Bacterial genomes are the genetic information of a bacterium, organized into genes and chromosomes made up of nucleotides. Bacteria have a circular chromosome structure and can evolve through genetic recombination or mutations. DNA sequencing has been used to sequence hundreds of bacterial genomes since the first in 1995.
A bacterial genome is the collection of the entire genetic information of a bacterium. In essence, it determines the appearance and functions of a bacterium, both externally and internally. This genetic information is organized into genes, which are encoded in the body’s deoxyribonucleic acid (DNA). These genes are further organized into chromosomes. In fact, the word “genome” is actually a portmanteau of the words “gene” and “chromosome.” Although there are many different variations on how genomes are structured in different organisms, all bacteria are haploid, meaning they have only one chromosome. Hence, all the genetic information contained in a bacterial genome is contained in a single chromosome.
The chromosome of a bacterium is made up of base pairs of nucleotides, the basic structural components of DNA. There are only four nucleotides in DNA: adenine, thymine, guanine and cytosine. These nucleotides always bind in the same way: adenine binds to thymine and guanine to cytosine. All four together form the so-called “base pair”.
Bacterial genomes can contain any number of base pairs. Some bacterial genomes contain fewer than 200,000 base pairs, while others contain over 12 million base pairs. As a result, the number of genes found in bacteria is highly variable, with anywhere between 575 and 5,500 genes reported in bacteria, depending on the species. By way of comparison, the human genome contains over 3 billion base pairs and approximately 23,000 genes.
Interestingly, most bacterial genomes are organized in a circular chromosome structure. Humans, by contrast, have a linear chromosome structure. The circular structure of a bacterial genome, however, allows its DNA replication to start and stop at the same location, a feature not seen in the linear genomes of other organisms.
As asexually reproducing organisms, bacteria are essentially clones with DNA identical to their parent’s genome. Changes to a bacterium’s genome, also known as evolution, can be made by genetic recombination or by mutations. Mutations can occur either from errors in DNA replication or from exposure to mutagens that affect how nucleotides interact with each other. Bacteria can also exchange and recombine genetic information with each other through separate processes known as transformation, transduction, or conjugation, depending on how the genetic exchange occurs. Changes to the genome of a bacterium can lead to the formation of a new species.
DNA sequencing is the determination of the order of nucleotides in an organism’s DNA. This information has been a key component in biological research. The first bacterial genome to be fully sequenced was that of Haemophilus influenzae, which can cause a variety of opportunistic diseases such as pneumonia, meningitis and respiratory infections. It was successfully sequenced in 1995 by the Institute for Genomic Research. Since this breakthrough, hundreds of bacterial genomes have been sequenced, and scientists continue to research and sequence the genomes of other bacteria.
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