Viruses are extremely small. Look at the size comparison with a bacteria cell and a eukaryotic cell! The smallest viruses are only 20nm in diameter smaller than a ribosome! Viruses are the simplest biological systems. A virus is an infectious particle consisting of genes (DNA or RNA) packaged in a protein coat (capsid). Most virologists would probably agree that viruses are not alive but lead a kind of borrowed life. Adolf Mayer In 1883 Adolf Mayer did extensive research on the plant disease Tobacco Mosaic Disease. He discovered that it could be transmitted by rubbing sap from an infected plant onto a healthy plant. He thought it was caused by a really small bacteria. Later it was found that the infectious agent was a virus and that it could only reproduce in the host cell a characteristic of all viruses. In 1935, Wendell Stanley crystallized the TMV virus. In 1883 Adolf Mayer did extensive research on the plant disease Tobacco Mosaic Disease. He discovered that it could be transmitted by rubbing sap from an infected plant onto a healthy plant. He thought it was caused by a really small bacteria. Later it was found that the infectious agent was a virus and that it could only reproduce in the host cell a characteristic of all viruses. In 1935, Wendell Stanley crystallized the TMV virus. Viruses are NOT cells they need host cells in order to survive. There are 3 main parts: - Nucleic Acid (DNA or RNA) - Capsid (protein coat surrounding the genetic info) - Envelope (glycoprotein membrane around capsid; not all have this ) Viruses are NOT cells they need host cells in order to survive. There are 3 main parts: - Nucleic Acid (DNA or RNA) - Capsid (protein coat surrounding the genetic info) - Envelope (glycoprotein membrane around capsid; not all have this ) Genome of a Virus: A Virus can have double stranded DNA, double stranded RNA, single stranded DNA, or single stranded RNA for their genome. Genome of a Virus: A Virus can have double stranded DNA, double stranded RNA, single stranded DNA, or single stranded RNA for their genome. Different viruses have differently shaped capsids Remember that capsids are the PROTEIN part of the virus. Capsids are made up of subunits called capsomeres. Viruses that infect bacteria are called bacteriophages or just phages. Viruses require a host to reproduce (they are obligate intracellular parasites ). They can reproduce one of two ways: - Lytic Cycle - Lysogenic Cycle The viruses invade the host cell and use the host cells materials to build more viruses (to make viral proteins). Viruses attack specific tissues; ex. influenza attacks respiratory system; HIV virus attacks certain types of white blood cells Viruses require a host to reproduce (they are obligate intracellular parasites ). They can reproduce one of two ways: - Lytic Cycle - Lysogenic Cycle The viruses invade the host cell and use the host cells materials to build more viruses (to make viral proteins). Viruses attack specific tissues; ex. influenza attacks respiratory system; HIV virus attacks certain types of white blood cells Host Range: Each type of virus can only infect a limited number of host cells called its host range. The viruses identify the cells like a lock and key fit between the proteins on the outside of the virus and the receptor molecules on the hosts surface. Examples: West Nile can infect mosquitoes, birds, horses, and humans; measles virus can infect only humans. Host Range: Each type of virus can only infect a limited number of host cells called its host range. The viruses identify the cells like a lock and key fit between the proteins on the outside of the virus and the receptor molecules on the hosts surface. Examples: West Nile can infect mosquitoes, birds, horses, and humans; measles virus can infect only humans. A viral infection begins when the genome of the virus enters the host cell. Once inside, the viral genome commandeers its host, reprogramming the cell to copy the viral genome and manufacture viral proteins. The host provides nucleotides, enzymes, ribosomes, tRNAs, amino acids, ATP, and other components for making viral proteins. The simplest type of viral replicative cycle ends with the exit of viruses from the infected host cell, a process that usually damages or destroys the host cell. This cellular damage and death cause many of the symptoms associated with viral infection. A viral infection begins when the genome of the virus enters the host cell. Once inside, the viral genome commandeers its host, reprogramming the cell to copy the viral genome and manufacture viral proteins. The host provides nucleotides, enzymes, ribosomes, tRNAs, amino acids, ATP, and other components for making viral proteins. The simplest type of viral replicative cycle ends with the exit of viruses from the infected host cell, a process that usually damages or destroys the host cell. This cellular damage and death cause many of the symptoms associated with viral infection. -Viruses that use this cycle are called virulent and kill the host immediately by lysing the cell and releasing a large number of new viruses -Some bacteria have a defense against these viruses called restriction nucleases (restriction enzymes).these enzymes cut up foreign DNA. Bacteria prevent their OWN DNA from being digested by methylating it -Viruses that use this cycle are called virulent and kill the host immediately by lysing the cell and releasing a large number of new viruses -Some bacteria have a defense against these viruses called restriction nucleases (restriction enzymes).these enzymes cut up foreign DNA. Bacteria prevent their OWN DNA from being digested by methylating it The viral genomes and viral proteins are assembled into viruses during this phase (not the lysogenic!) -Viruses that go through both the lysogenic and the lytic cycle are called