lecture 1 – What is a virus?
1. How many viruses do we carry at any given time? Why don’t these make us sick?
We constantly carry hundreds to thousands of different viruses—collectively called the human
virome—in our skin, gut, and other body sites.
Most of them do not cause disease because
our immune system keeps them under control,
many are host-specific or replication-defective, and
some simply pass through in food, air, or water (e.g., plant viruses in vegetables).
2. How can viruses be beneficial? Why do we care that they comprise the most biodiversity on
the planet?
Some viruses are beneficial because they
regulate ecosystems by controlling microbial populations,
cycle nutrients in oceans,
can protect hosts from bacterial infection, and
sometimes replace helpful bacterial functions in the gut.
3. What is the definition of a virus?
A virus is an infectious, obligate intracellular parasite consisting of genetic material (DNA or
RNA) often surrounded by a protein coat sometimes a lipid membrane(envelop).
It cannot reproduce or make proteins on its own; it must use(hijack) a host cell’s machinery to
replicate.
4. How does viral replication differ from that of bacteria?
Bacteria grow and divide by binary fission—each cell duplicates its genome and splits into two identical cells.
Viruses do not divide; they are assembled from pre-formed parts (genome + capsid ± envelope) inside the host cell.
So, viruses make components first, then assemble many particles, not one-by-one cell division.Viruses replicate by assemly of
preformed components into many particles not by Binary fussion.
5. How did the concept of ‘virus’ evolve, and what was the role of filtration?
In the late 1800s, scientists (Ivanovsky 1892, Beijerinck 1898) showed that tobacco mosaic Disease agent passed through
Chamberland porcelain filters which trap(yakalamak) bacteria
These “filterable agents” proved that viruses were smaller than bacteria and required living cells to reproduce.
The concept changed from “poisonous fluid” to distinct infectious particles visible later by electron microscopy (1939).
6. How are viruses classified?
1. Nucleic acid type – DNA or RNA, single or double stranded
2. Capsid symmetry – helical or icosahedral
3. Presence/absence of envelope(lipid membrane)
4. Particle size and morphology(dimensions of virus partciles)
7. How does the discovery of new viruses today differ from 100 years ago?
Then: Viruses were discovered by disease signs and filtration or animal inoculation.
Now: We use metagenomics, high-throughput sequencing, and bioinformatics to find viral sequences directly in
environmental or clinical samples—even without culturing.
8. What two simple facts provide an underlying simplicity and order to viruses?
All viruses are obligate intracellular parasites – they can function only after replication in a host cell.
Study questions BIOL GU4310 2021 Lecture 1
