A2.

3 Viruses (HL)
A2.3 Viruses AHL
Guiding questions
• How can viruses exist with so few genes?
• In what ways do viruses vary?
Statement Guidance
A2.3.1 Structural Relatively few features are shared by all viruses: small, fixed
features size; nucleic acid (DNA or RNA) as genetic material; a capsid
common to made of protein; no cytoplasm; and few or no enzymes.
viruses
A2.3.2 Diversity of Students should understand that viruses are highly diverse
structure in in their shape and structure. Genetic material may be RNA
viruses or DNA, which can be either single- or double-stranded.
Some viruses are enveloped in host cell membrane and
others are not enveloped. Virus examples include
bacteriophage lambda, coronaviruses and HIV.

A2.3.3 Lysogenic cycle Students should appreciate that viruses rely on a host cell
of a virus for energy supply, nutrition, protein synthesis and other life
functions. Use bacteriophage lambda as an example of the
phases in a lytic cycle.
A2.3 Viruses AHL
Guiding questions
• How can viruses exist with so few genes?
• In what ways do viruses vary?

Statement Guidance
A.2.3.4 Lysogenic Use bacteriophage lambda as an example.
cycle of a
virus
A2.3.5 Evidence for The diversity of viruses suggests several possible origins.
several Viruses share an extreme form of obligate parasitism as a
origins of mode of existence, so the structural features that they have
viruses from in common could be regarded as convergent evolution. The
other genetic code is shared between viruses and living organisms.
organisms
A2.3.6 Rapid Include reasons for very rapid rates of evolution in some
evolution in viruses. Use two examples of rapid evolution: evolution of
viruses influenza viruses and of HIV. Consider the consequences for
treating diseases caused by rapidly evolving viruses.
A2.3.1 Structural Features Common to Viruses
Viruses seem to be everywhere; nearly all living species can be infected by them.
• Viruses are thought to be the most abundant biological entity on our planet, but are not
considered to be alive, because they do not carry out all the functions of life.
• They take over the machinery of infected living cells in order to carry out the activities
necessary to ensure their own reproduction and presence.

Viruses display tremendous diversity. There are certain characteristics , however, that are
shown by all viruses:
• They are of a small, fixed size – between 20 and 300 nanometers in diameter. Smaller
than almost all bacteria and much smaller than plant and animal cells.
• They contain a nucleic acid, either RNA or DNA, as genetic material - But they cannot
replicate on their own.
• They are enclosed by a boundary composed of protein called a capsid.
• This unique composition and structure determines the ability of a virus to infect a
particular host cell.
• A host cell is the cell a virus uses to carry out its metabolic and reproductive
functions. In some viruses, the capsid contains specialized proteins that allow the
genetic material of the virus to penetrate the host cell membrane.
• They do not contain cytoplasm inside the capsid
• They possess few, if any, enzymes - the viral enzymes that are produced are required for
replication of the virus' genetic material, for infecting host cells or for bursting host cells
to release the new viruses
A2.3.2 Diversity of structure in viruses
• Viruses are very diverse in shape and structure. They can be threadlike, polyhedral, and
spherical in shape.
A2.3.2 Diversity of structure in viruses
A2.3.2 Diversity of structure in viruses
A2.3.2 Diversity of structure in viruses

Important features of HIV are:

• it has an envelope outside the capsid
• two identical single strands of RNA, protected by the capsid
• within the viral RNA, reverse transcriptase is encoded, which allows the production of
DNA using the viral RNA as a model
• it is known as a retrovirus because it makes a double stranded DNA copy of its
RNA code
• the envelope spikes of HIV are made of protein and carbohydrate​s
A2.3.2 Diversity of structure in viruses

Even though there is tremendous diversity in viruses, they're all obligatory

intracellular parasites. (Obligate Intracellular Parasites)
• That is, they rely on living host cells to multiply and to carry out the metabolic
functions they require to exist.
• An obligate parasite or holoparasite is a parasitic organism that cannot
complete its life-cycle without exploiting a suitable host.
• Each virus has specific type of cells that it utilizes for its continued existence.
• This specificity arises from the attachment points of the virus
that correspond with the host cell, and the particular properties needed
in the host cell for the virus to carry out its necessary functions
A2.3.2 Diversity of structure in viruses
A2.3.3 Lytic Cycle of a Virus

Viruses must have a host cell to perform all their life functions.
• They cannot produce their own energy because they do not have
mitochondria and rarely have enzymes.
• They do not have vacuoles or lysosomes to digest potential nutrients.
• They only possess one nucleic acid, which means they cannot carry
out their own protein synthesis.

To reproduce, all viruses must:

• Attached to a site on a specific host cell

• Incorporate their genetic material into the cytoplasm of the host cell
• Use the host cell’s processes to produce components of themselves
• Assemble the viral components into new functioning virus entities
• Release the new virus entities into the host cell's environment.
A2.3.3 Lytic Cycle of a Virus

This life cycle is called

lytic because the new
virus particles are
released by the lysis
or rupturing of the
cell membrane by an
enzyme called
lysozyme. This
enzyme is coded for
the bacteriophage
DNA. Lysis only occurs
after the production
of fully functional
virus particles known
as virions.
A2.3.4 Lysogenic Cycle of a Virus

• Unlike the lytic cycle, the lysogenic cycle does not result in the
immediate release of new viruses.
• Instead, the DNA of the bacteriophage combines with the
bacterial DNA form a prophage.
• This prophage, or incorporated bacteriophage DNA, does not alter
the bacterial DNA.
• The bacterium continues its usual life functions, including
reproduction.
• The next generation of bacteria cells will carry
the prophage within their genome.
• The genome of a cell is the genetic makeup of that cell.
A2.3.4 Lysogenic Cycle of a Virus
A2.3.4 Lysogenic Cycle of a Virus

• The integration of the latent prophage with the bacterial DNA can result in the
continual production of bacterial cells containing the prophage.
• The prophage is totally inactive within the cells, unless an event occurs
that releases the bacteriophage DNA from the bacterial DNA.

