I have discussed “Thing one and Thing Two” in previous posts. However, I think it is befitting to talk about them some more, in light of how nuclear envelopes are put together. Nuclear envelopes are double membranes that encompass all of the chromatin within the nucleus. If you’ve been keeping up with my posts, you should know that chromatin is where all the chromosomal DNA is located. However, nuclear envelopes are not just spherical membranes or simple bags of DNA. They form tubular invaginations that form a complex network within the nucleus. If these tubules were released from a dying cell, would they look like this?
If so, how exactly is the chromatin arranged within them? Obviously, some of the tubules have empty spots where the contents have been completely destroyed. Others contain material that lights up just like DNA. Green color is intact DNA, orange indicates necrosis and/or apoptosis. These apoptotic bodies appear to be moving in a linear fashion along the long axis of these tubules until some of them exit to the outer tips. Apoptosis is programmed cell death involving the orderly sequestration and destruction of chromatin fragments. The question here is this: Are these apoptotic bodies randomly produced or do they tell us something about chromatin organization within the nucleus? Are they remnants of endosymbionts? (see last post under this one).
This linear arrangement of discrete bodies can be seen in other forms as well (see above). Sometimes they are generated from circular structures (lower right). Do they have any connection to nuclear tubules within the nucleus? Nuclear bodies like cajal bodies exist as little DNA manufacturing plants within the nucleus and may provide a clue as to what constitutes an apoptotic body, i.e., something that must be sequestered and destroyed when it no longer serves a useful purpose such as during cell death. Note how these bodies all line up in a linear fashion, much like you see in mitotic chromosomes. This linearity may be a clue as to why geneticists still assume that the DNA itself is linear within chromosomes. However, the photomicrographs below fly in the face of such assertions.
In my last post, I mentioned an elephant. It referred to how bacterial chromosomes could be integrated into eukaryotic DNA when the nuclear envelope only allows very small molecules to pass through it via nuclear pores. That is indeed quite an elephant! However, these nuclear pores are not always present throughout the cell cycle. During the onset of mitosis, the nuclear envelope begins to disintegrate as the chromatin begins to condense into chromosomes. Nuclear tubules may be opened up at that point just like what you see in the first photomicrograph in this post where apoptotic bodies have migrated to what appear to be exit points. Such “exit” points may also be used as “entry” points as well for large chunks of foreign DNA to enter nearby cells, hence the need for apoptosis to avoid genetic corruption leading to cellular dysfunction and possibly cancer. In multicellular organisms like plants and animals, cancer must be avoided at all costs because it screws up the viability of the entire organism. Thus, foreign DNA must be kept out at all costs. However, in more primitive unicellular organisms, foreign DNA may have provided an evolutionary advantage over similar competitors by providing them with new kinds of “nuclear bodies” that enhanced metabolic function or allowed them to digest previously unavailable sources of food, etc. In these cases, the foreign DNA may have been welcomed to travel into nuclear channels where they attached themselves to an adjacent nuclear body, thus increasing the length of the parent chromosome. In fact, such nuclear upgrades may have allowed the development of specialized cells that function together as a multicellular organism. When a complete multicellular organism is formed, apoptosis comes into effect to prevent further entry of either outside DNA or nuclear body DNA released from adjacent dying cells.
Ok, so what is going on with all those circles of “DNA”? How do they fit into the picture of nuclear bodies? Well, sorry. You’ll just have to wait for the next post! Or you could start from scratch and read this blog from top to bottom.