-bound organelles. Another important difference is the DNA structure and location.⁴ Eukaryote DNA consists of multiple molecules of double-stranded linear DNA found in the nucleus, while that of prokaryotes is double-stranded, often circular, and located within the cytoplasm. However, it is worth noting that linear plasmids and chromosomes have been found in certain prokaryotes.⁵

Key similarities between prokaryotes and eukaryotes

Figure 1: A comparison showing the shared and unique features of prokaryotes and eukaryotes. Credit: Technology Networks.

All cells, whether prokaryotic or eukaryotic, share these four features, as shown in Figure 1:

1. DNA
2. Plasma membrane
3. Cytoplasm
4. Ribosomes

What are the key differences between prokaryotes and eukaryotes?

Prokaryotes and eukaryotes vary in several important ways – these differences include structural variation – whether a nucleus is present or absent and whether the cell has membrane-bound organelles. The differences are summarized in Table 1, below.

Table 1: Differences between prokaryotes and eukaryotes.

	Prokaryote	Eukaryote
Nucleus	Absent	Present
Membrane-bound organelles	Absent	Present
Cell structure	Unicellular	Mostly multicellular; some unicellular
Cell size	Typically smaller (0.1–5 μm), however, a much larger (centimeter-long) bacterium has recently been discovered in a mangrove swamp.	Larger (10–100 μm)
Complexity	Simpler	More complex
DNA Form	Often circular, however, linear plasmids and chromosomes have been found in certain prokaryotes.	Linear
Examples	Bacteria, archaea	Animals, plants, fungi, protists

Transcription and translation in prokaryotes vs eukaryotes

In prokaryotic cells, transcription and translation are coupled, meaning translation begins during mRNA synthesis.⁶

In eukaryotic cells, transcription and translation are not coupled. Transcription occurs in the nucleus, producing mRNA. The mRNA then exits the nucleus, and translation occurs in the cell’s cytoplasm.

Prokaryote definition

Prokaryotes can be split into two domains, bacteria and archaea, and are unicellular organisms that lack membrane-bound structures. Prokaryotic cells tend to be small, simple cells, measuring around 0.1–5 μm in diameter.⁷   

Figure 2: The key structures present in a prokaryote cell. Credit: Technology Networks.

While prokaryotic cells do not have membrane-bound structures, they do have distinct cellular regions. In prokaryotic cells, DNA bundles together in a region called the nucleoid (Figure 2). In prokaryotes, molecules of protein, DNA and metabolites are all found together, floating in the cytoplasm. Primitive organelles, found in bacteria, do act as micro-compartments to bring some sense of organization to the arrangement.⁸

Prokaryotic cell features

Here is a breakdown of what you might find in a prokaryotic bacterial cell (Figure 2).

• Nucleoid: A central region of the cell that contains its DNA. 
• Ribosome: Ribosomes are responsible for protein synthesis.
• Cell wall: The cell wall provides structure and protection from the outside environment. Most bacteria that include this have a rigid cell wall made from carbohydrates and proteins called peptidoglycans.
• Cell membrane: Every prokaryote has a cell membrane, also known as the plasma membrane, that separates the cell from the outside environment.
• Capsule: Some bacteria have a layer of carbohydrates that surrounds the cell wall called the capsule. The capsule helps the bacterium attach to surfaces and protects it from harmful substances or conditions.
• Pili: Pili, also referred to as fimbriae, are rod-shaped structures involved in multiple roles, including attachment and DNA transfer.
• Flagella: Flagella are thin, tail-like structures that assist in movement.

Examples of prokaryotes

Bacteria and archaea are the two types of prokaryotes.

Do prokaryotes have a nucleus?

Prokaryotes do not have a nucleus. Instead, prokaryote DNA can be found, bundled but free-floating, in a central region called the nucleoid. Prokaryote DNA is usually found as a single chromosome of circular DNA. These organisms also lack other membrane-bound structures such as the endoplasmic reticulum.

Do prokaryotes have mitochondria?

No, prokaryotes do not have mitochondria. Mitochondria are only found in eukaryotic cells. This is also true of other membrane-bound structures like the nucleus and the Golgi apparatus (more on these later).

Eukaryote definition

Eukaryotes are organisms whose cells have a nucleus and other organelles enclosed by a plasma membrane (Figure 3). Organelles are internal structures responsible for a variety of functions, such as energy production and protein synthesis.

Figure 3: The key structures present in a eukaryote cell. Credit: Technology Networks.

Eukaryotic cells are large (around 10–100 μm) and complex. While most eukaryotes are multicellular organisms, there are some single-cell eukaryotes.⁹

Eukaryotic cell features

Within a eukaryotic cell, each membrane-bound structure carries out specific cellular functions. Here is an overview of many of the primary components of eukaryotic cells.

• Nucleus: The nucleus stores the genetic information in chromatin form.
• Nucleolus: Found inside of the nucleus, the nucleolus is the part of eukaryotic cells where ribosomal RNA is produced.
• Plasma membrane: The plasma membrane is a phospholipid bilayer that surrounds the entire cell and encompasses the organelles within.
• Cytoskeleton: The system of protein fibers and other molecules that gives shape to the cell, aiding in the correct positioning of organelles.
• Cell wall: The cell wall is only found in certain eukaryotes, such as plant cells, and is a rigid covering that provides structural support and protection to the cell.
• Ribosomes: Ribosomes are responsible for protein synthesis.
• Mitochondria: Mitochondria, also known as the powerhouses of the cell, are responsible for energy production.
• Cytoplasmic space: The cytoplasmic space is the region of the cell between the nuclear envelope and plasma membrane.
• Cytoplasm: The cytoplasm is defined as the total inner-cellular volume, with the exception of the nucleus, and includes the cytosol and all organelles.
• Cytosol: The cytosol, which consists of a gel-like substance, accounts for all the material in the cytoplasm, excluding the contents of the various membrane-bound organelles. 
• Endoplasmic reticulum: The endoplasmic reticulum is an organelle dedicated to protein maturation and transportation.
• Vesicles and vacuoles: Vesicles and vacuoles are membrane-bound sacs involved in transportation and storage.

Other common organelles found in many, but not all, eukaryotes include the Golgi apparatus, chloroplasts and lysosomes.

Examples of eukaryotes

Animals, plants, fungi, algae and protozoans are all eukaryotes.