Biological Sciences
Prokaryotes and Viruses
Prokaryotes are single-celled organisms that lack a nucleus and other membrane-bound organelles. They include bacteria and archaea. Viruses are non-living infectious agents that can only replicate inside the cells of living organisms. They consist of genetic material (DNA or RNA) surrounded by a protein coat. Both prokaryotes and viruses play significant roles in various ecological and biological processes.
Written by Perlego with AI-assistance
Related key terms
6 Key excerpts on "Prokaryotes and Viruses"
eBook - ePub- Michael G. Cordingley(Author)
- 2017(Publication Date)
- Harvard University Press(Publisher)
In 2008 Raoult and Forterre proposed the division of all biological entities into “two groups of organisms: ribosome-encoding organisms, which include eukaryotic, archaeal and bacterial organisms, and capsid-encoding organisms, which include viruses.” The three domains of cellular life, Eukarya, Bacteria, and Archaea, all possess the capacity to synthesize proteins and to conduct their own metabolism. As discussed earlier, the molecular machinery of all living cells responsible for assembling proteins from amino acids has at its core the ribosome. Viruses have no such machinery and rely on ribosome-encoding cellular life-forms to make proteins for them. On the other hand, cellular life-forms do not appear to require, nor do their genomes encode, self-assembled capsids. This definition goes a long way toward distinguishing viruses from living organisms based on their distinct gene content. While accurate, I believe it is still inadequate because it fails to capture the quintessential nature of viruses. It also ignores the existence of many biological entities that share the essential features of viruses but lack a capsid gene. In some cases their evolutionary origins can be traced directly back to capsid-encoding ancestral viruses. These exceptions are viruses that adopt a variety of lifestyles. Some are rather simple genetic replicators that appear to be evolutionary relics—primitive replicons called viroids—which may have originated early on and persisted through evolutionary time (Chapter 8 will discuss conjectures on the origin of viroids). Others may be mobile genetic elements capable of replicating themselves and moving to new host cells by a variety of mechanisms. In some instances, these mobile genetic elements are found in the genome of other viruses and can hitch a ride between cells as part of the chromosome of their ride
eBook - ePub- Rene Krata(Author)
- 2011(Publication Date)
- Barrons Educational Series(Publisher)
Further studies of tobacco mosaic virus revealed more about this mysterious virus. In 1898, Martinus Beijerinck found that the virus would only multiply in living plant cells. In addition, he found that it could be completely dried and yet would still remain capable of causing the disease when introduced into plant tissue. In 1935, Wendell Stanley crystallized the virus and determined that it was mostly protein. This was followed in 1936 by the work of Frederick Bawden and Norman Pirie, who determined that the viral particles were actually made of both protein and nucleic acid.Properties of VirusesThe early studies on viruses clearly demonstrated that viruses were very different from bacteria. Bacteria can grow on their own in a wide variety of locations; viruses can only reproduce inside of cells. Bacteria are cells, and for most bacteria, if you dry them out completely, they will die. This is not true of many viruses that can exist in a dried state for long periods of time and still cause disease. Bacteria are made of four different types of macromolecules: lipids, nucleic acids, proteins, and carbohydrates. Many viruses, on the other hand, are just nucleic acid and protein. Also, as demonstrated by the early work with bacterial filters, viruses are smaller than bacteria. Clearly, viruses and bacteria are different from each other.Just how different bacteria and viruses are from each other can be summed up in a seemingly simple statement: Bacteria are cells; viruses are not cells. The implications of this statement are huge. Bacteria can live on their own because they have ribosomes for protein synthesis and metabolic pathways for the synthesis of ATP. They have membranes, structures like flagella and pili, and hundreds of different enzymes. Viruses don’t have any of this. Viruses are very tiny and very simple. The individual viral particles are called virions , and the simplest virions consist only of nucleic acid and a protein coat. Because they don’t have any ribosomes or ATP-generating metabolism, viruses can only reproduce if they enter a living cell and steal what they need to multiply. In other words, they are obligate intracellular parasites
eBook - ePub- Patricia Barnes-Svarney, Thomas E. Svarney(Authors)
- 2014(Publication Date)
- Visible Ink Press(Publisher)
Bacillus anthracis , the agent of anthrax, is a large, gram-positive, nonmoving (nonmotile), spore-forming, rod-shaped bacteria that is nasty for humans. The three major, clinical forms of human anthrax are as follows: the bacteria contracted through the skin (the most common, entering through a cut or scrape on the skin, forming lesions, blisters, then a black ulcer); lungs (breathing it into the warm, moist environment of the lungs allows the bacteria to “sprout,” spreading to the lymph system—which usually takes one to six days); or gastrointestinal tract (through the ingestion of anthrax). The symptoms for each form vary; it is usually treated with antibiotics, although the effectiveness of the treatment is often dependent on how the bacteria entered the body. But no matter what, if left untreated, anthrax can spread to the bloodstream, often leading to septicemia (blood poisoning) and death.VIRUS BASICS
What is a virus?
A virus is an infectious, protein-coated fragment of DNA or RNA. Viruses replicate by invading host cells and taking over the cell’s “machinery” for DNA replication. Viral particles can then break out of the cells, spreading disease. Viruses lie dormant in any environment (land, soil, air) and on any material. They infect every type of cell from plants and animals to bacteria and fungi.Are viruses living organisms?
