Although viruses and bacteria are very different types of microbes, both have genomes made of nucleic acids. These nucleic acids contain the instructions necessary for cells and viruses to exist and reproduce. But since bacteria are living cells, the expression of their genes directs a much more complex process, the business of life. Since viruses are acellular, or non-living infectious agents, their genomes direct a somewhat different ‘show.’ The following is a comparison of the expression of cellular versus viral genes.
* Metabolism of Bacterial Cells *
Bacteria, as living cells, conduct cellular business through the process of transcription and translation; the creation of RNA compliments of the bacterial genome that are then used to build the proteins necessary for bacterial cell structure and metabolism.
Segments of bacterial DNA are ‘unzipped’ whenever the genes encoded in that DNA must be expressed. RNA molecules are then built through the polymerization of new nucleotides, based on the DNA template.
The RNA is used to build the protein molecules that bacterial DNA codes for, collectively called the proteome. Some of these proteins are used to build structures of the bacterial cell; others function as enzymes required for cellular metabolism.
Metabolism is the process of breaking down food molecules to extract energy. Food energy is transformed into ATP molecules, the biological energy currency used to get work done in the cell. The enzymes used to carry out metabolism, the processes of life, are collectively referred to as an organism’s metabolome. The conversion of food energy to ATP is what enables bacteria to grow and reproduce.
* Replication of Viral Particles *
As acellular parasitic infectious particles, viruses are unable to transcribe their own genetic material and unable to build their own protein molecules. They can’t make anything on their own, and don’t metabolize, or transform energy at all. However, viruses are very adept at using the transcription and translation machinery of the cell that they infect to achieve the expression of their genome and build more viruses.
The viral genome, either made of DNA or RNA, consists of instructions for building the viral proteome. In order for more viruses to be made by an infected host cell, the host is directed to make more copies of the viral genome and then translate those genetic instructions into proteins used to make viral capsids, the protective structure surrounding the viral genome. The subunits of the capsid, called capsomeres, spontaneously arrange around a copy of each viral genome. The new viruses then exit the host cell to go infect other cells and continue the cycle.
The genome of DNA viruses must first be transcribed into RNA that is then translated at the bacterial ribosomes, where proteins are made. RNA viruses already exist as a transcript. These transcripts must be copied by the bacteria, but the process of transcription (making RNA based on the instructions in DNA) is skipped for RNA viruses.
* Sources *
Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings.
Park Talaro, K. (2008) Foundations in Microbiology.