........................................................... Monday, 19th October, 2020.
THE INVISIBLE WORLD
From prehistoric times human beings have lived with microorganisms earlier than they recognized them. Evidence continues to show that our ancestors had notions about the invisible world. They always thought somehow there is something that was able to cause diseases. They used this kind of reasoning to come up with herbs, burnt to ashes and applied those ashes on the cuts which they used to make on the skin of a patient. Several historical evidence shows that acient people knew that diseases can be communicable. This is seen in the biblical cases when people attempted to contain the leprosy by quarantining lepers. What they never knew was the mystery behind the powers which caused those diseases. They quarantined lepers fearing that the air that lepers breath would cause other people to get sick as well. However, always thought disease was the result of angering the gods or unfaithfulness to the people. That's according to the African culture.
Microorganisms are small living entities that can not be seen with naked eyes. Microorganisms are everywhere in and out of your body, infact the skin is the home of trillions of them and there is more than a thousand species of microorganisms in the mouth. Most of those microorganisms are harmless, those are the ones we share a mutual symbiotic relationship with and they form a backbone of food chains and food webs. Those ones we can't live without. Some of those are used in biofuels, medicines and foods like; bread, cheese and beer. Some microorganisms are helpful whilst some are harmful and can even kills. Therefore, it is important that we launch a discussion on the invisible world.
Although much about the invisible world is known now than ever in the development of mankind, the bigger part of the invisible world remains unexplored. Therefore, this research is the description of mesosomes, plasmids and later states whether viruses are living things or not.
MESOSOMES
Mesosomes are folded invaginations in the plasma membrane of bacteria. However, Information continues to circulate that mesosomes are produced as the result of chemical fixation which is the technique used to prepare samples for electron microscopy. They were first observed in 1953 by George B. Chapman and James Hillier, who referred to them as "peripheral bodies." They were termed "mesosomes" by Fitz-James in 1960. Initially, it was thought that mesosomes might play a role in several cellular processes, such as cell wall formation during cell division, chromosome replication, or as a site for oxidative phosphorylation. Mesosomes were also hypothesized to aid in photosynthesis, cell division, DNA replication, and cell compartmentalisation. By the mid to late 1980s, with advances in cryofixation and freeze substitution methods for electron microscopy, it was generally concluded that mesosomes do not exist in living cells. However, a few researchers continue to argue that the evidence remains inconclusive, and that mesosomes might not be artifacts in all cases. Mesosomes may be formed as the result of chemical fixation which can be malfunction on the plasma membrane. However, during endocytosis the cell membrane still folds inwards naturally to allow certain substances to pass. And this would increase the surface area of the plasma membrane.
Plasmids
A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; however, plasmids are sometimes present in archaea and eukaryotic organisms. In nature, plasmids often carry genes that benefit the survival of the organism and confer selective advantage such as antibiotic resistance. While chromosomes are large and contain all the essential genetic information for living under normal conditions, plasmids are usually very small and contain only additional genes that may be useful in certain situations or conditions. The term plasmid was introduced in 1952 by the American molecular biologist Joshua Lederberg to refer to "any extrachromosomal hereditary determinant.The term's early usage included any bacterial genetic material that exists extrachromosomally for at least part of its replication cycle, but because that description includes bacterial viruses, the notion of plasmid was refined over time to comprise genetic elements that reproduce autonomously. Later in 1968, it was decided that the term plasmid should be adopted as the term for extrachromosomal genetic element, and to distinguish it from viruses, the definition was narrowed to genetic elements that exist exclusively or predominantly outside of the chromosome and can replicate autonomously.
Plasmids may be classified in a number of ways. Plasmids can be broadly classified into conjugative plasmids and non-conjugative plasmids. Conjugative plasmids contain a set of transfer genes which promote sexual conjugation between different cells. In the complex process of conjugation, plasmids may be transferred from one bacterium to another via sex pili encoded by some of the transfer genes. Non-conjugative plasmids are incapable of initiating conjugation, hence they can be transferred only with the assistance of conjugative plasmids. An intermediate class of plasmids are mobilizable, and carry only a subset of the genes required for transfer. They can parasitize a conjugative plasmid, transferring at high frequency only in its presence.
