Prokaryotic cells and eukaryotic cells are the two types of cells that exist on Earth. There are several differences between the two, but the biggest distinction between them is that eukaryotic cells have a distinct nucleus containing the cell's genetic material, while prokaryotic cells don't have a nucleus and have free-floating genetic material instead.
From prokaryotic cells to eukaryotic cells
All living things can be divided into three basic domains: Bacteria, Archaea and Eukarya. The primarily single-celled organisms found in the Bacteria and Archaea domains are known as prokaryotes. These organisms are made of prokaryotic cells — the smallest, simplest and most ancient cells.
Organisms in the Eukarya domain are made of the more complex eukaryotic cells. These organisms, called eukaryotes, can be unicellular or multicellular and include animals, plants, fungi and protists. Many people are unclear on whether yeasts or fungi are prokaryotes or eukaryotes. Both are eukaryotes and share similar cell structure to all other eukaryotes.
Eukaryotes developed at least 2.7 billion years ago, following 1 to 1.5 billion years of prokaryotic evolution, according to the National Institutes of Health (NIH). Scientists hypothesize that the nucleus and other eukaryotic features may have first formed after a prokaryotic organism swallowed up another, according to the University of Texas. According to this theory, the engulfed organism would have then contributed to the functioning of its host.
What do prokaryotes and eukaryotes have in common?
Although prokaryotic and eukaryotic cells have many differences, they share some common features, including the following:
- DNA: Genetic coding that determines all the characteristics of living things.
- Cell (or plasma) membrane: Outer layer that separates the cell from the surrounding environment and acts as a selective barrier for incoming and outgoing materials.
- Cytoplasm: Jelly-like fluid within a cell that is composed primarily of water, salts and proteins.
- Ribosomes: Organelles that make proteins.
How do prokaryotes and eukaryotes differ?
Nucleus/DNA: Eukaryotic cells have a nucleus surrounded by a nuclear envelope that consists of two lipid membranes, according to Nature Education. The nucleus holds the eukaryotic cell's DNA. Prokaryotic cells do not have a nucleus; rather, they have a membraneless nucleoid region (open part of the cell) that holds free-floating DNA, according to Washington University.
The entire DNA in a cell can be found in individual pieces known as chromosomes. Eukaryotic cells have many chromosomes which undergo meiosis and mitosis during cell division, while most prokaryotic cells consist of just one circular chromosome. However, recent studies have shown that some prokaryotes have as many as four linear or circular chromosomes, according to Nature Education. For example, Vibrio cholerae, the bacterium that causes cholera, has two circular chromosomes.
Organelles in Eukaryotic Cells: Eukaryotic cells have several other membrane-bound organelles not found in prokaryotic cells. These include the mitochondria (convert food energy into adenosine triphosphate, or ATP, to power biochemical reactions); rough and smooth endoplasmic reticulum (an interconnected network of membrane-enclosed tubules that transport synthesized proteins); golgi complex (sorts and packages proteins for secretion); and in the case of plant cells, chloroplasts (conduct photosynthesis). All of these organelles are located in the eukaryotic cell's cytoplasm.
Ribosomes: In eukaryotic cells, the ribosomes are bigger, more complex and bound by a membrane. They can be found in various places: Sometimes in the cytoplasm; on the endoplasmic reticulum; or attached to the nuclear membrane (covering on the nucleus).
In prokaryotic cells, the ribosomes are scattered and floating freely throughout the cytoplasm. The ribosomes in prokaryotic cells also have smaller subunits. All ribosomes (in both eukaryotic and prokaryotic cells) are made of two subunits — one larger and one smaller. In eukaryotes, these pieces are identified by scientists as the 60-S and 40-S subunits. In prokaryotes, the ribosomes are made of slightly smaller subunits, called 50-S and 30-S.
The difference in types of subunits has allowed scientists to develop antibiotic drugs, such as streptomycin, that attack certain types of infectious bacteria, according to the British Society for Cell Biology. On the downside, some bacterial toxins and the polio virus use the ribosome differences to their advantage — they're able to identify and attack eukaryotic cells' translation mechanism, or the process by which messenger RNA is translated into proteins.
Reproduction: Most eukaryotes reproduce sexually (although some protists and single-celled fungi may reproduce through mitosis, which is functionally similar to asexual reproduction). Prokaryotes reproduce asexually, resulting in the offspring being an exact clone of the parent. Some prokaryotic cells also have pili, which are adhesive hair-like projections used to exchange genetic material during a type of sexual process called conjugation, according to Concepts of Biology. Conjugation can occur in bacteria, protozoans and some algae and fungi.
Cell Walls: Most prokaryotic cells have a rigid cell wall that surrounds the plasma membrane and gives shape to the organism. In eukaryotes, vertebrates don't have a cell wall but plants do. The cell walls of prokaryotes differ chemically from the eukaryotic cell walls of plant cells, which are primarily made of cellulose. In bacteria, for example, the cell walls are composed of peptidoglycans (sugars and amino acids), according to Washington University.