FUNDAMENTALS OF MICROBIOLOGY

Lesson 14. AN OVERVIEW OF DIFFERENCE BETWEEN PROKARYOTES AND EUKARYOTES

Module 4. Structure and function of prokaryotic cells

Lesson 14

AN OVERVIEW OF DIFFERENCE BETWEEN PROKARYOTES AND EUKARYOTES

14.1 Introduction

Cells in our world come in two basic types, prokaryotic (Fig. 14.1) and eukaryotic (Fig. 14.2). ‘Karyose’ comes from a Greek word which means ‘kernel’, as in a kernel of grain. In biology, we use this word root to refer to the nucleus of a cell. ‘Pro’ means ‘before’ and ‘eu’ means ‘true’, or ‘good’. So ‘Prokaryotic’ means ‘before a nucleus’ and ‘eukaryotic’ means ‘possessing a true nucleus’. This is a big hint about one of the differences between these two cell types. Prokaryotic cells have no nuclei, while eukaryotic cells do have true nuclei. This is far from the only difference between these two cell types.

14.2 What Prokaryotic and Eukaryotic Cells Have in Common

• Both have DNA as their genetic material.
• Both are covered by a cell membrane.
• Both contain RNA.
• Both are made from the same basic chemicals: carbohydrates, proteins, nucleic acid, minerals, fats and vitamins.
• Both have ribosomes (the structures on which proteins are made).
• Both regulate the flow of the nutrients and wastes that enter and leave them.
• Both have similar basic metabolism (life processes) like photosynthesis and reproduction.
• Both require a supply of energy.
• Both are highly regulated by elaborate sensing systems (‘chemical noses’) that make them aware of the reactions within them and the environment around them.

That's what prokaryotic and eukaryotic cells have in common. But there are significant differences between them too.

14.3 Prokaryotic and Eukaryotic Differences

If we take a closer look at the comparison of these cells, we see the following differences:

• Eukaryotic cells have a true nucleus, bound by a double membrane. Prokaryotic cells have no nucleus. The purpose of the nucleus is to sequester the DNA-related functions of the big eukaryotic cell into a smaller chamber, for the purpose of increased efficiency. This function is unnecessary for the prokaryotic cell, because it is much smaller size means that all materials within the cell are relatively close together. Of course, prokaryotic cells do have DNA and DNA functions. Biologists describe the central region of the cell as its ‘nucleoid’ (-oid = similar or imitating), because it's pretty much where the DNA is located. But note that the nucleoid is essentially an imaginary ‘structure’. There is no physical boundary enclosing the nucleoid.

• Eukaryotic DNA is linear; prokaryotic DNA is circular (it has no ends).

• Eukaryotic DNA is complexed with proteins called ‘histones’, and is organized into chromosomes; prokaryotic DNA is ‘naked’, meaning that it has no histones associated with it, and it is not formed into chromosomes. Though many are sloppy about it, the term ‘chromosome’ does not technically apply to anything in a prokaryotic cell. A eukaryotic cell contains a number of chromosomes; a prokaryotic cell contains only one circular DNA molecule and a varied assortment of much smaller circlets of DNA called ‘plasmids’. The smaller, simpler prokaryotic cell requires far fewer genes to operate than the eukaryotic cell.

• Both cell types have many, many ribosomes, but the ribosomes of the eukaryotic cells are larger and more complex than those of the prokaryotic cell. Ribosomes are made out of a special class of RNA molecules (ribosomal RNA, or rRNA) and a specific collection of different proteins. A eukaryotic ribosome is composed of five kinds of rRNA and about eighty kinds of proteins. Prokaryotic ribosomes are composed of only three kinds of rRNA and about fifty kinds of protein.

• The cytoplasm of eukaryotic cells is filled with a large, complex collection of organelles, many of them enclosed in their own membranes; the prokaryotic cell contains no membrane-bound organelles which are independent of the plasma membrane. This is a very significant difference, and the source of the vast majority of the greater complexity of the eukaryotic cell. There is much more space within a eukaryotic cell than within a prokaryotic cell, and many of these structures, like the nucleus, increase the efficiency of functions by confining them within smaller spaces within the huge cell, or with communication and movement within the cell.

Examination of these differences is interesting. As mentioned above, they are all associated with larger size and greater complexity. This leads to an important observation. Yes, these cells are different from each other. However, they are clearly more alike than different, and they are clearly evolutionarily related to each other. Biologists have no significant doubts about the connection between them. The eukaryotic cell is clearly developed from the prokaryotic cell.

One aspect of that evolutionary connection is particularly interesting. Within eukaryotic cells you find a really fascinating organelle called a mitochondrion. And in plant cells, you'd find an additional family of organelles called plastids, the most famous of which is the renowned chloroplast. Mitochondria (the plural of mitochondrion) and chloroplasts almost certainly have a similar evolutionary origin. Both are pretty clearly the descendants of independent prokaryotic cells, which have taken up permanent residence within other cells through a well-known and very common phenomenon called endosymbiosis.

The salient differences among Bacteria, Archaea and Eukarya are listed in Table14.1.

Table 14.1 Differences among bacteria, archaea and eukarya