Which of the following attributes is common to both prokaryotic cells and eukaryotic cells?

A) The use of DNA as the information storage molecule. B) Membrane-enclosed organelles. C) Prokaryotic and eukaryotic cells are generally about the same size. D) The use of proteins as information storage molecules. E) A membrane-enclosed nucleus.

The correct answer is A) The use of DNA as the information storage molecule.

Diagram of prokaryote and eukaryote cell
Diagram of prokaryotic and eukaryotic cell (Science Primer (National Center for Biotechnology Information). Vectorized by Mortadelo2005. [Public domain])
Prokaryotic cells are much smaller in size than eukaryotic cells and they also do not form tissues and do not make up multicellular organisms.

A prokaryotic cell can be recognized by the lack of membrane-bound organelles. They do, also all have a cell membrane and a cell wall. The plasma membrane is always made of phospholipids, although the lipid structure varies between the archaeans and bacteria.

Prokaryotes do share some features with eukaryotes. For instance, both types of cells contain DNA which contains all the genes which code for proteins. Both eukaryotic and prokaryotic cells have a cytoplasm and ribosomes. The ribosomes are needed for protein synthesis to occur.

Eukaryotic cells can be recognized as such by having membrane-bound organelles. The genetic material, the DNA of a eukaryote is housed in the nucleus which is an organelle that has a double membrane known as a nuclear envelope.

Other membrane-bound organelles that are found in eukaryotic cells are the endoplasmic reticulum, Golgi apparatus, and mitochondrion. Cells that photosynthesize also have a chloroplast present.

The mitochondrion and chloroplast are both organelles that have their own DNA which lends support to the idea that eukaryotes evolved from a type of prokaryotic cell.

The prokaryotic cell

This type of cell is found in organisms of the domains archaea and bacteria. The cells are in the range of 0.1 to 5 µm in size. The prokaryotic cells are believed to be older than eukaryotic cells in terms of evolution, and in fact, the archaeans are thought to be the ancestor of the eukaryotes.

All of the prokaryotes can be recognized as such by having no membrane-bound organelles present in the cell. Prokaryotic cells also do all have a cell wall present surrounding the plasma membrane.

This cell wall may consist of peptidoglycan of varying thickness in the bacteria, while the archaeans have various proteins and complex carbohydrates present making up the wall of the cell.

There are species of bacteria that have another layer, a capsule, surrounding the cell wall. This capsule is often made of a combination of polysaccharides and polypeptides. The plasma membrane of bacteria is made of phospholipids in which fatty acids are present while those of the archaeans are somewhat different in structure.

Prokaryotes also never form multicellular tissues but remain as individual cells, yet they can form layers that stick together to form thick layers known as biofilms. This makes them difficult to remove from surfaces.

Genetic material

A feature that prokaryotic cells do share with eukaryotic cells is the presence of DNA as the genetic material. The DNA stores the information for the cell and provides the code for making proteins. This is a feature seen in all types of living cells.

The lack of membrane-bound organelles in the prokaryotes means that the DNA is not surrounded by a membrane but rather occurs in a specific location in the cell that is known as a nucleoid.

The DNA is copied and translated into proteins in the process of protein synthesis. Transfer RNA and ribosomal RNA important in this process are present in prokaryotes as well as eukaryotes.

Prokaryotes, like eukaryotes, also have a cytoplasm in which ribosomes are found. These are the sites at which the synthesis of polypeptides occurs.

The eukaryotic cell

These cells are much larger than prokaryotes with size ranging from 10 to 100 µm. Eukaryotic cells may be unicellular organisms or come together and organize to form multicellular life forms.

This cell type is found in plants, animals, fungi, and protists and is easy to recognize by the presence of membrane-bound organelles. Eukaryotes are believed to have evolved after the prokaryotes.

The DNA of the cell is found within the nucleus which has a double membrane known as the nuclear envelope.

It does have openings in this membrane that are called the nuclear pores. These are made of proteins which carefully control what molecules can enter and leave the nucleus.

Cell membrane and cell wall

All of the eukaryotic cells have a cell membrane that consists of a double layer of phospholipids in which various proteins and other molecules such as cholesterol are scattered.

The proteins often act as receptors or channels through which molecules can pass through the plasma membrane.

Protists and animal cells do not have a cell wall but plants and fungi do, and the structures making up the wall differ between the two groups. Fungi have chitin present while plants have cellulose and hemicellulose present in the cell wall.

Endoplasmic reticulum and Golgi apparatus

Several different organelles are present in eukaryotic cells. They all have a nucleus, endoplasmic reticulum (ER), Golgi apparatus and mitochondria present.

The ER consists of a series of membranous sacs called cisternae, and there are two types that occur, the smooth ER which has no ribosomes attached to it, and the rough ER which does have ribosomes attached to it.

The smooth ER is important in forming lipids such as the steroids. The rough ER is involved in further alteration and modification of polypeptides formed during protein synthesis at the ribosomes.

Substances formed in the ER are often sent to the Golgi apparatus which further modifies and then transports substances to where they are needed in the cell. This is accomplished by vesicles that are released from one side of the organelle.

Endosymbiotic theory

Chloroplasts and mitochondria are two organelles that are both found in plant cells and some protists. They both contain DNA and are surrounded by a double membrane.

There is a theory that these two organelles were once independently living bacterial cells that formed a symbiotic relationship with a eukaryotic cell. This is the endosymbiotic theory and it is used as evidence that eukaryotes evolved from prokaryotic cells.

The mitochondrion is where some of the reactions of aerobic cellular respiration occur, and the chloroplast is where photosynthesis occurs. Only plant cells and certain protists can photosynthesize, and thus only these cells have chloroplasts present, while mitochondria are also found in animal and fungal cells.

References

  1. RL Dorit, WF Walker, RD Barnes (1991).  Zoology. USA: Philadelphia, Saunders College Publishing.
  2. PH Raven, RF Evert, SE Eichhorn (1987). Biology of Plants. Worth Publishers.
  3. M Lynch, GK Marinov (2017). Membranes, energetics, and evolution across the prokaryote-eukaryote divide. Elife.
  4. C Rye, R Wise, V Jurukovski, J DeSaix, J Choi, Y Avissar (2017). Biology. Houston: USA, Rice University.
  5. D Sagan, L Margulis, C Sagan (2019). Life. Retrieved from Encyclopedia Britannica.

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