Viruse
Structure features of
s
viruses.
Viruses are non living
They are not considered
living because
1. They are not made of
cells
2. They cannot keep
themselves in a stable
state
3. They do not grow
4. They can only replicate
themselves
5. They cannot perform
independent
metabolism
Diversity in structure of Viruses vary in size, shape, genetic material type,
viruses genes present and enveloping
The main function of
virus is
Bacteriophage lambda are obligate intracellular viruses that specifically
infects bacteria. Lambda is a phage which infects bacteria. After it infects
the bacteria, alternative strategies can be followed.
Viruses that infect plants and animals usually follow lytic cycle. As the result they spread
from cell to cell within the host organism. Usually, animal host fights off these viruses for
e.g. humans have antibodies which will help to fight off the viruses.
Phage attachment to host
cells
Phage attachment is generally very
random . The bacterium must have a
compatible receptor to let enter the
phage.
Phage DNA enters the
host cell
Phage protein •Inside the bacteria the viral DNA forms a circle.
synthesis There is a small nick or cut at one strand. The
replication enzymes are used to make more
copies of the DNA by the free carbon end. The
other strand is displaced automatically, and the
other strand is copied discontinuously.
The host cells ribosomes
are used to translate
phage RNA into phage
proteins
Assembly of new phage
viruses
Enzymes produced by the phage gradually weakens the
bacterial cell wall and eventually the bacterial cell lyse
releasing approximate 100 -200 phage progeny into the
environment. These new phage particles can infect new host
cells damaging host cells, viruses are said to be virulent.
Lysogenic
cycle.
The process of phage attachment
Phage attachment to host remains the same as lytic cycle.
cells
Phage attachment is generally very
random . The bacterium must have a
compatible receptor to let enter the
phage.
Phage DNA entry into the host
cell
Phage DNA integration with the host
cell
The prophage genome is then replicated passively along with the host genome
during DNA replication. As the phage genome is generally small , the bacterial
hosts are normally relatively unharmed by this process
The host cell divides creating 2 daughter cells
which each contain the prophage. When the
virus is a prophage, it is ‘temperate’ meaning it
is in a dormant state. It does not kill the host and
does not actively infect other cells.
Return to lytic phase – If the
bacterium containing prophage is
exposed to stress, such as UV light,
low nutrient condition, or chemicals,
phage may spontaneously extract
themselves from the host genome
and enter the lytic cycle
While the virus remains in the
lysogenic phase, it is called
temperate phage, and it can be
beneficial to the bacterial cell----
HOW?
•Possible Origins of Viruses
•Viruses show an incredible variety in
their structures and genetic material,
hinting at multiple potential origins.
Some theories on how viruses might
have arisen include:
•Viruses could have originated from
ancient RNA or DNA molecules that
became enclosed in a protective
protein coat. These encapsulated
genetic materials, or "proto-viruses,"
might have evolved over time,
eventually becoming the viruses we
recognize today.
•Viruses may have evolved from
viroids, small infectious agents made of
short RNA strands that infect flowering
plants (angiosperms).
•Viruses might have arisen from
transposons—genetic elements capable
of moving within an organism’s
genome.
•Another possibility is that viruses
evolved from ancient cells that lost
their ability to survive independently
and became reliant on other cells for
reproduction.
Rapid rates of evolution
in viruses
The evolution of viruses refers to the process by which they undergo changes and adapt over
time. Viruses evolve significantly faster than most organisms due to several factors:
1.High Mutation Rates: Many RNA viruses, including retroviruses, exhibit high mutation
rates because they replicate their genetic material using error-prone enzymes like reverse
transcriptase and RNA polymerases, which are far less accurate than DNA polymerases. For
instance, RNA-based viruses such as HIV, influenza, and hepatitis C can mutate up to 10,000
times faster than DNA viruses. These mutations are passed on to subsequent generations.
2.Genetic Exchange: Viruses can exchange genetic material through processes like
recombination and horizontal gene transfer. This enables them to quickly acquire new traits
or adapt to different environments.
3.Rapid Reproduction: Viruses have short generation times and high reproductive rates,
producing many offspring in a brief period. This increases the opportunities for evolutionary
changes to occur.
Viruses with higher mutation rates pose greater challenges for treatment and vaccination
because they are harder for the host's immune system to recognize and control
