DNA Structure, Replication, and Technology
The Theme of Levels of Organization in DNA Structure, Replication, and Technology
DNA is convenient for living things to use due to its clearly defined levels of organization. At the molecular level, there are 4 nucleotide bases: adenine, guanine, cytosine, and thymine, which are all linked to a ribose sugar and phosphate backbone. These nucleotides are joined in various sequences that are highly important for determining the genetic information that they encode. These strings of double-stranded nucleotides form what we call DNA in most organisms. If left unpackaged, the amount of DNA in a eukaryotic organism would not fit into a eukaryotic cell.
Therefore, higher levels of organization are necessary, such as the wrapping of DNA around histone proteins, forming a nucleosome core. These nucleosomes are further wrapped around each other to condense into what we call a chromatin fiber. The chromatin fiber condenses into what we identify as a chromosome in eukaryotic cells. This level of condensation only exists during mitosis and meiosis, where it is important that there be no stray bits of DNA floating around. When you move, you make sure to put everything into boxes, and that is exactly what the cell is doing. When it divides, it wants to make sure each daughter cell has the right amount and type of DNA necessary for survival.
However, most of the life of the cell is spent outside of mitosis (in interphase), so the levels of packaging are quite variable. Depending on the needs of the cell, certain regions of the chromosome are heavily packaged to silence them and prevent transcription from taking place, while other regions that make important proteins are loosely packaged and open for RNA transcription. This variable packaging process is important for regulating RNA transcription of various genes.
Another level of organization in eukaryotic cells is the presence of the nucleus. The fact that all the DNA in the cell is found only in the nucleus, though some exists in the mitochondria—that is a story for another day—while all the machinery for making proteins like ribosomes are only in the cytoplasm, provides another tight level of regulation. The amazing part of eukaryotic cells is the intricate ballet that leads to protein expression. This ballet in eukaryotic cells causes them to evolve at a slower rate than prokaryotic cells and viruses; there are many checkpoints that must be cleared for protein expression to take place in the eukaryotic cell.
Prokaryotic cells, with all due respect, are like our sloppy cousins because they lack the levels of organization that eukaryotes do. They do not have a nucleus, so their DNA, RNA, and ribosomes can all easily interact. In fact, their transcription and translation machinery occurs simultaneously, which potentially leads to damaging consequences if there are any errors in RNA transcription. Their chromosomes are also not tightly packaged because prokaryotes lack histones, and they typically have extrachromosomal plasmids that can integrate into the chromosome or can be removed. It almost seems like prokaryotes were designed to make mistakes because, unlike most eukaryotes, survival of prokaryotes is more heavily biased toward success of the species. Therefore, mutations that lead to death of specific individuals are more tolerated in prokaryotes than in eukaryotes, primarily due to the shorter life span of prokaryotes. Prokaryotes are more accepting of death, like emo vampires.