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2-39 Eukaryotic cells have much in common with prokaryotic cells

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  • There are many basic similarities between eukaryotic and prokaryotic cell structures, but some structures are different.
  • Many enzymes that carry out the same function between eukaryotes and bacteria are homologous.
  • The basic structure and function of DNA is similar, but eukaryotes have a more highly organized genome that organizes itself into chromosomes during cell division.
  • The overall sequence of transcription and translation is very similar, although the details can be different.
  • Membranes are highly conserved throughout all life forms.

After the recent journey through the bacterial cell, you may have started to wonder about your own cells or other eukaryotic cells. How many properties do we share with bacteria? How are we different? It turns out, as you might expect, we share some basic things in common, but other structures are very different.

It turns out that much of what you now know about bacterial cells also applies to those of eukaryotes. It is safe to say we are more similar than we are different. The basic building blocks of the cell, such as nucleic acids, amino acids and sugars are identical. Macromolecular organizations such as chromosomes and membranes have many similarities. Many proteins in eukaryotes, especially those that carry out essential cell functions, have homologs in bacteria that share a high degree of sequence and structural similarity. An example that illustrates this point is the respiratory enzyme cytochrome oxidase. As shown in Figures 2-55 and 2-61, a comparison of cytochrome oxidase from the bovine and Rhodobacter sphaeroides reveals a near identical arrangement of the catalytic proteins and high sequence similarity. However, the cytochrome oxidase in the bovine has a number of other polypeptides that serve a structural role.

Figure 2-55 Comparison of cytochrome oxidase from bacteria and bovine.

Comparison of cytochrome oxidase from bacteria and bovine.

Molecular models of cytochrome oxidase from Rhodobacter sphaeroides (A) and bovine (B) are compared. Each protein is a complex of several distinct proteins, but the four polypeptides shown in color have a high degree of similarity in both their sequence (see Figure 2-61) and structure. The structural similarity should be obvious in this view. Such structurally similarity cannot have arisen by chance, but must reflect the evolution of each from a single ancestor. The gray polypeptides in the bovine cytochrome oxidase are not found in the bacterial protein.

Figure 2-61 Sequence comparison of cytochrome oxidase from three species

Sequence comparison of cytochrome oxidase from three species

A sequence comparison of cytochrome oxidase showing the high degree of identical amino acids between these very different species: cows and two different bacteria. The colored boxed indicate where the amino acids are identical or similar among the three sequences and the different colors refer to different classes of amino acids.

The DNA in all organisms is chemically similar but the organization of the helix into higher order structures varies. Eukaryotes contain a larger number of histones (chromosome-binding proteins) and in many cases during cell division, compaction of the chromosome takes place, so that they are visible using light microscopy.

The basic mechanism of converting genetic information into proteins is also rather similar. Most all of the central components of this process show sequence similarities across biology, and substantial functional similarities as well. The fact that eukaryotes have nuclei means that mRNA must be transported outside of that structure before translation can begin, but the process is otherwise rather similar.

The most conserved of all structures is probably the membrane. Membranes enclose all living systems and every membrane contains amphipathic lipids. The exact chemical structure of the lipids is often different depending upon the species and its environment, but the overall arrangement of the membrane is the same. Membranes perform remarkably similar functions in all species: keeping the cytoplasm in and the environment out. Cholesterol is common in the membranes of eukaryotes, but is uncommon in bacterial and archaeal membranes.

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