Viruses and Prokaryotes
Viruses Are they living things? Viruses can reproduce, however, they cannot reproduce without a host cell. They also do not contain cytoplasmic materials and they do not have a form of metabolism. Therefore, viruses are not categorized as living things. However, viruses are still classified to identify them. Viruses do have taxonomical classification into orders, families, genera, and species. Classification is based on size, shape, and type of genetic material.
Viral Structure Viruses are small particles that are capable of infecting the cells of a biological organism. Viruses are simple in their structure. They are composed of genetic material (either DNA or RNA) contained within a capsule composed of protein (the capsid). There is no cytoplasm within the capsid, though some viruses do have a membranous envelope around the capsid. The DNA or RNA contains the genetic information for synthesizing proteins that can make new viruses. Viruses will infect cells and take control of the cell to synthesize new viral particles.
Viral Infections Viruses have specificity in terms of infection. They will recognize certain cell types or specific organisms. The disease can then spread from host to host. A large-scale outbreak of disease which is confined to a limited geographic region is known as an epidemic. An outbreak that occurs over a widespread geographic area (often global), is known as a pandemic. Depending on the type of viral infection, viruses can cause a variety of symptoms in the host organism. Sometimes, they can even cause cancer in the host organism
Viral Replication Outside of a host cell, a virus does not carry out any life functions. To replicate, viruses must infect living cells by inserting their genetic information into a host cell. The host cell will read the genetic information and replicate viral particles, creating new viruses. The viruses will then destroy and exit the host cell to infect other host cells. Thus, continuing the cycle of infection. There are two types of cycles: The Lytic Cycle The Lysogenic Cycle
Lytic Cycle Steps in a lytic cycle: 1. Virus binds to the surface of a host cell and inserts its genetic information (or the whole virus) into the cell. 2. Viral DNA/RNA acts as instructions for the host cell to make new viral particles. 3. New viruses (copies of the original virus) are assembled within the host cell. 4. Host cell lysis (rupturing of a cell) occurs and releases new viral particles to infect other cells
Lysogenic Cycle Steps in a lysogenic cycle: 1. Virus binds to the surface of a host cell and inserts its genetic information (or the whole virus) into the cell. 2. Viral DNA/RNA inserts itself into the host chromosome. 3. Each time the host cell divides, a copy of the viral information is replicated and passed on to the daughter cells. 4. This can eventually result in the lytic cycle when the viral DNA becomes active in the host cells. This can result in some bacterial DNA being transferred by the resulting viruses. This is known as transduction.
Viral Applications Gene therapy A method of treating diseases in which genes are introduced into cells to replace, supplement, or repair a defective gene. Since certain viruses are capable of inserting their DNA into host chromosomes, this mechanism can be used to insert wanted genes into a host.
Prokaryotes Recall: Prokaryotes are single celled organisms that lack membrane-bound organelles. Prokaryotes have been on earth for more than 3.5 billion years. The domains bacteria and archaea both consist of prokaryotes. Formerly, bacteria and archaea were thought to be very similar since they were all prokaryotic. However, since then, it has been discovered that one group of the bacteria behave more like eukaryotic cells than the other group.
The Domain Bacteria Genetic studies shows that bacteria diverged early into many branches with varying characteristics (e.g. photosynthetic, pathogenic, etc.) Bacteria contain a single chromosome. Some also have plasmids which are small circular loops of DNA. The plasmids usually contain information which is not crucial but may be advantageous.
Bacterial Structure Bacteria can be characterized and distinguished generally by these four characteristics: Cell shape Cell wall structure Motility Nutritional Requirement
Bacteria Cell Shapes The basic shape of bacteria are as follows: Cocci (coccus for singular) Spherical bacteria E.g. Staphylococcus aureas Bacilli (bacillus for singular) Rod-shaped bacteria E.g. Bacillus cereus Spirochetes Spiral-shaped bacteria E.g. Leptospira
Bacteria Cell Wall Structure Almost all bacteria have a cell wall surrounding their cell membrane. The cell wall is for support and protection. There are two types of bacterial cell walls. They both contain peptidoglycan (sugar layer) but one contains significantly less peptidoglycan and has an outer membrane instead.
Bacteria Cell Wall Structure Gram positive bacteria have a large layer of peptidoglycan in their cell walls. Gram negative bacteria have a smaller layer of peptidoglycan in their cell walls and have an extra cell membrane layer.
Bacteria Motility Some prokaryotes are motile while others are not. For motility, some bacteria use flagella to propel themselves toward food, light, or oxygen. Others which are spiral-shaped can use a corkscrew motion to move. Some bacteria also secrete a mucous that they can glide on.
Bacteria Nutrition Nutritional Mode Heterotroph Photoautotroph Photoheterotroph Chemoheterotroph Definition Organism that requires a source of organic materials from the environment, or through ingestion of other organisms. Organism that is able to use light as an energy source and convert carbon dioxide into carbon compounds. Organism that is able to use light as an energy source but requires organic material as a source of carbon. Organism that uses inorganic energy sources such as sulfur.
Bacteria Reproduction Bacteria reproduce through binary fission
Bacteria Reproduction Binary fission is considered asexual reproduction that generates clones of the original cell. However, genetic recombination is still possible through the following processes: Transformation Fragments of free DNA from the environment is taken in by the bacterium Transduction Viral infections can result in transfer of genetic information from one bacteria to the next through the viral chromosome Conjugation Bacteria are joined together and genetic information in the form of circular DNA (a plasmid) is transferred
The Doman Archaea The domain archaea consists of prokaryotes which different from other living things. Little is known about archaea due to the fact that many inhabit extreme environments. Like bacteria, they have a cell wall around their cell membranes. However, the cell walls of archaea usually lack peptidoglycan. These cell walls are more resistant to physical and chemical disruptions.
Examples of Archaea Groups Sample types Methanogens Halophiles Extreme Thermophiles Psychrophiles Key Features Live in low-oxygen environments Generate energy by converting chemical compounds into methane gas Salt-loving organisms that live in high salt environments Get energy from organic molecules Some able to use light Hot-loving organisms that can live in extremely hot environments Optimal range is between 70 C to 95 C Cold-loving organisms that can live in extremely cold environments Optimal range is between -10 C to -20 C