All living things use DNA or RNA to store genetic information. But eukaryotes are different from bacteria and archaea in the organization of their DNA as well as in how they replicate DNA and pass it down to their offspring.
The name eukaryote points to one of the most obvious things that set eukaryotes apart from other forms of life. In Greek, it means “true kernel,” referring to the nucleus in which eukaryotes keep their DNA tightly coiled. In bacteria and archaea, a single circular chromosome floats within the cell. It is not constrained by a nucleus, nor is it tightly spooled around histones. That does not mean this DNA is simply a loose tangle. Bacteria and archaea produce proteins that keep sections of DNA organized in twisted loops. Like eukaryotes, bacteria and archaea can regulate gene expression by unwinding and winding their DNA.
Also like eukaryotes, bacteria and archaea have genetic regulatory regions upstream from their genes. Transcription factors can trigger dramatic changes in gene expression through regulatory cascades. Bacteria and archaea can alter their gene expression in response to signals from their environment. As a result, some species can produce spores when conditions turn stressful. Others can produce toxins when they sense other microbes competing for resources.
Overall, however, gene regulation is less complex in bacteria and archaea than it is in eukaryotes. Bacteria and archaea lack enhancers, for example, which can be located thousands of base pairs away from genes they control in eukaryotes. Bacteria and archaea have self-splicing introns but lack the abundant spliceosomal introns found in eukaryotes, which require a group of proteins called the spliceosome to remove them from transcripts. Bacteria and archaea thus lack the alternative splicing found in eukaryotes. As a result, they always produce the same protein from any given gene.
Replication in bacteria and archaea is also simpler. They do not perform mitosis or meiosis. They do not have full-blown sexual reproduction, in which males and females produce gametes that combine in a new offspring. Instead, bacteria and archaea typically grow until they are large enough to divide. They then build a second copy of their circular chromosome and then the two DNA molecules are dragged to either end of the dividing cell. The two daughter cells are identical to the original, except for any mutations that arise during DNA replication.
Bacteria and archaea have many of the same kinds of mutations found in eukaryotes, such as point mutations and insertions. But they cannot acquire genetic variation as a consequence of sexual reproduction the way we see in some eukaryotes (i.e., through independent assortment of chromosomes).
Beneficial mutations that increase the survival or reproductive rate of bacteria can sweep quickly through a population of microbes, thanks to natural selection.
One reason antibiotic resistance can spread so quickly is that bacteria are not limited simply to passing down their genes to their descendants (known as vertical gene transfer). It’s also possible for one individual microbe to “donate” DNA to another, through a process called horizontal gene transfer.