Biology Reference
In-Depth Information
Fig. 4.8
that all organisms are derived from one common ancestor and are related to one
another - the basic predictions of Darwin's evolutionary theory.
Figure 4.8 contains some similar terms that are often confused, even by some
biologists. The essential point to remember is that the terms “prokaryote” and
“eukaryote” describe basic structural differences between two types of cell, while
the terms “Archaea”, “Bacteria” and “Eukarya” describe evolutionary relationships
as deduced from DNA sequences. Eukaryotic cells in general are larger and more
complex than prokaryotic cells, and contain a number of internal organelles, such
as a nucleus, mitochondria, lysosomes, endoplasmic reticulum, Golgi bodies and
a complex cytoskeleton, which are not found in prokaryotic cells. Plant cells con-
tain, in addition, structures termed plastids, of which the chloroplast is the most
studied example. Figure 4.9 illustrates these striking structural differences between
prokaryotic cells and eukaryotic cells.
Eukaryotic cells have their DNA separated from the cytoplasm by a nuclear mem-
brane, but prokaryotic cells have their DNA in direct contact with the cytoplasm.
The cytoplasm is defined as everything outside the DNA. This difference affects
the relation between transcription and translation, because transcription takes place
only in the nucleus of eukaryotes, while translation takes place only in the cyto-
plasm. In contrast, transcription and translation take place in the same compartment
in prokaryotic cells, and so these processes can be coupled together - a messen-
ger RNA molecule can be translated into protein while it is still being synthesized
from a DNA template. Most prokaryotes are unicellular, and so the close coupling
of transcription with translation in the same compartment allows these cells to grow
