6.2: Some chromosomes are circular, others are linear.

As a method for storing genetic information, DNA has complete market saturation. All life on earth uses it. This is pretty remarkable considering the tremendous diversity of life that exists on our planet—from single-celled bacteria to multicellular plants and animals. One way in which different organisms’ DNA varies is in how it is organized into chromosomes.

The most important part of a eukaryotic chromosome may be the DNA molecule, which carries information about how to accomplish the processes needed to support the life of the organism. But eukaryotic chromosomes (and some prokaryotic chromosomes) are made of more than just DNA. The eukaryotic chromosome is composed of chromatin, a linear DNA strand bound to and wrapped tightly around proteins called histones, which keep the DNA from getting tangled and enable it to be tightly and efficiently packed in an orderly manner inside the nucleus. Plants and animals usually have between 10 and 50 chromosomes (although there are species with as few as 2 and others with more than a hundred).

Most prokaryotes—the bacteria and archaea—have less DNA than eukaryotes. They carry their genetic information in a single, circular chromosome, a strand of DNA that is attached at one site to the cell membrane (FIGURE 6-3). And when it is time for them to reproduce, they use a method called binary fission, which means “division in two” (FIGURE 6-4). This process begins with replication, the method by which a cell creates an exact duplicate of each chromosome.

Replication in prokaryotes begins as the double-stranded DNA molecule unwinds from its coiled-up configuration. Once the strands are uncoiled, they split apart like a zipper, with bases exposed on each of the two separated, single-stranded circular molecules of DNA. As the double-stranded molecule unzips, enzymes bind to the DNA and attach free-floating nucleotides to the growing DNA backbone, matching A to T and G to C, thus creating two identical double-stranded DNA molecules.

The two newly created circular chromosomes attach to the inside of the plasma membrane at different spots from each other. The original cell, called the parent cell, then pinches in until it divides into two new cells, called daughter cells. Each of the daughter cells has an identical two-stranded copy of the original two-stranded circular chromosome.

In some prokaryotes, such as the bacteria called E. coli that live in our digestive system, the complete process of binary fission can occur very quickly—often in as little as 20 minutes. Binary fission is considered asexual reproduction, because the daughter cells inherit their DNA from a single parent cell and thus are genetically identical to the parent.