I.Chromosome Stucture A.Strand of DNA B.Histones 1.Grouped in fours 2.DNA wraps around these four histones 3.Four histones + DNA = nucleosomes C.30nm Fiber 1.Histone groups link together D.Looped Domain 1.30nm fibers form loops E.Chromosome 1.Coiling of looped domains.
I.What Is the structure of a virus? A.Inner Structures 1.ds DNA or RNA, ss DNA or RNA 2.Usually either a circle or single strand B.Outer Structures 1.Capsid a)Protein shell surrounding genome b)Built from multiple proteins called capsomeres
2.Other Structures a)Viral envelope - membranous envelope around capsid to help infect cells 1)Derived from host cell membrane – has host specific proteins 2)Also contain specific viral proteins b)Protein Tail 1)Attached to the head 2)Example: bacteriophages
II.Virus Reproduction Host range – types of hosts a virus can infect A.General Features 1.Binding a)Viral surface protein binds to protein surface receptor on outside of the cell 2.Genome Entry a)Can be injected b)Entire virus can be taken in by receptor mediated endocytosis c)Virus may fuse with cell membrane
3.Reproduction a)Virus takes control of cell 1)Uses cell’s own proteins to make new virus proteins b)Viral proteins spontaneously assembleinto new viruses 4.Release a)Viruses lyse the cell pouring out into body or b)Viruses take cell membrane with them similar to vacuoles coming off ER or golgi body
A.Reproductive Cycle in Detail 1.The Lytic Cycle – Death of the host cell –Virulent phage – phage that only uses lytic cycle a)Follows the same steps as shown in A.
2.Why have bacteriophages killed off all bacteria? a)Natural selection favors mutants that have receptors that no longer can be bound by phages b)Bacteria contain restriction enzymes which cut up foreign DNA 1)Bacteria’s DNA is methylated to protect itself from restriction enzymes c)Some phages don’t kill host bacteria 3.The Lysogenic Cycle –Phages use both lytic and lysogenic cycles –Called temperate phages
a)After insertion of viral DNA, the viral DNA is incorporated into bacteria DNA 1)Viral protein does the cutting and insertion of viral DNA into bacterial DNA 2)Called prophage » one gene prevents transcription of viral DNA 1)When bacteria reproduce, viral DNA is reproduced and passed along to new cells b)Viral DNA can leave the bacterial DNA and start the lytic cycle 1)Occurs in response to an environmental signal
B.Reproduction of Animal Viruses 1.Most have envelope and fuse with cell membrane to enter cell
2.Retroviruses (HIV) a)RNA virus that carries reverse transcriptase which makes DNA from RNA a)Works in cytoplasm to make DS DNA from RNA b)Integrase a)In nucleus c)Protease a)In cytoplasm
C.Evolution of Viruses 1.Probably started as DNA in the environment from damaged cells a)Plasmids are circular DNA found in bacteria and yeast b)Transposons – DNA segments that can move from one location to another 2.Somehow evolved protein coat to help get into cells 3.Interestingly, DNA of a virus is more closely related to host DNA than to a different type of virus. a)Suggests multiple evolutions
III.Scary Stuff A.Viral diseases in Animals 1.Causes of viral symptoms a)Kill or damage cells b)Cause cells to produce toxins c)Contain proteins that act as toxins. 2.Why do people recover from some viruses but not others? a)Can the cells surrounding the infected cells reproduce? 1)Colds- epithelial cells reproduce easily 2)Polio – nerve cells don’t reproduce
3.What is a vaccine? a)A heat-killed or attenuated (modified to not be disease causing) version of the virus. b)Vaccine induces an immune response towards the virus c)This immune response primes the immune system in case it encounters the virus again. 4.Why can’t we cure viral infections? a)There is very little in a virus to target with a chemical to kill since it uses the cell’s own machinery to replicate. b)Inside cells c)Most affected replication of the virus or try to target reverse transcriptase
B.Emergent Viruses 1.New viruses or ones the appear apparently out of nowhere a)AIDS b)Ebola c)SARS 2.Where do they come from? a)Mutations of new viruses b)Spread from small populations (gay men and HIV) c)Spread from animals 1)When an animal is exposed to multiple viruses, they can recombine making novel viruses that can infect humans.
3.Why are they bad? a)We have no immunity to them since we have never been exposed to them before 1)epidemics 2)Pandemics b)Ease of travel makes the virus’ ability to spread easier….infect more people quicker. C.Viral diseases in Plants 1.Significant economic effect 2.Types of transmission a)Horizontal – enters plant through physical damage already present b)Vertical – inherits disease from parents 3.Spread via plasmodesmata
C.Viroids and Prions 1.Viroids a)Small circular RNA molecules that infect plants 1)Only a few 100 nucleotides long 2)Do not encode proteins but replicate using host proteins 3)Cause errors in regulatory systems that control plant growth
1.Prions a)Short infectious proteins 1)Cause mad cow disease and other brain diseases b)Most likely transmitted in food c)Problems 1)Prions act slowly »incubation periods of years »Means identification of disease doesn’t occur for a long time…more time to infect 2) Seem to be indestructible »Not destroyed or deactivate by heating »ie. cooking
d)How do prions replicate? a)Appear to come from normal proteins b)When a mutated/misfolded protein gets into the cell it causes the normal proteins to be misfolded c)Somehow this group of misfolded proteins get together which cause other proteins to misform.