An Overview of Biological Basics - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 25

TABLE 2.1 A comparison of Prokaryotes with Eukaryotes

Characteristic Prokaryotes Eukaryotes

Genome

No. of DNA molecules One More than one

DNA in organelles No Yes

DNA observed as chromosomes No Yes

Nuclear membrane No Yes

Mitotic and meiotic division of the nucleus No Yes

Formation of partial diploid Yes No

Organelles

Mitochondria No Yes

Endoplasmic reticulum No Yes

Golgi apparatus No Yes

Photosynthetic apparatus Chromosomes Chloroplasts

Flagella Single protein, simple

structure

Complex structure,
with microtubules

Spores Endospores Endo- and exospores

Heat resistance High Low

Source: Millis NF in Comprehensive Biotechnology, M. Moo-Young, ed., Vol. I, Elsevier Science, 1985.

Evidence suggests that a common or universal ancestor gave rise to three distinctive
branches of life: eukaryotes, eubacteria (or “true” bacteria), and archaebacteria. Table 2.2
summarizes some of the distinctive features of these groups. The ability to sequence the
genes of whole organisms will have a great impact on our understanding of how these fami-
lies evolved and are related.
The cellular organisms summarized in Table 2.2 are free-living organisms and are all DNA
based. Viruses cannot be classified under any of these categories, as they are not independent
(or free-living) organisms. Still, viruses are all nucleic acid (either DNA or RNA) based.
Prions, on the other hand, are not even nucleic acid based. Let's consider first some of the
characteristics of these rather simple “organisms.”

2.1.3. Viruses
Viruses are very small and are obligate parasites of other cells, such as bacterial, yeast,
plant, and animal cells. Viruses cannot capture or store free energy and are not functionally
active except when inside their host cells. The sizes of viruses vary from 30 to 200 nm. Viruses
contain either DNA (DNA viruses) or RNA (RNA viruses) as genetic material. DNA and
RNA molecules will be discussed in x 2.3.6 in more detail. In free-living cells, all genetic