A2.3 Viruses PDF

A2.3 Viruses HL only A2.3.1 Structural features common to viruses Viruses are non- cellular agents that infect cells and reproduce inside them Viruses probably have multiple origins. (Cells comes from LUCA) Viruses share few features: Small size: 20 - 300 nm Virus must be smaller than...

A2.3 Viruses HL only A2.3.1 Structural features common to viruses Viruses are non- cellular agents that infect cells and reproduce inside them Viruses probably have multiple origins. (Cells comes from LUCA) Viruses share few features: Small size: 20 - 300 nm Virus must be smaller than host cells so they can enter them.They lack cytoplasm and other features Fixed size: they do not growth, not increase in size Nucleic acid as genetic material: DNA or RNA using the universal genetic code. They produce their proteins using the host cells.  Capsid made of protein: before viruses are release from their host cells, their genetic material is enclosed in a protein coat called capsid. This capsid is made of repeating protein subunits that form a symmetrical shape. Viruses with no cytoplasm: few or no enzymes. The enzymes are used to replicate the viruses A2.3.2 Diversity of structure in viruses 1. Diversity of genetic material DNA (double strand) or RNA (single strand) Variation in length of this molecule: circular with no ends or lineal with two ends Variation in how viruses replicated: for RNA viruses a. positive -sense RNA viruses use their genes directly as messenger RNA b. Negative - sense RNA viruses transcribe their genes to make messenger RNA c. Retroviruses make double -stranded DNA copies of their RNA and then transcribe to produce RNA viruses 2. Enveloped and non- enveloped viruses To be released from the host cell, viruses burst in a process call lysis. Common in viruses that infect animal cells. Some viruses in this process are covered by a membrane that comes from the cell membrane. This cell membrane help the viruses to contact the host cell and infect it. Other viruses do not become enclosed in a membrane (non- enveloped viruses) They infect bacteria and plants. Enveloped and non- enveloped viruses A2.3.3 Lytic cycle of a virus Bacteriophage lambda has proteins in the tails that The viral infection becomes increasingly bind to the outer surface of its host, Escherichia coli. widespread within the body and the effects of This virus is virulent when follows lytic cycle the disease become more severe. Humans because destroys the host. produce antibodies that destroys all copies of a The DNA change from linear to circle DNA. virus within the body. Lysogenic cycle: the viral DNA becomes If virus kill the host cell they need to spread into integrated into the bacterial DNA molecule another host. and does not kill the cell. Lytic cycle: the virus reproduced and then bursts out of the host cell killing it. Viruses that infect plants or animal cells follow lytic cycle. Lytic cycle Maltoporin: protein in the inner membrane of bacteria Lysogenic cycle of a virus  Bacteriophage lambda COVID-19 HIV Type of Bacteriophage: DNA virus Coronavirus RNA Retrovirus: converts virus uses bacteria or Archae as uses animal cells as RNA genome into host host DNA Enveloped Non-enveloped Enveloped Enveloped or non-env. Genetic double -stranded of DNA Single- stranded of 2 copies of single- material RNA strand of RNA Distinctive features Host Escherichia coli Human cells T-helpers cells in Evidence for several origins of viruses from other organisms All viruses have the same genetic code, with few differences. All viruses are descending from a single ancestral virus. Viruses are obligate parasites. They need a host cell to replicate. Genetic code in inherit from LUCA Viruses must have evolved from cells Hypothesis: 1. Progressive hypothesis 2. Regressive hypothesis Progressive hypothesis Regressive hypothesis Viruses are build up in a series of steps by Viruses develop from cells in a series of steps taking and modifying cell componentes. by loss of components. Retrotransposons are sequences of nucleotides Some virus are small and simple (polio virus) that occur widely in the genome of eukaryotes. complex viruses (smallpox virus) The influenza virus enveloped virus / single-stranded RNA The influenza virus can also be transmitted between species, particularly between birds and humans. Proteins can change and be put together in new combinations, creating novel strains of the virus that have the potential to cause a pandemic. Rapid evolution of the influenza virus explains how a person can contract influenza repeatedly and also why protection depends on vaccination every year. Each vaccine contains several strains of influenza virus. The HIV virus Retrovirus that uses reverse transcriptase to convert its single-stranded RNA genome to DNA. There are two factors together give HIV the highest known mutation rate of any virus. Mutations in the env gene that codes for this protein allow HIV to evolve to use different cell types in the human body as hosts. HIV evolve to become resistant to antiretroviral drugs.