temperate phages. The lysogenic cycle does NOT kill the host immediately. Viruses can remain dormant for several years. -The viral DNA gets incorporated into the host cells genome and is then called a prophage (its get copied with the host cells DNA during replication, so all the newly made cells have the viral DNA component) -The viral genome can exit the lysogenic cycle and go into the lytic cycle when signaled to (stress, environmental factors such as chemicals or high energy radiation, etc) -Viruses that go through both the lysogenic and the lytic cycle are called temperate phages. The lysogenic cycle does NOT kill the host immediately. Viruses can remain dormant for several years. -The viral DNA gets incorporated into the host cells genome and is then called a prophage (its get copied with the host cells DNA during replication, so all the newly made cells have the viral DNA component) -The viral genome can exit the lysogenic cycle and go into the lytic cycle when signaled to (stress, environmental factors such as chemicals or high energy radiation, etc) One key variable is the type of nucleic acid (DNA or RNA) that serves as a viruss genetic material. Another variable is the presence or absence of a membranous envelope derived from the host cell membrane. The viral envelope is usually derived from the hosts plasma membrane, although some viruses have envelopes that are derived from the nuclear envelope of the host. The viruses that use RNA as the genetic material are quite diverse, especially those that infect animals. One key variable is the type of nucleic acid (DNA or RNA) that serves as a viruss genetic material. Another variable is the presence or absence of a membranous envelope derived from the host cell membrane. The viral envelope is usually derived from the hosts plasma membrane, although some viruses have envelopes that are derived from the nuclear envelope of the host. The viruses that use RNA as the genetic material are quite diverse, especially those that infect animals. RNA viruses mutate more easily than DNA viruses because they lack proofreading enzymes. RNA viruses are very diverse; several different classes. Retroviruses (Class VI) RNA gets converted into DNA using the enzyme reverse transcriptase; RNA polymerase then transcribes the DNA into RNA RNA viruses mutate more easily than DNA viruses because they lack proofreading enzymes. RNA viruses are very diverse; several different classes. Retroviruses (Class VI) RNA gets converted into DNA using the enzyme reverse transcriptase; RNA polymerase then transcribes the DNA into RNA Retroviruses have the most complicated replicative cycles. Retroviruses are single stranded RNA viruses that can act as a template for DNA Retroviruses carry an enzyme called reverse transcriptase that transcribes DNA from an RNA template. This provides RNA DNA information flow. Viruses that would most likely have reverse transcriptase are RNA viruses that do the lysogenic cycle. Viral genomes can encode reverse transcriptase, so it can be used in host cells even if it was not in the host cell originally Human immunodeficiency virus (HIV), the virus that causes AIDS (acquired immunodeficiency syndrome), is a retrovirus. After HIV enters the host cell, reverse transcriptase molecules are released into the cytoplasm and catalyze the synthesis of viral DNA. The newly made viral DNA enters the cells nucleus and is inserted as a permanent provirus into a chromosome. (Provirus viral genome that is permanently inserted into a host genome) Retroviruses have the most complicated replicative cycles. Retroviruses are single stranded RNA viruses that can act as a template for DNA Retroviruses carry an enzyme called reverse transcriptase that transcribes DNA from an RNA template. This provides RNA DNA information flow. Viruses that would most likely have reverse transcriptase are RNA viruses that do the lysogenic cycle. Viral genomes can encode reverse transcriptase, so it can be used in host cells even if it was not in the host cell originally Human immunodeficiency virus (HIV), the virus that causes AIDS (acquired immunodeficiency syndrome), is a retrovirus. After HIV enters the host cell, reverse transcriptase molecules are released into the cytoplasm and catalyze the synthesis of viral DNA. The newly made viral DNA enters the cells nucleus and is inserted as a permanent provirus into a chromosome. (Provirus viral genome that is permanently inserted into a host genome) Theory of How Viruses Came About : Researchers believe that viruses developed from plasmids or transposons. They arose AFTER cells and came from fragments of DNA or RNA that could move from one cell to another. Theory of How Viruses Came About : Researchers believe that viruses developed from plasmids or transposons. They arose AFTER cells and came from fragments of DNA or RNA that could move from one cell to another. Plasmid an extra piece of DNA that is separate from chromosomes; they can copy independently and can be transferred from one cell to another; common in bacteria, but are also found in eukaryotic cells (Ex. yeast cells) Transposons pieces of DNA that can move from one place in the genome to another A viral infection can produce symptoms by a number of different routes. Some viruses damage or kill cells by triggering the release of hydrolytic enzymes from lysosomes. Some viruses cause the infected cell to produce toxins that lead to disease symptoms. Others viruses have molecular components, such as envelope proteins, that are toxic. In some cases, viral damage is easily repaired (respiratory epithelium after a cold), but in others, infection causes permanent damage (nerve cells after polio). Many of the temporary symptoms associated with a viral infection, such as fever and aches, result from the bodys own efforts