5.2: Introduction
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The first organisms on Earth were likely unicellular prokaryotes. These organisms could reproduce themselves by increasing the amount of cytoplasm, making a copy of their single loop of DNA, and forming a new cell wall to divide the original cell into two identical cells. This form of cell division is called binary fission (binary meaning two, fission meaning to split apart). This is a form of asexual reproduction, where an organism reproduces an exact genetic copy of itself. This is also sometimes called clonal reproduction.
In binary fission, one cell enlarges and the DNA is replicated, dividing into two cells.
As eukaryotic cells evolved, cell division became more complex. There were now organelles to replicate and organize. Perhaps more importantly, division of the DNA became more complicated. Instead of a single loop of DNA, eukaryotic DNA occurs in linear strands that are then wound around histone proteins. This combination of DNA and proteins is called chromatin, because it appeared dark when stained. When eukaryotic cells divide, they further condense the chromatin into tightly wound complexes called chromosomes. This organizational structure is necessary, as eukaryotic cells need instructions to make and maintain all of the new cell components, often resulting in large amounts of DNA. For example, there are approximately six feet of DNA that need to be contained within the nucleus of each cell in your body. Six feet! If you lined up all of the DNA from each of your trillion or so cells, it would stretch across the solar system...twice! How to organize this massive amount of DNA so that it can be replicated and divided into two identical sets as a cell divides?