Unlike viruses, bacteria are cells. They are single-celled organisms. The bacterial genome consists of a single chromosome. But unlike the eukaryotic genome, the bacterial chromosome is not enclosed in a membrane. This characteristic makes bacteria prokaryotes. The chromosome is usually circular and is tightly wound in a region of the cell called the nucleoid.

A plasmid is a circular piece of DNA found in bacteria apart from the nucleoid. Plasmids carry additional pieces of genetic information. Plasmids replicate separately from the chromosome and can recombine with other plasmids. However, some plasmids, called episomes, can reversibly incorporate themselves in the bacterial chromosome. Episomes can replicate either as a part of the chromosome or separately.

Plasmids are usually beneficial to the bacterial cell. Some plasmids allow the bacterial cell to be resistant to certain antibiotics and can help the cell survive in unfavorable environments. As we’ll see in a moment, plasmids can be transferred from one cell to another.

Bacteria reproduce through a process called binary fission. The cell first replicates its chromosome. The two chromosomes are attached to the cell membrane and begin to move apart from each other. The membrane then starts to separate into two cells. In this process, the daughter cells contain duplicates of the parent’s chromosome. There is no change in the chromosome, except where mutations occur.

Bacteria are also able to transfer genetic information to each other. In the process of conjugation, genetic information is passed between bacterial cells that are temporarily joined. The cells are joined by a pilus, a thin structure that extends from one cell to another. The donor cell uses the pilus to attach to the recipient cell and draw it close. It then transfers plasmids and small pieces of bacterial chromosome through a bridge of cytoplasm into the recipient cell.

There are other ways for bacteria to change their genome. Transformation is the process by which bacteria are changed by the uptake of free foreign DNA. The foreign DNA, usually plasmids, can come from the environment or from dead bacteria.

Transformation is used widely in the lab to insert specific plasmids into bacteria. The bacteria could then replicate the plasmid as well as synthesize any proteins that the plasmid codes for.

Phages can also help change bacterial genomes. Transduction is the process by which phages transfer bacterial genetic information from one host cell to another. There are two types of transduction processes. One type is called general transduction and the other is called restricted or specialized transduction.

General transduction occurs as a result of the lytic cycle of phages. Recall that during the lytic cycle, the host’s DNA is cut up into small pieces. Sometimes the small pieces are packaged into the capsids of the new viruses. When the virus infects a new host cell, it injects the DNA fragment that came from the previous host cell.

Restricted transduction occurs as a result of the lysogenic cycle of phages. When the prophage separates from the chromosome, it could take with it bacterial genes that were located next to it. When the virus infects a new host cell, it carries with it a piece of bacterial DNA from the previous host cell.

The bacterial genome is organized so that functionally related genes are together. Operons are stretches of DNA that contain both regulatory sequences and structural genes that code for polypeptides. Regulatory sequences control the level of transcription of structural genes by blocking RNA polymerase, or by allowing it to bind to the DNA sequence. This organization of the genome into operons is important because it allows functionally related genes to be regulated together.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education