at defending itself against infection. Modern medicine has developed vaccines to mount defenses against the actual pathogen. A viral infection can produce symptoms by a number of different routes. Some viruses damage or kill cells by triggering the release of hydrolytic enzymes from lysosomes. Some viruses cause the infected cell to produce toxins that lead to disease symptoms. Others viruses have molecular components, such as envelope proteins, that are toxic. In some cases, viral damage is easily repaired (respiratory epithelium after a cold), but in others, infection causes permanent damage (nerve cells after polio). Many of the temporary symptoms associated with a viral infection, such as fever and aches, result from the bodys own efforts at defending itself against infection. Modern medicine has developed vaccines to mount defenses against the actual pathogen. Edward Jenner- Father of Immunology Edward Jenner- Father of Immunology Cow Pox Virus Cow Pox Vaccines are dead or harmless forms of the pathogen that are exposed to the body. The body makes antibodies to fight it off, and then if the person is ever exposed to the real pathogen, your body already has the materials to fight it off. Vaccines can prevent the onset of viral infections in humans. Antibiotics, which can kill bacteria by inhibiting enzymes or processes specific to bacteria, are powerless against viruses, which have few or no enzymes of their own. Most antiviral drugs resemble nucleosides and interfere with viral nucleic acid synthesis (they interfere with viral replication). Antibiotics, which can kill bacteria by inhibiting enzymes or processes specific to bacteria, are powerless against viruses, which have few or no enzymes of their own. Most antiviral drugs resemble nucleosides and interfere with viral nucleic acid synthesis (they interfere with viral replication). In recent years, several emerging viruses have risen to prominence including: HIV, Ebola, and West Nile. The emergence of these new viral diseases is due to three processes: First, the mutation of existing viruses is a major source of new viral diseases. RNA viruses tend to have high mutation rates because replication of their nucleic acid lacks proofreading. Second, a viral disease can spread from a small, isolated population to become a widespread epidemic. HIV/AIDS is considered an emerging virus because it was virtually unnoticed for decades before spreading around the world and becoming widespread and apparent in the 1980s. A third source of new human viral disease is the spread of existing viruses from other animals (bird flu, swine flu). In recent years, several emerging viruses have risen to prominence including: HIV, Ebola, and West Nile. The emergence of these new viral diseases is due to three processes: First, the mutation of existing viruses is a major source of new viral diseases. RNA viruses tend to have high mutation rates because replication of their nucleic acid lacks proofreading. Second, a viral disease can spread from a small, isolated population to become a widespread epidemic. HIV/AIDS is considered an emerging virus because it was virtually unnoticed for decades before spreading around the world and becoming widespread and apparent in the 1980s. A third source of new human viral disease is the spread of existing viruses from other animals (bird flu, swine flu). Most plant viruses stunt plant growth and decrease their yield (think crops). Usually they are RNA viruses. They can be transmitted one of two ways: - Horizontal Transmission (external source) insects are major carriers of viruses; also farming tools - Vertical Transmission (inherited) this can occur in asexual propagation or in infected seeds Most plant viruses stunt plant growth and decrease their yield (think crops). Usually they are RNA viruses. They can be transmitted one of two ways: - Horizontal Transmission (external source) insects are major carriers of viruses; also farming tools - Vertical Transmission (inherited) this can occur in asexual propagation or in infected seeds Viruses can also spread in plants via plasmodesmata. Normal Plant Plant with a Viroid Symptoms of viroids Viroids are infectious nucleic acids that infect PLANTS. They are tiny molecules of naked, circular RNA they do NOT have capsids. They show that a single MOLECULE can act as an infectious agent and spread disease. They cant encode their own proteins, but they can interfere with the normal metabolism of the plant to hamper the growth of the infected plant. Prions are infectious proteins. They are misfolded versions of normal proteins. They can convert normal proteins to the prion (misfolded) versions. They usually attack proteins that are present in brain tissue. The prion aggregation interferes with normal cellular functions and causes disease symptoms. Ex. Mad Cow Disease (degenerative brain disease) and Scrapie in sheep Prions are infectious proteins. They are misfolded versions of normal proteins. They can convert normal proteins to the prion (misfolded) versions. They usually attack proteins that are present in brain tissue. The prion aggregation interferes with normal cellular functions and causes disease symptoms. Ex. Mad Cow Disease (degenerative brain disease) and Scrapie in sheep Viroids and Prions are the simplest infectious agents! HPV virus attacking epithelial cells Proto-oncogene being converted into an oncogene Viruses can convert normal cells (proto-oncogenes) into cancer cells (oncogenes). The results in the cell cycle being de-regulated and protein production can be altered. Tumor formation follows shortly after Examples of viruses that can cause cancer: - Retroviruses - Adenovirus (lung cancer) - Papovavirus (HPV) Examples of viruses that can cause cancer: - Retroviruses - Adenovirus (lung cancer) - Papovavirus (HPV)