Characteristic s of Life AND Viruses Image National Geographic Society Learning Goals List the key characteristics of all cellular life and discuss which are shared by viruses Describe the basic structure of a virus Using the SARS Cov2 viral infection as an example explain how viruses use the host cell to replicate Explain the difference between antivirals and vaccines Describe the three major hypotheses for the origin of viruses Discuss the importance of viruses Bacteriophage T4 infecting E coli Schmidt et al 2021 Bacteriophage T4 Miller et al 2003 A giant Tupanvirus Abrah o et al 2018 Which of the following is are alive a c b d Traits unifying cellular life on Earth Do viruses exhibit the traits that unify earth s living organisms Traits of Cellular Life on Earth 1 Homeostasis 2 Metabolism 3 Heredity 4 Evolution 5 Response 6 Reproduction 7 Cellular organization Traits of Viral life on Earth 1 Homeostasis 2 Metabolism 3 Heredity 4 Evolution 5 Response 6 Reproduction 7 Cellular organization But usually requires host enzymes But usually requires host enzymes But usually requires host enzymes 4000 3000 2000 1000 0 mya prokaryotic cells nucleus and other organelles BACTERIA ARCHAEA EUKARYA VIRUSES Why viruses lack cellular life traits Bacterium Viru s Viruses Tiny particles composed of genetic material single or double stranded DNA or RNA surrounded by a protein capsid Some contain an additional membrane surrounding the capsid Tiny 20 500 nanometers nm 10 9 m Most are only visible using electron microscopy Inert until they encounter a host cell Obligate parasites require a living host cell to replicate Pandoravirus Rotavirus Ebola virus Adenoviru s What do viruses do Viral particles enter host cells or inject genetic material into host cells in a variety of ways Use host cell s molecules and organelles to replicate the viral genome and manufacture new capsids Exit cell through a form of exocytosis or by bursting the host Eamer and Tremblay Inside Your Insides A Guide to the Microbes that Call You Home SARS CoV 2 infection cycle https youtu be xRTMXvZ75dY t 52 Antivirals vs Vaccines Specifically target viral replication cycle Inactivate virus particles Prevent virus attachment Prevent assembly or release of new virions Only work once a person has been infected i e when there are viruses to disrupt Stimulate the immune system to recognize virus if infection occurs Your body is ready to more rapidly respond control contain virus before it can spread too much Types Inactivated Live attenuated mRNA Subunit vaccines There are no common genes shared by all viruses viruses lack a common ancestor and do not constitute an evolutionarily related group Papillomaviridae Mimiviridae Ligamenvirales Small sets of viruses have a common ancestor creating evolutionary groups Many viruses show more homology to the genes of their hosts than to the genes from other viruses Viruses are selfish replicators that use host machinery to produce progeny Flaviviridae Picornavirales Herpesvirales Circoviridae Caudovirales Adenoviridae Tombusviridae Mononegavirales Retroviridae Orthomyxoviridae Nidovirales Tymovirales Geminiviridae Parvoviridae Many others including unclassified families etc Taxonomy of viruses from International Committee on Taxonomy of Viruses ICTV So where do viruses come from Origins of Viruses Progressive hypothesis Pieces of genetic material capable of moving within a genome mobile genetic elements e g transposons gained the ability to exit one cell and enter infect another retrotransposon gag pol TRANSCRIPTION AAAAAA TRANSLATION Reverse transcriptase and integrase REVERSE TRANSCRIPTION Origins of Viruses Regressive hypothesis Simplification of formerly free living organisms becoming parasitic to the point of losing ability to independently replicate Giant Mimi viruses a puzzling intermediate dsDNA Contain hallmark genes of cellular life including tRNA protein biosynthesis even metabolism Likely acquired through HGT No evidence yet of independent protein translation Can be infected by other viruses Recently many more discovered through DNA sequencing La Scola 2003 Science Origins of Viruses Virus first hypothesis Earliest life forms were viruses today s viruses are the descendants of the first self replicating entities the other two hypotheses assume cellular life came first What if all viruses disappeared Alright Examples of viral pathogens of humans Rhinovirus causes colds Zika virus Ebola Measles Influenza SARS CoV 2 HPV Oh no If all viruses suddenly disappeared the world would be a wonderful place for about a day and a half and then we d all die that s the bottom line All the essential things they do in the world far outweigh the bad things Tony Goldberg an epidemiologist at the University of Wisconsin Madison Benefits of viruses Ecosystem functioning 90 of ocean biomass is microbial and generates 50 of oxygen Primary regulator of these bacteria is viruses Benefits of viruses Density Dependence Enhance animal and plant health PCV 1 infected J and virus free J Benefits of viruses Enhance health of hosts Benefits of viruses Enhance health of hosts More examples Plant viral diseases cause crop losses but can produce interesting patterns in horticultural plants https apsjournals apsnet org doi pdf 10 1094 PDIS 11 11 0928 FE Fungal viruses can slow the growth of fungal pathogens like chestnut blight and cyanobacterial and algal viruses are the major check on the explosive growth of phytoplankton blooms Viruses in the human genome Approximately 8 of the genome consists of remnants from ancient viral infections originated from viruses An additional 40 consists of repeat regions likely to have Some can be copied and inserted in new places in the genome transposons generally regulated but can become a problem when regulatory mechanisms break down These sequences can play important roles in disease Example loss of TDP 43 regulation in some forms of amyotropic lateral sclerosis Benefits of viruses Enhance health of hosts Review Viruses are non cellular obligate parasites Viruses are tiny particles that contain genetic material enclosed inside a protein coat Viruses lack common ancestor There are three hypotheses to explain the origin of viruses All viruses are not bad news they are crucial for ecosystem functioning and for the health of organisms Viruses drive the evolution of their hosts