Plasmids can also be classified into incompatibility groups. A microbe can harbour different types of plasmids, but different plasmids can only exist in a single bacterial cell if they are compatible. If two plasmids are not compatible, one or the other will be rapidly lost from the cell. Different plasmids may therefore be assigned to different incompatibility groups depending on whether they can coexist together. Incompatible plasmids normally share the same replication mechanisms and can thus not be kept together in a single cell.
Specific Types of Plasmids
There are five main types of plasmids: fertility F-plasmids, resistance plasmids, virulence plasmids, degradative plasmids, and Col plasmids.
Fertility F-plasmids- Fertility plasmids, also known as F-plasmids, contain transfer genes that allow genes to be transferred from one bacteria to another through conjugation. These make up the broad category of conjugative plasmids. F-plasmids are episomes, which are plasmids that can be inserted into chromosomal DNA. Bacteria that have the F-plasmid are known as F positive (F+), and bacteria without it are F negative (F–). When an F+ bacterium conjugates with an F– bacterium, two F+ bacterium result. There can only be one F-plasmid in each bacterium.
Resistance plasmids- Resistance or R plasmids contain genes that help a bacterial cell defend against environmental factors such as poisons or antibiotics. Some resistance plasmids can transfer themselves through conjugation. When this happens, a strain of bacteria can become resistant to antibiotics.
Virulence Plasmids- When a virulence plasmid is inside a bacterium, it turns that bacterium into a pathogen, which is an agent of disease. Bacteria that cause disease can be easily spread and replicated among affected individuals. The bacterium Escherichia coli (E. coli)has several virulence plasmids. E. coli is found naturally in the human gut and in other animals, but certain strains of E. coli can cause severe diarrhea and vomiting. Salmonella enterica is another bacterium that contains virulence plasmids.
Degradative Plasmids- Degradative plasmids help the host bacterium to digest compounds that are not commonly found in nature, such as camphor, xylene, toluene, and salicylic acid. These plasmids contain genes for special enzymes that break down specific compounds. Degradative plasmids are conjugative.
Col Plasmids- Col plasmids contain genes that make bacteriocins (also known as colicins), which are proteins that kill other bacteria and thus defend the host bacterium. Bacteriocins are found in many types of bacteria including E. coli, which gets them from the plasmid ColE1.
Plasmids can belong to more than one of these functional groups.
Are viruses living things or not?
The question of whether viruses can be considered to be alive, of course, hinges on one’s definition of life. Where we draw the line between chemistry and life can seem a philosophical, or even theological argument. Most creation stories involve a deity that imbues inanimate matter with the ‘spark of life’. From a scientific perspective, attempting to find a working definition for ‘life’ seems to me to have little practical value, but it is fun to think about.
A virus is a wrapped nucleic acid’, either DNA or RNA and either double-stranded or single-stranded. The wrapping is virtually always a virus-encoded protein capsid and may or may not also include a lipid coat from the host. The viral nucleic acid is replicated and the viral proteins synthesised using the host cell’s processes. In many cases the virus also encodes some of the enzymes required for its replication, a well-known example being reverse transcriptase in RNA viruses. (Nigel Brown.2019).
However, a crucial point is that viruses are not capable of independent replication. They have to replicate within a host cell machinery. They do not contain the full range of required metabolic processes and are dependent on their host to provide many of the requirements for their replication. It should noted that there is a crucial difference between viruses and other obligate intracellular parasites, such as bacteria; is that viruses have to utilise the host metabolic and replication machinery. Intracellular bacteria may merely use the host as the environment in which they can supplement their limited metabolic capacity and they usually have their own replication machinery.
