"An organic being is a microcosm, a little universe formed of a host of self propagating organisms inconceivably minute, and as numerous as the stars in heaven."
- Charles Darwin
Animals and Plants ii, p483 (1868)
DNA in Prokaryotes and Eukaryotes
Eukaryote Membrane Structures
From the Merck Manual
Ciliate Shapes (Eukaryotes)
Bar at right is 1 mm.
Source: P. Eigner
used by permission
INNOVATIONS IN THE EUKARYOTIC CELL
|Membranes enclose a number of
specialized parts of the cell -- the nucleus and the various organelles
of a typical cell. The nuclear envelope controls the passage of
molecules between the nucleus and the rest of the cell using access
ports that recognize particular molecules.
||Error-detecting and correcting
schemes ensure the accuracy of the dna and rna copies. In bacteria,
insertions and changes to the dna code occur frequently, because the
dna is located in the cell cytoplasm and can come in contact with
portions of dna derived from food and viruses. In addition the
transcription of the bacterial dna is more prone to errors. Overall for
a bacterial dna, transcription accuracy is on the order of one
transcription error per ??? codons. In contrast, overall
transcription accuracy for a eukariotic cell is on the order of one
error per billion codons. [CHECK]. This still results in about ???
errors per cell per second. [CHECK]
||• Provides structure to the cell
• Provides an internal transport network
Uses the Kinesin linear motor molecule to transport large molecules between the cell wall and organelles.
Allows task specialization in the organelles.
|sex - mitosis/meiosis
The nucleus protects the DNA from damage by contact with food or cell invaders. When the DNAs genes are copied prior to building the various proteins and other complex molecules of life, the genes are processed within the nucleus to remove un-needed information, and to correct copying errors.
It is hard to overemphasize how this contrasts with bacteria. The DNA of bacteria comes in constant contact with the cell contents, and as a result is subject to both random and deliberate changes in the DNA code itself. That is how viral infection works: a virus injects its genetic material into a bacterial cell, and the material then inserts itself into the cell's own DNA. From this point the cell begins to reproduce the virus, using the cell's own genetic machinery.
Bacteria are designed in this loose genetic way because
strong points about bacteria is the ability to respond to environmental
changes by changing its genetic make-up. Bacteria can even share
of DNA from other bacterial species, perhaps through snips of DNA that
enter the cell as food. This is why bacterial species, such as the
E. Coli have so many different subspecies, both harmful and beneficial.
One biologist, Lynn Margulis, argued that the concept of species is not
really appropriate for bacteria because there is so much genetic
She notes: "Because bacteria that differ in nearly every
trait can receive and permanently incorporate any number of genes from
each other or from the environment, the concept of "species,"
to named eukaryotes, seems inappropriate for the Prokarya.
For higher species, that advantage is overshadowed by the need
to guard the genetic code's accuracy. The code is much more complex,
changes are very likely to be unwelcome. So for eukaryotes, the
is on limiting changes in the code and controlling the accuracy when
code is copied. This work takes place within the nucleus.
More remarks along these lines will be made in the next
Typical Eukaryotic Cell
Eigen's Paradox -- Error Correction Coding
Error correction mechanisms in prokaryotes and eukaryotes. Eigen's paradox.
When mid-19th Century biologists looked at cells through a microscope, they saw the cell protoplasm as an amorphous jelly. The prominent evolutionary proponent Ernst Haeckel viewed the protoplasm as the "life force" of living cells, and assumed that it, and therefore the essence of life, to be essentially simple[FOOTNOTE: Ernst Haeckel, The History of Creation (1876): Vol. I, On the Protoplasm Theory, p.99ff.  "protoplasm (the original slime) is the most essential (and sometimes the only) constituent part of the genuine cell." [p406] "the general explanation of life is now no more difficult to us than the explanation of the physical properties of inorganic bodies."John Theodore Merz, A History of European Thought in the Ninetenth Century (1907-1914): Vol. II. Chapter 10 "On the Vitalistic View of Nature" pp 444ff. The term "protoplasm" was coined by Hugo von Mohl in 1846 for the "visible but apparently structureless forms of cells and protoplasm".]. However, by the end of the 19th century it was generally understood through numerous scientific investigations that there is much more structure and content to the protoplasm[FOOTNOTE: George L. Goodale, Protoplasm and its History (Botanical Gazette Vol. XIV No. 335, Oct. 1889) Pdf (2.8 Megs) "Protoplasm is no longer regarded by any one in any sense as a comparatively simple substance. ... By better methods of staining, and by the use of homogeneous immersion [compound microscope] objectives, the apparently structureless mass is seen to be made up of parts which are easily distinguishable. There has been, and in fact is now, a suspicion that some of these appearances, under the influence of staining agents, are post-mortem changes, and do not belong to protoplasm in a living state. But it seems to be beyond reasonable doubt that protoplasm is marvellously complex in its morphological and physical as well as its chemical constitution.].
The basic problem is the resolution of light microscopes which can only see things down to a dimension of a few microns -- the size of small bacteria. Therefore the elaborate structure within a bacterial cell was almost completely invisible.
The structural content of the "protoplasm" is built up of cytoskeleton threads that are only a few nanometers in diameter (a few molecules across), and can be viewed only with electron microscopes, first built in the 1940s. Even then, the essentially colorless threads can only be viewed if they were tagged with dyes or doped with heavy atoms such as gold.
The cytoskeleton performs a number of functions in the cell (see Figure 8): it provides:
• Structure, support and spatial organization;
• Food and waste transport between cell organelles and the cell wall;
• Contraction, dilation and movement;
The food and waste transport involves the Kinesin transport molecule which is a linear motor that carrys waste and food along the microtubules which connect the cell wall and all internal organelles.
The Kinesin molecule transports food and wastes between the organelles of a eukaryotic cell, moving along microtubules of the cell's cytoskeleton (Figure 9).
Kinesin motors were first discovered by accident in 1981[FOOTNOTE: The description by Pamela Clapp neglects to note that Dr. Allen's wife Nina worked closely with him and participated in the discovery] by a Dartmouth professor, Robert D. Allen, when he used a television camera to view squid nerve fiber under a light microscope. By adjusting the image brightness it was discovered that details could be seen that are a tenth of the size that is normally visible in a light microscope, and for the first time, it was actually possible to see little round objects moving along the nerve fiber. These turned out to be kinesin.
Bacteria rely on ordinary diffusion to move food and waste within a cell. Diffusion depends on random movement due to molecular collisions, and is typified by the dispersion of a dye in a beaker of water (Figure 10):