Understanding the Basics Of Bacteria
Prokaryotes, which include bacteria and archaea, are usually unicellular organisms that lack cell nuclei. Individual prokaryotic cells may exist as single units or remain associated with each other after cell division forming pairs, chains, or clumps.
Bacteria are widespread on Earth, and several species are known to cause many types of infectious diseases. Many species of archaea are also known, though they are less common than bacteria.
The genes of bacteria are found within structures known as bacterial chromosomes. Although a bacterial cell usually cell usually has a single type of chromosome, it may have more than one copy of that chromosome.
The number of copies depends on the bacterial species and on growth conditions, but a bacterium typically has one to four identical chromosomes. Each bacterial chromosome is tightly packed within a distinct nucleoid region of the cell.
IMPORTANT: Unlike the eukaryotic nucleus, the bacterial nucleoid is not a separate cellular compartment bounded by a membrane. The DNA in the nucleoid is in direct contact with the cytoplasm of the cell.
Bacterial chromosomes have a single assembly of several proteins that are required or DNA replication.
Bacteria Reproduce Asexually by Binary Fission
A bacterial cell has only a single type of chromosome and simple division process called binary fission. Before it divides, the cell replicates its DNA. This produces two identical copies of the chromosome.
Next, the cell’s plasma membrane is drawn inward and deposits new cell-wall material, separating the two daughter cells. Each daughter cell receives one of the copies of the original chromosome.
Except when a mutation occurs, each daughter cell contains an identical copy of the mother cell’s genetic material. Binary Fission does not involve genetic contributions from two different parents.
Three Modes of Genetic Transfer Enhance Genetic Diversity in Bacteria
Even though bacteria reproduce asexually, they exhibit a great deal of genetic diversity. Within a given bacterial species, the term strain refers to a lineage that has genetic differences compared to another strain.
One strain of E. coli may be resistant to antibiotic while another strain may be sensitive to the same antibiotic.
Genetic diversity in bacteria primarily comes from two sources.
- Mutations can occur that alter the bacterial genome and affect the traits of bacterial cells.
- Diversity can arise by genetic transfer, in which genetic material is transferred from one bacterial cell to another.
Genetic transfer occurs in three different ways.
- Conjugation- involves a direct physical interaction between two bacterial cells. During conjugation, one bacterium acts as a donor and transfers DNA to a recipient cell.
- Transformation- DNA that is released into the environment when a bacterium dies is taken up by another bacterial cell.
- Transduction occurs when a virus infects a bacterial cell and then transfers some of that cell’s DNA to another bacterium.