Module 1 Viruses and Bacteria PDF

3. Basic structure and function of viruses and bacteria 3.1 Viruses Viruses are not classified as one of the 5 kingdoms of living organisms, since viruses display non-living characteristics and they do not consist of cells. The debate is still ongoing as to whether or not viruses are considered li...

3. Basic structure and function of viruses and bacteria 3.1 Viruses Viruses are not classified as one of the 5 kingdoms of living organisms, since viruses display non-living characteristics and they do not consist of cells. The debate is still ongoing as to whether or not viruses are considered living or non-living. 3.1.1 Basic structure: Viruses can be seen only through electron microscopes. They are around 20-300 nm in size (which is very small). The shapes include rod-shaped, spherical or more complex. See Figure 2 below. A typical virus, however, will contain DNA or RNA at its core (never both), enclosed by a protein coat called a capsid. Viruses are acellular and do not have cytoplasm or any organelles (only DNA or RNA enclosed by capsids) Graphic depiction of different types of viruses and their shapes: polyhedral, spherical, complex and helical. Figure 2: Shapes of viruses. 3.1.2 Function and characteristics of viruses Viruses cannot feed, excrete waste or respire. They cannot even reproduce; they do however replicate themselves. Movement is another problem: viruses cannot move independently, they require a host. It is due to these features that they are considered non-living, as all living things can eat, excrete and respire, as well as grow, reproduce and move. They are considered obligate internal parasites, meaning that they require another living organism or host in order to multiply. Obligate means compulsory or obliged. Internal means within a cell and parasite means an organism that lives on living material and causes harm. We say that a virus infects its host. You might even have been infected, or know somebody who has been infected, with the Covid-19 virus. Bacteriophages are viruses that infect bacteria. The diagram below illustrates what a bacteriophage looks like vs a typical virus (such as the flu virus). Graphic depiction which highlights the differences in structure between a typical virus and a bacteriophage. Figure 3: Structure of typical virus and bacteriophage. Viruses can infect bacteria, protists, plants and animals. They are called pathogens or are pathogenic if they are disease-causing viruses. A virus without a host becomes dormant. In humans, the following common diseases are caused by viruses: HIV/AIDS, chickenpox, herpes, and influenza (flu). The image below indicates what viruses look like. Viruses reproduce by converting the genetic material, the DNA or RNA, of the host cell into viral nucleic acids so that new viruses can be produced. 3.2 Bacteria Poor bacteria are considered bad and harmful by many people. Some are, but not all; in fact, most are useful and needed. Bacteria belong to the kingdom Monera and they are prokaryotic, meaning that their nuclear structure is not enclosed in a membrane. Bacteria can live almost anywhere on Earth and have been around the longest, dating back more than 3.5 billion years. 3.2.1 Structure of bacteria All bacteria basically have the same structure. See Figure 4 below. They are unicellular (meaning one- or single-celled) and can occur as single cells, filaments or colonies. The cell structure is simpler than that of other organisms, as there is no nucleus or any membrane-bound organelles. Instead, their control centre containing the genetic information is contained in a single loop of DNA. The DNA is in chromatin known as the nucleoid. The cell is, however, surrounded by a cell wall that consists of polysaccharides (carbohydrates), proteins and lipids (fats). In some cases, the cell wall is surrounded by a slime layer or capsule (especially in those that cause disease), which protects the bacterium. The plasma membrane is directly below the cell wall. Some bacteria are able to move in liquid with the help of threadlike structures called flagella. Graphic showing the structure of bacteria with labels indicating the various components of the bacteria's structure: The cell membrane, cell wall, cytoplasm with scattered ribosomes, nucleaoplasm with single circular chromosome (no nuclear membrane), slime capsule, storage granule, and flagella. Figure 4: The structure of bacteria. Bacteria are distinguished from one another based on their shape. These shapes include : coccus – round, bacillus – rod-shaped, spirillum – spiral-shaped, and vibrio – comma-shaped See Figure 5 below. Diagram showing the various shapes of bacteria. Specifically: Sphrical Cocci, Rod-shaped Bacilli, Comma-shaped Vibrio and Spiral spirilla Figure 5: Different shapes of bacteria. Fun fact A sore throat is sometimes due to Group A streptococcus. The name (the ‘coccus’ part) tells us that this type of bacteria is round. 3.2.2 Function and characteristics of bacteria There are many different types of bacteria: Most bacteria are saprotrophs, which means that they generally feed on dead organic matter (decomposers). They do this by producing enzymes that digest the food outside the cell. The digested food molecules are then absorbed back into the bacterial cell. Some bacteria produce their own food by photosynthesis or chemosynthesis. They are autotrophs. Heterotrophic bacteria cannot manufacture their own food. Parasitic and saprotrophic bacteria are part of this group. Parasitic bacteria that obtain their food from other living organisms are included in this group, as are saprotrophic bacteria, as they obtain their food from dead organic plants and animals. Mutualistic bacteria form a relationship with another organism, both benefiting from the relationship. Aerobic bacteria use oxygen for respiration (think ‘air’ when you hear ‘aer’), while anaerobic bacteria can respire without oxygen (‘anaer’ = ‘without air’). Bacteria reproduce asexually by binary fission. Binary fission is when one cell splits into two to form two new cells. When conditions are unfavourable, bacteria form endospores.