Viruses -The Boundary of Life At the boundary of life, between the macromolecules (which are not alive) and the prokaryotic cells (which are), lie the viruses and bacteriophages (phages). Viruses are found everywhere. Viruses consist of a core of nucleic acid, either DNA or RNA, and a protective coat of protein molecules and sometimes lipids.
Cells and VirusesCharacteristic Cell VirusStructure Cell membrane, cytoplasm; eukaryotes also contain nucleus and organellesReproduction Independent cell division either asexually or sexuallyGenetic Code DNAGrowth and Yes; in multicellular Development organisms, cells increase in number and differentiateObtain and Use yes EnergyResponse to yes EnvironmentChange Over Time yes
Naming Viruses • International Committee on Taxonomy of Viruses names them based on three characteristics: • Type of nucleic acid (DNA or RNA) • Is the nucleic acid double or single stranded • Presence or absence of nuclear envelope
Prokaryotes Vs. Eukaryotes Vs. Viruses• No membrane • Nucleus with • No nucleus bound nucleus membrane• Has a cell wall • Only plants have • No membranes• Only a few cell wall • No organelles organelles or • Contains many none at all. organelles • Cannot• Has a capsule • Has a lipid bi- reproduce on surrounding it layer membrane its own• Three main types. surrounding it. • Generally not • Specialized by thousands of considered different sizes alive by most and shapes. standards
Bacteriophages Have Multicomponent ParticlesThe head consists of an icosahedron that has very tightly packed DNA. Nearly 20 proteins are found in the head. An equally complex tail sheath forms a helical component. The head is connected to the tail sheath by a neck that is composed of four to five proteins. An end plate with lysozyme activity and pins at the base of the sheath contain several different proteins. Tail fibers used to recognize receptor proteins on the surface of the bacterial cell consist of numerous additional proteins. Phage assembly and infection processes require coordination of many genes.
Viruses and Living Cells• Viruses must infect a living cell in order to grow and reproduce• They also take advantage of the host’s respiration, nutrition and all the other functions that occur in living things• Therefore, viruses are considered to be parasites
Viral Reproduction• Steps of Lytic Cycle • Attachment • Entry • Replication • Assembly • Lysis/Release (lyses the cell)
How do viruses replicate?2 methods of replication: 1. Lytic Cycle – the virus enters the cell, replicates itself hundreds of times, and then bursts out of the cell, destroying it. 2. Lysogenic Cycle – the virus DNA integrates with the host DNA and the host’s cell helps create more virus DNA. An environmental change may cause the virus to enter the Lytic Cycle.
In the lytic cycle,the virusreproduces itselfusing the hostcells chemicalmachinery. Thered spiral lines inthe drawingindicate theviruss geneticmaterial. Theorange portion isthe outer shellthat protects it.
In the lysogeniccycle, the virusreproduces byfirst injecting itsgeneticmaterial,indicated by thered line, into thehost cellsgeneticinstructions.
Viruses Enter Living Cells Viruses enter bacterial cells by punching a hole in the cells wall and injecting its DNA
Viruses Enter Living Cells Viruses enter plant cells through tiny rips in the cell wall. Viruses enter animal cells by endocytosis.
Viriods • Much smaller than viruses • Just consist of small sRNA molecule • No protein coat • Infect plants
Prions• Proteinaceous infectious agents• Contain only protein, no nucleic acid• Linked to number of fatal diseases in humans and animals• Obligate intracellular parasite• How does it replicate if no nucleic acid? • Prion protein converts host protein to prion protein
Prions• Cannot be killed by UV light or nucleases, can be killed by proteases and heat• Usually cannot be transmitted across species
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