Viruses exist at the boundary between living and non-living things. They consist of nucleic acids and proteins but cannot reproduce outside of a host cell. Viruses come in different shapes and sizes and have unique structures suited to their method of infection. They hijack the machinery of living cells to reproduce either through lytic or lysogenic cycles, ultimately resulting in the destruction of the host cell. While viruses rely on host cells, they are not considered alive as they do not have their own metabolism or respond to their environment.

1. Viruses

2. Living or Non-Living?
•Infectious particles of nucleic
acid and proteins
•Cannot “live” (reproduce)
outside a host

3. 1st virus discovered-Tobacco
Mosaic Virus (TMV)

4. 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.

5. Are viruses alive?

6. Cells and Viruses
Characteristic Cell Virus
Structure Cell membrane, cytoplasm;
eukaryotes also contain
nucleus and organelles
Reproduction Independent cell division
either asexually or sexually
Genetic Code DNA
Growth and Yes; in multicellular
Development organisms, cells increase
in number and differentiate
Obtain and Use yes
Energy
Response to yes
Environment
Change Over Time yes

7. 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

8. 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

9. VIRUS STRUCTURE

11. Adenovirus, a naked virus, with a polyhedral
capsid and a fiber at each corner

12. Influenza virus, surrounded by an envelope
with spikes

13. Virion Structure
Lipid Envelope Nucleic Acid
Protein
Capsid
Virion
Associated
Spike
Polymerase
Projections

15. Types and shapes

16. Helical
Rod like with capsid proteins winding
around the core in a spiral
Tobacco Mosaic Virus

17. Polyhedral
Has many sides
Most polyhedral capsids have 20
sides and 12 corners

18. Polyhedral capsid attached to
a helical tail.

19. Flu
virus

20. T4 Bacteriophage

21. HIV

23. A retrovirus injects the enzyme, reverse transcriptase into the cell
to copy viral RNA into DNA.

24. Bacteriophages

25. Head
DNA
Tail
Tail fiber

26. Bacteriophages Have Multicomponent Particles
The 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.

27. Viral Infection

28. 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

29. Viral Reproduction
• Steps of Lytic Cycle
• Attachment
• Entry
• Replication
• Assembly
• Lysis/Release (lyses the
cell)

30. 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.

31. In the lytic cycle,
the virus
reproduces itself
using the host
cell's chemical
machinery. The
red spiral lines in
the drawing
indicate the
virus's genetic
material. The
orange portion is
the outer shell
that protects it.

32. In the lysogenic
cycle, the virus
reproduces by
first injecting its
genetic
material,
indicated by the
red line, into the
host cell's
genetic
instructions.

33. Viruses Enter Living Cells
Viruses enter bacterial cells by punching a hole
in the cells wall and injecting its DNA

34. Viruses Enter Living Cells
Viruses enter plant cells through tiny rips
in the cell wall.
Viruses enter animal cells by endocytosis.

35. Viriods
• Much smaller than viruses
• Just consist of small sRNA
molecule
• No protein coat
• Infect plants

36. 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

37. Prions
• Cannot be killed by UV light or
nucleases, can be killed by
proteases and heat
• Usually cannot be transmitted
across species