Genetics of Bacteria Help

By — McGraw-Hill Professional
Updated on Aug 23, 2011

Characteristics of Bacteria

Each cellular organism is classified as either prokaryote or eukaryote. A prokaryote is generally a single-celled organism whose DNA is not confined within a true nucleus. A eukaryote can be single-celled or multicelled and has its genetic material isolated from the rest of the cell by a nuclear membrane. All bacteria are prokaryotes. There are two main prokaryotic groups—the Eubacteria (true bacteria) or Bacteria and the Archaebacteria (ancient bacteria) or Archaea. These two groups are distinguished based on their cellular structures and DNA sequences. The Eubacteria contains most of the organisms we typically identify as bacteria, such as the common gut microbe Escherichia coli, and the causative agent of strep throat, Streptococcus pyogenes. The Archaea contains organisms that have been hypothesized to be evolutionarily older, or at least distinct, from the Eubacteria, such as the methanogens (bacteria that produce methane). All other forms of life (fungi, plants, animals) are eukaryotes in the Eukarya group. Bacteria are part of the larger category of organisms called microorganisms. This category includes bacteria as well as some smaller eukaryotes, such as the fungi, protozoans, and algae.

Most bacterial cells have a wall surrounding their plasma membrane. The wall contains a unique chemical called peptidoglycan (also called murein). There are two main types of bacteria based on the structure of their walls, and, in part, on the amount of peptidoglycan in their walls. The Gram-positive bacteria have a thick peptidoglycan layer, while the Gram-negative bacteria have a thinner peptidoglycan layer with an extra outer membrane. The antibiotic penicillin is effective against bacteria because it disrupts the synthesis of peptidoglycan.

Bacteria do not reproduce sexually (i.e., by formation of haploid gametes produced by meiosis, and fusion of gametes to form diploid zygotes) and do not use mitosis as a mechanism for cellular division. The bacterial chromosome does not condense, it has no centromere, and no spindle develops. Instead, the circular bacterial chromosome is replicated and, as the cell elongates, new cell wall material is laid down and the chromosome copies move apart in a process called binary fission. Bacteria can divide much more rapidly than eukaryotes (once every 20 min under ideal conditions, in contrast to 24–48 h or longer for many eukaryotic cells). Bacteria are typically about 1 μm wide and 1–5 μm long. Some bacteria can be as wide as 50 μm and several millimeters long, but this is rare. As mentioned earlier, bacteria are primarily single-celled organisms; however, there are exceptions. The Actinomycetes contain bacterial species that form filamentous, multicellular structures and, more recently, many different types of bacteria previously known only as single-celled organisms have been observed to form multicellular, slimy structures called biofilms. Biofilms are difficult to destroy due to difficulty in penetrating their multilayer, complex structure. They have been observed to accumulate in pipes used for food processing, medical catheters, or in patient tissues—resulting in difficult-to-treat diseases. In fact, symptoms of cystic fibrosis are thought to be due, in part, to biofilm accumulation in the patient's lungs.

Most of the genetic information of a bacterial cell resides in a single, circular, double-strandedDNAmolecule, commonly referred to as the bacterial chromosome, in a region of the cell called the nucleoid. Some bacterial DNA is complexed with basically charged proteins to form a kind of bacterial chromatin analogous to the association of histones and other chromosomal proteins with DNA in eukaryotic chromatin. Some bacteria may also contain small, self-replicating DNA circles called plasmids. There seldom are any membrane-bound organelles in bacterial cells.

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