BIO101 Notes Quick Revision

LESSON NO. 66

Virus name The name “virus” has been derived from a Latin word meaning “slimy liquid” or “poison.” Viruses are microscopic parasites that lack the capacity to thrive and reproduce outside of a host body. These can multiply only in living cells of animals, plants, or bacteria. Latin word meaning poison or venom given by Beijerinck (1897). WHAT ARE VIRUSES? Viruses are submicroscopic, obligate intracellular parasites. Most are too small to be seen by optical microscopes, and they have no choice but to replicate inside host cells. This simple but useful definition goes a long way toward describing viruses and differentiating them from all other types of organism. However, this short definition is not completely adequate. Viruses are unique in nature. They are the smallest of all self-replicating organisms, historically characterized by their ability to pass through filters that retain even the smallest bacteria. In their most basic form, viruses consist solely of a small segment of nucleic acid encased in a simple protein shell. Viruses have no metabolism of their own but rather are obliged to invade cells and parasitize subcellular machinery, subverting it to their own purposes. Virus Size ▪ Submicroscopic Smaller than bacteria, Smaller than the smallest cell LESSON NO.67 Importance of virology UNIQUE PROPERTIES ▪ Living as well as non-living characteristics ▪ Unable to cultivate in laboratory easily ▪ Simplest structure ▪ Complex interaction with host ▪ Replication within host To study an important class of pathogens Many types of pathogens cause disease in humans. The most familiar are viruses and bacteria. Viruses cause diseases ranging from AIDS and smallpox to the common cold. They are essentially fragments of nucleic acid (DNA or RNA) instructions, wrapped in a protective shell of proteins and (in some cases) membrane. They use the basic transcription and translation machinery of their host cells for their replication. Of all the bacteria we encounter in our lives, only a small minority are dedicated pathogens. Much larger and more complex than viruses, bacteria are usually free-living cells, which perform most of their basic metabolic functions themselves, relying on the host primarily for nutrition. Some other infectious agents are eukaryotic organisms. These range from single-celled fungi and protozoa, through large complex metazoa such as parasitic worms. Some rare neurodegenerative diseases, including mad cow disease, are caused by an unusual type of infectious particle called a prion, which is made only of protein. Although the prion contains no genome, it can nevertheless replicate and kill the host. Even within each class of pathogen, there is striking diversity. Viruses vary tremendously in their size, shape, and content, and the same is true for the other pathogens. The ability to cause disease (pathogenesis) is a lifestyle choice, not a legacy shared only among close relatives Virus life cycle Bacteria, fungi, and eukaryotic parasites are cells themselves. Even when they are obligate parasites, they use their own machinery for DNA replication, transcription, and translation, and they provide their own sources of metabolic energy. Viruses, by contrast, are the ultimate hitchhikers, carrying little more than information in the form of nucleic acid. The information is largely replicated, packaged, and preserved by the host cells. Viruses have a small genome, made up of a single nucleic acid type—either DNA or RNA. The genome is packaged in a protein coat. Viruses replicate in various ways. In general, replication involves (1) disassembly of the infectious virus particle, (2) replication of the viral genome, (3) synthesis of the viral proteins by the host cell translation machinery, and (4) reassembly of these components into progeny virus particles. To study viral cultivation in lab ▪ Tissue culture” ▪ Embryonated egg ▪ Live animal LESSON NO.68 General Properties of VIRUS A virus is a sub-microscopic particle that can infect living cells. Viruses are much smaller than prokaryotes, ranging in size from about 20–300 nanometers (nm), though some can be larger. Prokaryotes are typically 0.5–5.0 micrometers (µm) in length. For example, if a virus was about the size of three soccer balls lying side-by-side, then a prokaryote would be about the size of soccer field. Virus Size Foot and Mouth Disease Virus 27 nm Pox Virus 300 nm Structure An individual virus is called a virion. It is a tiny particle much smaller than a prokaryotic cell. Because viruses do not consist of cells, they also lack cell membranes, cytoplasm, ribosomes, and other cell organelles. Without these structures, they are unable to make proteins or even reproduce on their own. Instead, they must depend on a host cell to synthesize their proteins and to make copies of themselves. Although viruses are not classified as living things, they share two important traits with living things. They have genetic material, and they can evolve. Simple Viruses ▪ a protective protein coat ▪ a nucleic acid genome Deoxyribonucleic Acid (DNA) or Ribonucleic Acid (RNA) ” WHY VIRUSES NEED HOST? ▪ No enzymes of their own ▪ No metabolism ▪ Require host machinery COMPLEX VIRUSES Simple viruses only contain protein coat and nucleic acid genome. Complex viruses also contains some additional contents e.g. ▪ Lipid ▪ Polysaccharides ▪ Trace elements Lesson no.69 Structure of enveloped and non-enveloped viruses. Non-enveloped viruses are composed of capsid protein and nucleic acid (DNA or RNA), viz. nucleocapsid. Whereas enveloped viruses are composed of an envelope and nucleocapsid. Some viruses may have spikes to help attach to the host cell. Most viruses infect only specific host cells. MORPHOLOGY ▪ Viruses come in a variety of shapes. ▪ Helical Viruses: Protein subunits interact with each other and with the nucleic acid to form a coiled, ribbon like structure: e.g. tobacco mosaic virus, influenza virus, rabies virus. Ebola virus. Structures with helical symmetry. ▪ Polyhedral shapes: The capsid shell is made of repeating