LK - Virus Structure & Evolution Flashcards
Virus Definition (3)
- Biological entity, consisting of protein and nucleic acid (and lipids in some)
- Non-cellular, no organelles, no ribosomes, so needs living cell to reproduce
- Reproduction by production of subunits, followed by assembly
Virus structure (4)
- Between 50% and 90% protein, forms capsid (protein ‘shell’)
- Capsid proteins protect nucleic acid, allow for recognition of host cell, facilitate infection
- Nucleic acid is either DNA or RNA, single-stranded or double-stranded, circular or linear
- Genome size varies from 4 to ~2500 coding genes
What are Filamentous Viruses? (3)
- Simplest form
- DNA is coiled
- All proteins are the same
What are Isometric Viruses? (2)
- Equal measurements throughout
- Contains different proteins with different functions
What are Enveloped Viruses? (2)
- Contains envelope proteins present in a lipid envelope
- Contains multiple coils
What are 3 features of Bacteriophages?
- Only infect bacteria
- Contain genetic material surrounded by capsid for protection only
- Tail fibers land on the bacteria and penetrate and eject viral material
What are Viroids? (4)
- No protein capsids (pure RNA)
- Circular ssRNA
- No protein-coding genes
- Mostly plant pathogens
What are the 3 main theories regarding the origin of viruses?
- Regressive theory
- Escape theory
- Ancient theory
What is the Regressive Theory and what is a limitation of it?
Viruses descend from free-living organisms, becoming parasitic
- Parasites often lose functions, so would explain simplicity, small genome
BUT huge gap between viruses and even the simplest parasite
What is the Escape Theory and what is a limitation of it?
Viruses originate from genetic material ‘escaping’ from genomes of larger organisms
- Other ‘selfish’ genetic entities known: plasmids, transposons
BUT doesn’t explain virus structure
What is the Ancient Theory and what is a limitation of it?
Viruses have their origin in self-replicating molecules in pre-cellular world
- Would explain fundamental difference between viruses and cellular life
BUT raises a conundrum: how did these ancient virus-ancestors replicate without host cells?
What is the name given to the last universal common ancestor?
LUCA
What would be present on the ToL if each of the theories were correct?
- If viruses formed from the regressive theory or escaped theory, they should be added in the form of many separate branches to the ToL
- If viruses predate cellular life, they should branch off well before split between Bacteria, Archaea and Eukarya
How are viruses classified?
Viruses: evolutionary history unknown
Virus classification based on structural traits:
- Capsid
- Nucleic acid type
- Absence/presence envelope
Are viruses alive?
7 Pillars of Life
- Organisation- Yes
- Metabolism- No
- Response to Stimuli- No
- Homeostasis- No
- Growth- No
- Reproduction- Yes
- Evolution- Yes
‘Less alive’ than bacteria, trees, dinosaurs, and you and me
