The cell is organized into a number of different compartments by a continuous system of membranes. This system of compartmentalization is absolutely essential for cellular function and for producing the biological characteristics of an organism in conjunction with the information coded in DNA.
The image is, of course, simplistic. Even within one organelle's membrane, there are further regional divisons in structure and function, and there are many fascinating localized structures that are not shown in the picture.
What is important here, though, is another point. The information needed to produce these membranous compartments is only partly coded for in DNA. Part of it resides in the membrane itself, is heritable, and is not coded for anywhere in the DNA.
Molecular Biology of the Cell, the definitive guide to mainstream molecular biology, decribes why this is so in its twelfth chapter, “Intracellular Compartments and Protein Sorting.”
Each membranous organelle is to some degree distinguished by the types of lipids it contains, but these are in turn determined by its unique set of proteins. The proteins themselves are major determinants of the oragenelle's function. These proteins have signal sequences associated with them that direct them to specific organelles.
But here's the catch! What facilitates the matching of the signal sequence to the membranous organelle for which the protein is destined? The proteins in the membrane. That's right, if there are no protein transporters in, for example, the endoplasmic reticulum, then those signal sequences that destine a protein to the endoplasmic reticulum can't be recognized by anything and have no meaning at all.
In the case of the endoplasmic reticulum, the phenomenon is even more striking, since the endoplasmic reticulum actually has to make those proteins before they can get transported anywhere anyway.
Thus, it is not just the nuclear DNA that defines the membrane, but the membrane itself. Here's the conclusion of the authors of Molecular Biology of the Cell (p. 704) in their own words:
Thus, it seems that the information required to construct an oragenlle does not reside exclusively in the DNA that specifies the organelle's proteins. Information in the form of at least one distinct protein that preexists in the organelle membrane is also required, and this information is passed from parent cell to progeny cell in the form of the organelle itself. Presumably, such information is essential for the propagation of the cell's compartmental organization, just as the information in DNA is essential for the propagation of the cell's nucleotide and amino acid sequences.
Thus, there exists absolutely critical information that is biologically heritable that is not coded for in the DNA.
To what extent do variations in this information contribute to variation between and within species?
This is a mystery that will only begin to be unraveled when more scientists drop the myopic and completely false view that organisms are largely vehicles meant as containers for self-propagating genes — a view that was always preposterous but is now more than ever completely proven false — and join the ranks of computational systems biologists who look at physiology as an integrated whole and attempt to actually ask and answer such questions.
That's not to say that most scientists actually hold such a myopic view, but scientists who study heredity essentially universally study variations in DNA sequences. The fact that we know of many examples where variations in DNA contribute to variations in heritable characteristics and do not have a similar body of knowledge about non-genetic heritable information may simply be a result of the many people asking the first type of question and almost no one asking the second type of question rather than being a result of a greater relative frequency of that type of biological inheritance.
On the other hand, though we still have a great deal to learn about gene expression, we do have a very impressive body of knowledge about how cells access, utilize, control, and even in some cases restructure their genomes in response to their own needs and the needs of the organisms of which they are part. That will be the subject of the next post in this series.