Revisiting the Definition of Living Thing

Transcription

1 Biology of Viruses (Ch 0 p77 and 88-9) What do you already know about viruses? Revisiting the Definition of Living Thing How did we define a living thing? H0 DOMAIN ARCHAEA virus So, if the Cell Theory defines what is alive then. Lets Look at the Anatomy of a Virus Viruses consist of only two macromolecules:. Nucleic Acid ( either or RNA). Protein A protein coat (capsid) and protein spikes.(some viruses have carbohydrates, too) Lets Look at the Anatomy of a Virus What things (organelles, etc) does a virus NOT have? Would you claim that a virus is a living thing? Why or why not? Viruses straddle the line between what is alive and what is inanimate -viruses are amazingly small; most range from 0-00 nm (that is about 000x smaller than bacteria!) -viruses do not have cells, not even one cell, nor do they have any of the organelles the average cell has (ie. no mitochondria, no nucleus, no chloroplasts, no cytoskeleton, etc) -viruses do not respire or dispose of waste nor do they need any nutrients and they cannot reproduce on their own -viruses have no membranes, or cytoplasm.

2 Biology of Viruses A virus is a microscopic parasite that infects cells of biological organisms. Viruses are obligate intracellular parasites. What does that mean? Biology of Viruses A virus cannot reproduce on its own. Viruses have (or RNA) but they lack organelles (machinery) needed to perform transcription and translation. They are obligate parasites, because they MUST invade a cell of the host organism and usurp the transcriptional/translational machinery in order to reproduce Bacteriophage (phage) was the first virus to be characterized. Bacteriophage is a virus that attacks bacteria One bacterium Who does phage invade? What is in this picture? Phage Viral Typical lifecycle of a virus.virus enters host.virus latches on to host cell (different viruses attack different cells) flu virus: cells of the nose and throat rabies: cells of the brain and nervous system HIV: white blood cells (immune system cells).virus injects its nucleic acid into host cell 4.Viral or RNA hijacks the host cell s ribosomes and cellular enzymes and uses them for its own reproduction 5. Virus may exit cell (usually killing cell in process) and infect other cells Simplified Virus Lifecycle Virus attaches to host cell and injects its nucleic acid Virus hijacks host cell machinery, to read its own code and make viral proteins New virus particles EXIT host cell. New viruses exit host cell. Move on to infect other cells.

3 Generalized lifecyle of a bacteriophage Phage virus contains as its nucleic acid Types of Viruses Three major types (categorized by type of nucleic acid they contain and/or accessory enzymes):.. RNA. Retroviruses: RNA + a special enzyme called Transcriptase (RT) How does each type of virus replicate itself inside of the host? How viruses replicate. viruses: is added (inserted) into host (in nucleus). Can be read by RNA polymerase and translated into viral proteins.. RNA viruses: In cytosol, RNA is replicated by viral enzyme, one copy goes to host ribosome.viral proteins made. One copy serves as part of next virus.. RNA + Transcriptase viruses: a few more steps involved. RNA first converted into. Then, the steps are similar to viruses (#). VIRUS Protein spike RNA virus, ie. Influenza (genome) Plasma membrane of host cell Entry Uncoating Protein coat Envelope RNA synthesis by viral enzyme Avian Flu N mrna 4 Protein synthesis 5 RNA synthesis (other strand) Template New viral proteins 6 Assembly New viral genome H 7 Exit H5N is a subtype of the species called avian influenza virus (bird flu). Avian flu is a disease and avian flu virus is a species. The avian flu virus subtypes are labeled according to an H number and an N number.

4 Retrovirus (RNA and Transcriptase) HIV, the AIDS Virus HIV is a retrovirus normally Retroviruses: Transcription RNA protein - Transcription. RNA Transcription. RNA (RNA to ribosome; viral proteins made) protein A retrovirus is an RNA virus that reproduces by means of a molecule (but it contains RNA only!) It copies its RNA to using reverse transcriptase (RT) Envelope Protein Protein coat RNA (two identical strands) transcriptase (a) HIV Figure 0.0a Retroviruses Retroviruses Retroviruses contain RNA and the enzyme REVERSE TRANSCRIPTASE Transcriptase (RT) changes the way we think about the Central Dogma Transcriptase (RT) Viral entry into host cell RNA is reverse transcribed into the viral is inserted into the host genome this is also an enzyme-mediated step (integrase) com/kuby/content/anm/ kb0an0.htm How HIV reproduces inside a cell strand Doublestranded transcriptase 5 Nucleus Cytoplasm Chromosomal Provirus 4 Transcriptase (RT) RT is also referred to as RNA-directed polymerase RT is an enzyme that is able to transcribe RNA into. com/kuby/content/anm/ kb0an0.htm Viral RNA and proteins 6 RT has a high error rate (up to about in,000 bases) when transcribing RNA into. This high error rate allows retroviruses to mutate rapidly. (b) The behavior of HIV nucleic acid in an infected cell Transcriptase (RT) Figure 0.0b 4