All living things have around six character-istics—they adapt to their environment, have a cell makeup, have metabolic processes help them obtain and use energy, they move in response to their environment, they grow and develop, and they reproduce.Thus, technically, viruses are not “alive.” In particular, they cannot grow, they cannot reproduce (replicate) on their own—and need a host cell to become active to provide these functions. In other words, they are inert outside their living host cell. As such, they are considered to be between life and nonlife and are not considered living organisms. In fact, British biologist Sir Peter Medawar (1915–1987), a Nobel Prize recipient in Physiology or Medicine, described viruses as “…a piece of bad news wrapped in a protein.” He was referring to the fact that viruses cause influenza, smallpox, infectious hepatitis, yellow fever, polio, rabies, AIDS, and many other diseases.Viruses are kind of bizarre, alien creatures when you think about it—almost like robots. They are not really alive because they can’t reproduce or grow without a host.
eBook - ePub- Laurie Ann Callihan, David Callihan(Authors)
- 2013(Publication Date)
- Research & Education Association(Publisher)
Many species have only a single cell, others are multicellular. (Although viruses are sometimes considered to be living, they are non-cellular and cannot fulfill the characteristics of life without invading the cell of another organism.) Cell structure varies according to the function of the cell and the type of living thing. There are two main types of cells: prokaryotic and eukaryotic. Prokaryotes have no nucleus or any other membrane-bound organelles (cell components that perform particular functions). The DNA in prokaryotic cells usually forms a single chromosome, which floats within the cytoplasm. Prokaryotic organisms have only one cell and include all bacteria. Plant, fungi, and animal cells, as well as protozoa, are eukaryotic. Eukaryotic cells contain membrane-bound intracellular organelles, including a nucleus. The DNA within eukaryotes is organized into chromosomes. A single organism can be unicellular (consisting of just one cell), or multicellular (consisting of many cells). A multicellular organism may have many different types of cells that differ in structure to serve different functions. Individual cells may contain organelles that assist them with specialized functions. For example, muscle cells tend to contain more mitochondria (organelles that make energy available to the cells) since muscle requires the use of extra energy. Animal cells differ in structure and function from photosynthetic cells, which are found in plants, some bacteria, and some protists. Photosynthetic cells have the added job of producing food so they are equipped with specialized photosynthetic organelles. Plant cells also have a central vacuole and cell walls, structures not found in animal cells. Fig. 3-1 Viruses and Cells. Viruses are much smaller than cells, ranging from approximately 0.05-0.1 micrometers. The prokaryotic cell has no nucleus or other membrane-bound organelles and is approximately 1-10 micrometers in diameter
eBook - ePubMarine Microbiology
Ecology & Applications
- Colin Munn, Colin B. Munn(Authors)
- 2019(Publication Date)
- CRC Press(Publisher)
Virus particles, termed “virions,” are composed of nucleic acid surrounded by a protein coat (capsid). A fundamental difference in the makeup of viruses compared with cells is that viruses contain only one type of nucleic acid, either DNA or RNA, whereas cells contain both. Also, they do not contain ribosomes and therefore cannot synthesize proteins. Therefore, viruses are obligate intracellular parasites, that rely on the biochemical machinery of the cell they infect to complete the flow of genetic information, synthesize proteins, and replicate their virions. Since all forms of cellular life are susceptible to virus infection, we can reasonably speculate that every type of marine organism—from bacteria to whales—is a host to at least one type of virus. Many organisms are known to be hosts to several different viruses. Table 7.1 shows a list of representative marine virus families and their hosts; as can be seen, virions show great variation in size and exist in various morphological forms. Various schemes have been developed to classify the different types of viruses. In the early days of virology in the mid-twentieth century, viruses were classified largely on the basis of their hosts (plants, animals, or humans). As knowledge of these accumulated in the 1960s and 1970s, the structure and replication of viruses became the main criteria for classification. The Baltimore classification scheme devised in the 1970s divided viruses into seven groups, based on the nature of their genome and their replication strategy. Since the 1990s, the International Committee on Taxonomy of Viruses (ICTV) has developed rules for classifying and naming viruses using taxonomic divisions and Latin species names, but these differ from the binomial format used for cellular organisms. The organization reflects the phylogenetic ancestry, structure, replication, hosts, and transmission vectors of different viruses
eBook - ePub- Michael J. Benton(Author)
- 2019(Publication Date)
- Wiley-Blackwell(Publisher)
3 The Origin of EukaryotesIn This Chapter
For the first half of Earth’s history, life consisted of prokaryotes (Archaea and Bacteria), but the evolution of eukaryotes (cells with nuclei) changed the biological world forever. Eukaryotes have complex cells resulting from a symbiosis, or stable evolutionary partnership, between an archaeal host cell and one or more bacterial endosymbionts. These are major steps in evolution and we discuss how they happened. While most eukaryotes are single‐celled microbes, multicellularity evolved several times and gave rise to the most familiar eukaryotic groups, including animals, plants, and fungi. In this chapter, we also explore how biologists and paleontologists make sense of the huge diversity of species on Earth – the secret has been the revolution in cladistics and genomics in reconstructing the tree of life.Single‐Celled Life
The microbes that were Earth’s first life evolved into two different major groups or domains: Archaea and Bacteria. They shared much the same body plan, however, and we group them together as prokaryotes (Figure 3.1 ). Prokaryotes were and are very successful in an incredible range of habitats, from stinking swamps to the hindgut of termites and from hot springs in the deep sea to the ice desert of Antarctica, and deep in rocks underground. They occur in numbers averaging 500 million/l in surface ocean waters, 1 billion/l in fresh water, and about 300 million on the skin of the average human. The evolutionary success of prokaryotes is due to their metabolic versatility, resilience to a broad range of environmental conditions, and large population sizes. With such large populations, adaptive evolution to new nutrient sources and environmental challenges typically proceeds much faster in prokaryotes than in eukaryotes – the cellular lineage to which we belong.A prokaryotic cell. The DNA (blue cords) is twisted and folded to fit into the cell and floats free in the cell cytoplasm (orangeFigure 3.1
Learn about this page
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
