Genetics is the branch of science concerned with the passing of genes from parent to daughter cells and the composition of genes in an organism. Microbial genetics is a field that studies the genetics of microorganisms, both single and multicellular, including bacteria, viruses, archaea, protozoa, and fungi.
A gene is a unit of heritable material that codes for a particular trait. Physical traits can be controlled by a single gene or many different genes. For example, human eye color is controlled by at least 10 different genes. In contrast, the bacterium Thermus aquaticus has a single gene that codes for a heat-resistant enzyme that allows the bacterium to replicate its DNA at very high temperatures. A chromosome is a structure that contains deoxyribonucleic acid (DNA), the genetic material that is passed from one generation to the next. Genes are segments of DNA that code for specific proteins that affect cellular and organism function. This information is written in the DNA in the form of nitrogenous bases: adenine, thymine, cytosine, and guanine. These bases are arranged in groups of three, called codons, and each codes for an amino acid. The amino acids, when linked together, form the protein for which the DNA codes.
The passing of genetic material to offspring requires a reproductive event. Reproductive events can be asexual, resulting in a duplicate of the parent cell, or sexual, resulting in each parent passing a random assemblage of genes to the offspring. A large percentage of microorganisms, the prokaryotes (organisms without a distinct nucleus), reproduce asexually. Eukaryotic microorganisms, however, can reproduce sexually, asexually, or both.The genotype is the genetic makeup of an organism. The phenotype represents the observable characteristics of an organism that result from genetic and environmental influences. Phenotypes can change over the lifetime of an individual organism, but an organism’s genotype remains conserved. Phenotypic change, also called phenotypic plasticity, happens because of differential gene expression, which means certain genes can be turned on or off depending on age or environment. Phenotypic plasticity is more common in immobile organisms, such as plants, because mobile organisms can move away from hostile environments to more habitable ones. However, bacteria are known to change their shapes and sizes in response to influences in the environment. One such example is the bacterium Campylobacter jejuni, a species that causes food poisoning in humans. Normally spiral-shaped, C. jejuni may change to a spherical coccus shape when oxygen is present in higher concentrations.
Anatomy of a Gene
An organism's genotype is stored as deoxyribonucleic acid (DNA), an organic molecule containing coded instructions for the life processes of an organism. In some microorganisms, such as adenovirus or poxvirus, DNA takes the form of a double helix, where the nucleotides of two strands are bonded together. Some viruses, such as parvoviruses, contain only a single strand of DNA. Most eukaryotes have linear nuclear DNA, meaning the DNA strands have a definite beginning and an end. The DNA of prokaryotes exists in circular strands in the cytoplasm. Bacteria also have plasmids that contain genetic information apart from their chromosomes. Plasmids in most bacteria are tightly coiled, double-stranded DNA molecules. However, a few linear variants have been observed in the genus Borrelia.
A nucleotide is an organic compound consisting of a sugar, a phosphate, and a nitrogenous base. Nucleotides form the basis of a genetic sequence. The double-helix structure is formed by the joining of two single DNA strands. The sugar and phosphate form the backbone of each strand. Extending from each sugar is the nitrogenous base, and the bond between paired bases forms the center of the double helix. Nucleotides from each DNA strand bind together throughout the entire length of the double helix.There are two types of nitrogenous bases in nucleic acids, purines and pyrimidines. A purine includes two joined rings containing carbon and nitrogen. Adenine and guanine are purines. A pyrimidine includes a single ring containing carbon and nitrogen. Thymine, uracil, and cytosine are pyrimidines. In DNA, adenine always pairs with thymine while cytosine always pairs with guanine. In ribonucleic acid (RNA), thymine is replaced with uracil. The overall length of the bonded nucleotides remains the same for the entire double helix by bonding a purine with a pyrimidine. Viral genomes can exist as single-stranded DNA (ssDNA), double-stranded DNA (dsDNA), single-stranded RNA (ssRNA), or double-stranded RNA (dsRNA), and may be linear or circular.