VIRUSES Virus (Latin): meaning poisonous slime of plant or animal origin Viruses are non-living. 1. Acellular: lack cell membrane and organelles 2. No metabolic activity 3. Lack energy generating system 4. They are obligate parasites - only capable of reproduction in a host cell. 5. Genetic material: DNA or RNA, housed within a protein coat called a capsid Virus size: nm

2 Virus Characteristics (cont.) There may be a further layer referred to as the envelope, which surrounds the capsid - this is found in animal viruses and is derived from the membrane of the host animal cell. Influenza viruses (100nm each), with capsid and membrane visible Protein spikes may protrude from the viral particle - these are involved in the attachment of the particle to the host cell.

3 v=7iVm1uEIyP0 Viruses are classified based on 1. Capsid’s size and shape (Cylindrical, Polyhedron, Enveloped, Binal (complex) shape) 2. Types of genetic materials (i.e. DNA or RNA) 3. The number of strands in Nucleic Acid - either single stranded or double stranded form 4. Nature of host (plant, animal or bacteria) 5. Presence or absence of envelope VIRUS CLASSIFICATION Polyhedron Binal (irregular) shape Cylindrical/ helical Spherical

4 VIRAL SHAPES EXAMPLES

5 BACTERIOPHAGE STRUCTURE capsid (protein coat) nucleic acid tail tail fibres cell membrane viral RNA/DNA injected into host cell Bacteriophage: is a virus that infects bacteria

6 Largest Virus Found in 2011! Pandoravirus (~1  m) Irregular shape, Look like a bacterium DNA as genetic material Infects amoebas Not a threat to human Found in sediments in Chile, Australia

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8 Virus Reproduction Viruses undergo replication within a host cell incubation period: time between when the virus infects a cell and when the virus ruptures the cell 2 possible cycles: lytic and lysogenic 1. The LYTIC Cycle (shorter incubation period) E.g. Bacteriophage T4 infects E. coli

9 A. Attachment B. Insertion C. Replication D. Assembly E. Lysis & Release bacterial DNA - Host’s DNA/RNA get digested - Virus injects its nucleic acid into host cell Viral DNA Bacterial host cell The host’s metabolism replicates the viral RNA/DNA New viral particles are assembled virus Cell lysed due to viral enzyme; new phage go on infect other hosts

10 Bacteria are NOT defenceless against phage infections Bacteria use restriction enzymes that act like scissors to cut up phage’s DNA, making it useless - Bacteria with effective restriction enzymes survive and reproduce - Any phages that develop resistance to the restriction enzymes also have a reproductive advantage  Bacteria and virus arms race!

11 Summary of lytic cycle (HW- self note) 1. Attachment & 2. Insertion: 3. Replication: 4. Assembly: 5. Lysis & Release:

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13 A. Attachment & Entry bacterial DNA/RNA virus Cells undergoing this cycle can be triggered into reproducing in the lytic cycle Virus Reproduction 2. The LYSOGENIC cycle (longer incubation period)

14 B. Integration provirus Viral nucleic acid becomes a part of the bacterial nucleic acid Virus Reproduction 2. The LYSOGENIC cycle (longer incubation period)

15 C. Multiplication of Genome Virus Reproduction 2. The LYSOGENIC cycle (longer incubation period)

16 May now undergo the lytic cycle D. Cell Division Virus Reproduction 2. The LYSOGENIC cycle (longer incubation period)

17 Attachment: The virus invades a host cell and injects its DNA (or RNA) into the host. The viral DNA is merged with the host DNA. The viral DNA remains dormant for a period of time (provirus) The viral DNA is extracted from the HOST by a "trigger". The host cell replicates the parts of the virus and other materials in viral DNA  LYTIC CYCLE New viral particles are assembled in the host cell. The LYSOGENIC Cycle (longer incubation period)

18 Bring 2 cycles together

19 Compare lytic and lysogenic cycles Lytic cycleLysogenic cycle Fate of virus DNA/RNA after infecting the host Take over host DNA Becomes part of host DNA Incubation time in host cell Short; new viruses released within host’s lifetime or in the next generation Long; new viruses released after several generations of host Onset of viral diseases symptoms Immediatedelayed

20 VIRUSES and DISEASES COLD SORE (Kissing Disease) Caused by herpes simplex virus (a db stranded DNA virus). The sores appear during lytic cycle The sores disappear (when the viruses is in its provirus stage) on the skin throughout the person’s lifetime. Virus is periodically reactivated in times of physical or emotional stress (newly added note) A different kind of sore: canker sore

21 Retro-viruses VIRUSES and DISEASES

22 Normal genetic information transfer is from DNA to RNA Retro-viruses use the reverse process (i.e. RNA to DNA): -After injecting RNA into host cell, retro-viruses have an enzyme called Reverse transcriptase that can then convert their RNA into DNA Retro-viruses -The viral DNA merges with host’s DNA and can replicate for many years showing no harm to the host. - Can switch to lytic cycle any time. What triggers this switch is not yet known. VIRUSES and DISEASES HIV video: What’s the inference for showing HIV video under retro-viruses topic?

To sum it up… 23

24 AIDS (acquired immune deficiency syndrome): Caused by HIV- a retro virus The HIV destroys the body’s T- lymphocytes, which help fight off infections from other diseases When leaving the host cell, new HIV take a piece of the host cell membrane to disguise themselves from the host body’s immune system. HIV (AIDS) VIRUSES and DISEASES HIV video:

25 can be used to make lots of copies of desirable genes VIRUSES and BIOTECHNOLOGY

26 VIRUSES and BIOTECHNOLOGY Viruses well-known for inserting their DNA/RNA into host’s DNA can be used in ‘gene therapy’ to repair bad genes The virus must be disarmed To correct for a genetic disorder permanently, a stem cell is used. -The virus carrying the desired gene is thus let infect the patient’s stem cells. - The result is the desired gene is permanently expressed once the stem cells are injected into the bone marrow of the patient

27 Vaccines Vaccines are mixtures of dead or weakened forms/parts of dangerous viruses When inside a host, the antigens on the surface of the weakened viruses triggered white blood cells to produce antibodies, without causing an infection Antibodies are chemicals produce by your body to recognize foreign antigens antigens These antibodies has long lasting memories that allows the immune system to react quickly if it contacts the real virus NPR Flu Attack video:

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29 White Blood Cell Foreign Invader antibody antigen macrophage Vaccines

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31 Smaller than virus, with no capsid Single stranded, circular RNA that do not code for proteins (unlike viruses; viral RNA do code for proteins) Viroids Mis-shaped potatoes caused by viroids Viroids under microscope

32 Are single proteins Found in brain of infected animals Once eaten, prions go to the animal’s brain Cause diseases by interacting with normally shaped proteins causing them to become abnormal and infectious Responsible for brain diseases such as mad cow diseases in cow, scrapie in sheep and Creutzfeldt-Jakob disease in human BSE = bovine spongiform encaphelopathy or mad cow disease Prions

33 News Update Univ. of Cadiff- School of Med

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