However, some people argue that no organism is entirely self-supporting, this means, life is absolutely interdependent. There are many examples of obligate intracellular organisms, prokaryote and eukaryote that are critically dependent on the metabolic activities of their host cells. Humans likewise depend on the metabolic activity of nitrogen-fixing bacteria and photosynthetic plants along with that of our microbiota. Perhaps the most satisfying definition, that explicitly excludes viruses, emerges from the ‘metabolism first’ model and concerns the presence of membrane-associated metabolic activity – a tangible ‘spark’ of life.This definition also confers the status of life on mitochondria and plastids, however. The endosymbiosis that led to mitochondria is thought to have given rise to eukaryotic life. Mitochondria have metabolic activity on which we depend, they have machinery to manufacture proteins and they have genomes. Most would accept that mitochondria are part of a life form, but they are not independent life.(David Bhella.2020)
A group of scientists examined the phylogenomic relationships of viruses to living organisms through analysis of viral proteomes and assigned protein fold superfamilies. The authors concluded that viruses originated in ‘proto-virocells’ that were cellular in nature and they implied that viruses and modern bacteria evolved from common ancestors. They further claim that this means that viruses are indeed living organisms. However, If viruses are alive, should we not also consider a DNA molecule to be alive? Plasmids can do what viruses does but they are simply DNA molecules. Although they may be essential for the host’s survival in certain environments, Virulent plasmids can cause disease or kill another organism for the survival of the bacteria. If plasmids are not considered to be alive why then can viruses be alive?
The contention that viruses have no place in the tree of life is often supported by the assertion that viruses do not have a comparable history. Viruses are polyphyletic. Viruses are at a terrible disadvantage in this comparison. A recent study has investigated viral origins by analysis of the evolution and conservation of protein folds in the structural classification of proteins (SCOP) database. This work identified a subset of proteins that are unique to viruses. The authors conclude that viruses most likely originated from early RNA-containing cells. They all have surprisingly complex replication (life) cycles, however; they are exquisitely adapted to deliver their genomes to the site of replication and have precisely regulated cascades of gene expression. Viruses also engineer their environment, constructing organelles within which they may safely replicate, a feature they share with other intracellular parasites. While a virion is biologically inert and may be considered ‘dead’ in the same way that a bacterial spore or a seed is, once delivered to the appropriate environment, they claim that viruses are very much alive.
Finally it maybe productive to note that mesosomes where recognized as artifacts by the late 1970s and are no longer considered to be part of the normal structure of bacterial cells. However, these extensions are in the form of vesicles, tubules, and lamellae. Therefore I argue that, mesosomes still exists in bacterial cells especially during the formation of vesicles. Am in total agreement with the fact that the technique of chemical fixation can result in the formation of mesosome-like structures which further show the destruction of the plasma membrane. However, it does not mean mesosomes doesn't exist under natural circumstances. The case of the proposal and then disproof of the mesosome hypothesis has been discussed from the viewpoint of the philosophy of science as an example of how a scientific idea can be falsified, hypothesised then rejected.
Arguments over the life/not life status of viruses are often rooted in evolutionary biology and theories of the origins of life. All cellular organisms can claim a direct lineage to a primordial cell or cells, a continuous chain of cell divisions along which the ‘spark’ has been passed. Fundamental to the argument that viruses are not alive is the suggestion that metabolism and self-sustaining replication are key definitions of life. Viruses are not able to replicate without the host cell. However the answer to whether viruses are living or not depends on each person's perspective of life. Much is known about the invisible world now than before. However, much remains unexplored.
REFERENCES
1. Sinkovics J, Horvath J, Horak A (1998). "The origin and evolution of viruses (a review)". Acta Microbiologica et Immunologica Hungarica. 45 (3–4): 349–90. PMID 9873943.
2. Smillie C, Garcillán-Barcia MP, Francia MV, Rocha EP, de la Cruz F (September 2010). "Mobility of plasmids". Microbiology and Molecular Biology Reviews. 74 (3): 434–52. doi:10.1128/MMBR.00020-10. PMC 2937521. PMID 20805406.
3. Thomas CM, Summers D (2008). Bacterial Plasmids. Encyclopedia of Life Sciences. doi:10.1002/9780470015902.a0000468.pub2. ISBN 978-0470016176.
PATRICK NKEMBA