subunits of viral protein (may be one kind of subunit or several, according to the virus). - All faces of the icosahedron are identical. like the influenza virus ▪ Complex shapes: Most viruses have either icosahedral or helical capsids, but some viruses do not fit into either category. Poxviruses and large bacteriophages are two important examples Lesson no.70 Bacteriophages Introduction Even bacteria can get a virus! The viruses that infect bacteria are called bacteriophages, and certain bacteriophages have been studied in detail in the lab (making them some of the viruses we understand best). We'll take a look at two different cycles that bacteriophages may use to infect their bacterial hosts: ▪ The lytic cycle: The phage infects a bacterium, hijacks the bacterium to make lots of phages, and then kills the cell by making it explode (lyse). ▪ The lysogenic cycle: The phage infects a bacterium and inserts its DNA into the bacterial chromosome, allowing the phage DNA (now called a prophage) to be copied and passed on along with the cell's own DNA. A bacteriophage is a virus that infects bacteria A bacteriophage, or phage for short, is a virus that infects bacteria. Like other types of viruses, bacteriophages vary a lot in their shape and genetic material. ▪ Phage genomes can consist of either DNA or RNA, and can contain as few as four genes or as many as several hundred. ▪ The capsid of a bacteriophage can be icosahedral, filamentous, or head-tail in shape. The head-tail structure seems to be unique to phages and their close relatives (and is not found in eukaryotic viruses). Bacteriophage infections Bacteriophages, just like other viruses, must infect a host cell in order to reproduce. The steps that make up the infection process are collectively called the lifecycle of the phage. Some phages can only reproduce via a lytic lifecycle, in which they burst and kill their host cells. Other phages can alternate between a lytic lifecycle and a lysogenic lifecycle, in which they don't kill the host cell (and are instead copied along with the host DNA each time the cell divides). Let's take closer look at these two cycles. As an example, we'll use a phage called lambda (\lambdaλlambda), which infects E. coli bacteria and can switch between the lytic and lysogenic cycles. LESSON NO.71 BACTERIOPHAGES: Properties A bacteriophage also known informally as a phage is a virus that infects and replicates within bacteria and archaea. General Characteristics Virus architecture ,Virus particle called virion , Consists of nucleic acid surrounded by protein coat  › Shapes Capsid › Isometric › Helical › Complex › Two types of virion › Naked – without envelope › Enveloped – surrounded by lipid membrane SIZE: LESSON NO.72 MICROBES They are are also minute, unicellular organisms that called as germs or microorganisms. They are too small to be seen with the unaided eye. Therefore, a microscope is used to observe them. They are found all around us. They live in water, soil, and in the air. The human body is home to millions of these microbes too, also called microorganisms. Some microbes make us sick; others are important for our health. The most common types are bacteria, viruses and fungi. There are also microbes called protozoa. These are tiny living things that are responsible for diseases such as toxoplasmosis and malaria. MICROBIOLOGY It is the study of microorganisms. The field is concerned with the structure, function, and classification of such organisms and with ways of both exploiting and controlling their activities. CLASSIFICATION OF MICROBES Classification of microorganisms has been a challenge. In the currently accepted scientific classification of Life, there are three domains of microorganisms: ▪ Eukaryotes ▪ Bacteria ▪ Archaea LESSON NO.73 Classification of Microbes Introduction There is a need to classify microbes in order to study them conveniently, to identify them easily, to establish relationship between organisms and to establish the evolutionary relationships. Bases of Classification: ▪ Similarities among them ▪ Morphology used to be the main feature prior to the advent of molecular biology ▪ Biochemical similarities ▪ Genetic similarities ▪ Evolutionary relationships Five Kingdom Classification of Organisms The first person to divide living things into five broad kingdoms was North American ecologist Robert Whittaker. This researcher proved in 1959 that fungi were not plant organisms - previously it was thought that they were - and a decade later he proposed the creation of the fungi kingdom to differentiate them from plants. Whittaker's theory was widely accepted, and the scientific community thereby added a new group to the previous four-kingdom system, established by the American biologist Herbert Copeland in 1956. Animal Kingdom The kingdom Animalia is the most evolved and is divided into two large groups - vertebrates and invertebrates. These animals are multi-celled, heterotrophic eukaryotes with aerobic respiration, sexual reproduction and the ability to move. This kingdom is one of the most diverse and comprises mammals, fish, birds, reptiles, amphibians, insects, molluscs and annelids, among others. Plant Kingdom Trees, plants and other species of vegetation make up part of the Plantae kingdom - one of the oldest, and characterized by its immobile, multicellular and eukaryotic nature. These autotrophic things, whose cells contain cellulose and chlorophyll are essential for life on Earth since they release oxygen through photosynthesis. As regards their method of reproduction, this may be either sexual or asexual. Fungi Kingdom This name is used to designate the fungi kingdom which includes yeasts, moulds and all species of mushrooms and toadstools. These multicellular aerobic heterotrophic eukaryotes have chitin in their cell walls, feed off other living things, and reproduce through spores. Protista Kingdom This group is the most primitive of the eukaryotics and all the others are descendants of it. The Protista kingdom is paraphyletic - it contains the common ancestor but not all its descendants - and it