5 Drugs for the treatment of HIV. AZT (and similar drugs). Protease Inhibitors How do these drugs work? Or, what is their mechanism of action? AZT inhibits Transcriptase RNA - Transcription The mechanism of action of AZT is to inhibit Transcriptase (RT). strand Doublestranded and proteins 6 transcriptase Nucleus 5 Cytoplasm Chromosomal Provirus 4 AZT inhibits Transcriptase. AZT closely resembles Thymine(T). RT picks up AZT instead of T while trying to transcribe RNA to. Using AZT instead of T inhibits elongation of the transcript. - Transcription. RNA AZT Part of a T nucleotide Thymine (T) Figure 0. Drugs for the treatment of HIV. AZT. Protease Inhibitors How do protease inhibitors work to fight HIV? What is mechanism of action of protease inhibitors? First, we need to know what a protease is and what it does. Proteases are enzymes that cut other proteins at highly specific locations. Normally, the HIV proteases snip long pre-proteins into their active, shorter forms. These newly snipped proteins allow virus to mature/exit/re-infect. Inhibiting a viral protease can lead to inhibition of maturation/exit/further infection. Protease Inhibitors The protein, protease, is critical to the HIV lifecycle The function of protease is to participate in modification of viral proteins into active viral proteins Protease Inhibitors Inactive viral proteins (active forms needed form viral infection of next host cell) Protease Protease snips the inactive protein Active viral proteins Human Immunodeficiency Virus (HIV) infection can lead to Acquired Immunodeficiency Syndrome (AIDS) AIDS claims over 8,000 lives every day. 5 people die of AIDS every minute There are now over 40 million people living with HIV and AIDS (90% of those with HIV live in developing countries) HIV infects cells of the immune system, in particular, white blood cells (leukocytes), specifically the helper T cells. What are possible treatments for HIV infection? transcriptase inhibitors Protease inhibitors 5

6 So, by definition a virus certainly is not alive. Or is it? What do we know? Viruses are responsible for numerous life-threatening and/or life-disrupting diseases: Measles, Influenza, Chicken Pox, Herpes (oral and genital) Rabies, Ebola Virus, West Nile Virus and HIV (AIDS) Viruses cause diseases that are contagious Some of these diseases are currently without cure Life and living are words that the scientist has borrowed from the plain man. The loan has worked satisfactorily until comparatively recently, for the scientist seldom cared and certainly never knew just what he meant by these words, nor for that matter did the plain man. Now, however, systems are being discovered and studied which are neither obviously living nor obviously dead, and it is necessary to define these words or else give up using them and coin others. -British virologist Norman Pirie, c. 94 The last two Technology topics Cloning (by nuclear transplantation) Cloning by Somatic cell nuclear transfer How was Dolly the Sheep cloned?? Transgenic Animals (remember the pig article?) Somatic cell nucleus from Dolly s mom This is the sheep that carried Dolly Genetics Savings and Clone Little Nicky 6

7 Transgenic Animals Instead of injecting a nucleus from an adult animal into an egg, of interest is added to egg Egg is placed into surrogate female 98- The first transgenic mouse Engineered by Richard Palmiter (at the UW) and Ralph Brinster (U Penn) Gene of interest in a microscopic micropipette Mouse on right is expressing many, many copies of rat Growth Hormone US scientists make pigs with heart-healthy fats Reuters Sun. March 6, 006 WASHINGTON (Reuters) U.S. scientists said on Sunday they had genetically engineered pigs that make beneficial fatty acids and may one day serve as a healthier source of pork chops or bacon. The pigs produced omega- fatty acids, compounds that have been shown to improve cardiac function and reduce the risk of heart disease in people. The only way now for humans to get omega-s is through taking dietary supplement pills or by eating certain fish. Some fish, however, may have high levels of toxic mercury. Seeking another source of omega-s, researchers transferred a worm gene called fat- into pig cells in a laboratory. They used cloning technology to create embryonic cells that were implanted into the womb of a normal pig. The gene produced an enzyme that converted the less desirable omega-6 fatty acids that the pigs naturally produced Fat- Glycerol phospholipids 7