All living organisms have DNA, but not all of them, especially prokaryotes, such as bacteria, have nucleus in their cells. Why can’t DNA leave the nucleus then? Where is DNA located on organisms without nucleus? To answer these questions, one thing must be kept in mind: DNA stays in the nucleus forever and it will never leave the nucleus. We will explore more about this below to make the matter more obvious and easier to understand.
DNA on Prokaryotes
The cells of eukaryotes consist of nucleus and cytoplasm. Prokaryotes, on the other hand, don’t have nucleus and therefore their cell consists only of cytoplasm and everything in it. Normally, DNA stays in the nucleus; however, because prokaryotes’ cell lacks a nucleus, the DNA is located in the cytoplasm with other cell parts. Even so, the location of prokaryotes’ DNA in their cell is unique compared with the rest of the cytoplasm. This unique region is often called nucleotide and the DNA never actually leaves this region. Therefore, although prokaryotes’ cell doesn’t have a nucleus, their DNA can theoretically be assumed to occupy a nucleus-like region perpetually.
The core premise that DNA never leaves the nucleus is thus preserved, both on prokaryotes and eukaryotes. But why can’t DNA leave the nucleus (or nucleoid)? There are many reasons for that and the more you understand the reasons, the more obvious the DNA’s role in cell division process will be.
DNA Plays an Essential Role in the Cell Division Process.
One very obvious reason why DNA never leaves the nucleus is because it plays an essential role in the cell division process. Nucleus is the only cell part that can initiate cell division. In other words, the whole process of cell division starts at its nucleus (and centrosome on animals). When a cell divides, each daughter cell must carry the genetic information of the parent cell. Because genetic information is recorded in the DNA, DNA must perpetually stay in the nucleus.
In fact, the replication process of DNA occurs just before a cell divides, so it can be said that the initiation of a cell division process actually occurs in the DNA. Only after the DNA replication process concludes can the cell division process in the nucleus can be started.
DNA Replication Is Carried Out by a Series of Enzymes in the Nucleus.
DNA replication is catalyzed by a series of DNA polymerase enzymes that are only found in the nucleus. To safeguard the replication process, to avoid DNA damage, and to prevent any possible anomalies in the replication process, DNA must stay in the nucleus. The fact that DNA never leaves the nucleus can be regarded as a measure to protect DNA and its activities from any possible damaging effects in the cellular level.
The Physical Characteristic of DNA Requires It to Stay in the Nucleus.
Why can’t DNA leave the nucleus? Physically, DNA is attached to the inner membrane of the nucleus. There is no way DNA can thrive in the cell other than attaching itself on this membrane. Besides, DNA, even as a molecule, is way too big for the nuclear pores to allow it to escape. There are indeed nuclear materials that can leave and enter the nucleus through these pores through the process of diffusion. The size of chromosomal DNA is always much bigger than these pores and therefore can never escape the nucleus.
There Is No Mechanism to Push the DNA Out of the Nucleus.
The only mechanism that allows cellular materials to enter and exit the nucleus is called diffusion. DNA is not affected by this mechanism because it is attached to the inside of the nucleus and because its protein structure. There is no mechanism to cut the DNA from the nucleus and there is also no mechanism to strip away the proteins. Why can’t DNA leave the nucleus? With no mechanism to allow DNA to escape the nucleus, DNA will always stay in the nucleus.
Nuclear Receptors Act as a “Customs Agency” in the Nucleus.
Receptors on the nuclear envelope constantly regulate what can leave and enter the nucleus. Although there is a diffusion mechanism that allows cellular materials to enter and exit nucleus, these exit and entry movements don’t occur randomly and automatically. If a certain material is not permitted by these receptors to leave or enter the nucleus, it can’t. DNA is not among the materials that have the permission to leave and enter the nucleus. RNA, on the other hand, can leave and enter the nucleus to perform its functions, but this leaving and entering movements don’t occur without regulation. As a messenger, RNA exits and enters the nucleus because there is something that regulates it. RNA also has a passport in the form of the 5’ cap methylated G nucleotide that allows it to undergo this transportation process. Why can’t DNA leave the nucleus? Because DNA doesn’t have this passport.
Proteins are needed for the DNA replication process to occur properly. The synthesis of these proteins requires communication with other cellular parts outside the nucleus. This, however, doesn’t necessitate the DNA to leave the nucleus. The nucleus will send mRNA (messenger RNA) to leave it and to collect everything necessary for protein synthesis.
Is There DNA Outside the Nucleus?
One interesting question about DNA is that if DNA can never leave the nucleus, is there actually DNA that originally stays outside the nucleus? DNA does never leave the nucleus, but this doesn’t mean that there is no DNA outside the nucleus. Certain cellular parts, especially mitochondria (rod-shaped bodies that are found in the cytoplasm and convert food into energy) and plastids (organelles in photosynthetic plant’s cells that play a role in the synthesis of food), also have their own DNA. Just like the nuclear DNA, the DNA found in the mitochondria and plastids can replicate on their own. And just like the nuclear DNA, the mitochondrial and plastids DNA also cannot leave its respective host.