• While the virus remains in the lysogenic cycle, it is 'temperate': it does not kill
its host and it causes minimal harm.
• The virus remains undetectable as a prophage in the bacterial DNA.
• It is inherited by daughter cells but cannot spread by infecting uninfected
cells.

• A temperate virus existing as a prophage is called 'lysogenic' but it could

change through the lytic state and then cause lysis.
A2.3.4 Lysogenic Cycle of a Virus
A2.3.5 Evidence for several origins of viruses from other organisms.

Scientists estimate that 80% of the human genetic makeup comes from viruses.
This indicates a co-evolution of viruses and our species, especially our defense /
immune system. Many researchers believe viruses had played a key role in the
evolution of most life forms present today. Even though not able to live on their
own, viruses have had, and continue to have, a tremendous impact on our planet.

There are three leading hypothesis on the origin of the first viruses.

1. The virus first hypothesis states that viruses originated before cells. This idea is
based on the simplicity exhibited by a virus compared to a cell, and that the course
of evolution usually proceeds from the simple to the more complex. This
hypothesis also suggests that ancestors of modern viruses could have provided the
materials necessary for the development of the first cells.

2. The regressive hypothesis, also known as the reduction or

degeneracy hypothesis, states that viruses were once small cells that became
parasites of larger cells. Overtime, these small cells shed the structures that were
no longer needed via gene reduction.
A2.3.5 Evidence for several origins of viruses from other organisms.

3. The escape hypothesis, also known as the vagrancy hypothesis, states that
portions of genetic material, DNA and RNA, escaped from larger organisms such as
bacteria and subsequently became surrounded by an outer boundary.

Each of these hypothesis has to overcome serious challenges to gain adequate

credibility. The diversity that viruses demonstrate indicates their may have been
different origins for viruses at different times.

However, there are some interesting comments about the structural, functional
and genetic features of viruses that indicate convergent evolution may have
occurred to a certain extent.

Features that suggest evidence of convergent evolution in viruses include the fact
that they all:
• Are obligate parasites, none can replicate or carry out the functions of life
individually
• Have a protein outer boundary, the capsid, with no cytoplasm
• Have genetic material, DNA or RNA, inside a capsid, and the code of this
genetic material is shared between viruses and all of the earth's organisms
A2.3.6 Rapid evolution in viruses

Viruses can show extremely rapid rates of evolution. Even during an infection of one
person, a virus can undergo heritable changes - it can evolve. The influenza virus and
HIV are two types of virus that display rapid rates of evolution. Like other rapidly
evolving viruses, this viruses have large population sizes, short generation times
and high mutation rates.

There are three main reasons for this rapidity:

1. Evolutionary change can only happen between one generation and the next, so
it is limited by generation time. In humans, the average generation time is about
25 years but in viruses, it can be less than an hour.
2. Evolution depends on genetic variation. The ultimate source of this variation is
mutation and mutation rates tend to be high in viruses. Mutations are changes in
the genetic code.
3. Evolution is the result of natural selection acting on variation in a population.
The intensity of natural selection of viruses tend to be high. The host organism,
whether a bacterium, plant or animal, has mechanisms for detecting and
destroying invading viruses. Viruses with a new variant of protein in the capsid or
in the enveloping membrane can evade the immune system and multiply, whereas
those with the previous form are destroyed. As a consequence, natural selection
is powerful, and this encourages rapid evolution.
A2.3.6 Rapid evolution in viruses

Recombination of genetic material in viruses

Although viruses are not alive, they still evolve in similar ways to living organisms. To
continue their existence, they must be able to survive the immune system of organisms
they infect. One way they can accomplish this is if they undergo genetic change. Genetic
changes occur commonly in viruses either by antigenic drift or by antigenic shift.
A2.3.6 Rapid evolution in viruses

Genetic change in influenza virus through A) drift and B) shift. In drift, a slight change develops
(represented by the yellow pentagons as opposed to hexagons on the glycoprotein spikes) a result of one
or more mutations (represented by the * on the RNA segment). Shift results from the shuffling of the
genomes of two different strains. Note the new strain has RNA from both parent strains (blue and pink).
The glycoproteins (HA), however, are those of the human strain (yellow) allowing the new strain to bind to
and infect human cells
A2.3.6 Rapid evolution in viruses

Vaccines can be helpful in the prevention and / or control of many viruses.

• Because of antigenic drift, the influenza vaccine has to be adjusted each year
so that it is effective. This is a successful approach but the changes arising
from antigenic drift are small and not particularly rapid.

Conversely, antigenic shifts are rapid and bring about major changes.
• Vaccine adjustments are not as effective in this case because the changes are
not predictable enough to enable successful alterations of the vaccine.

HIV is a unique virus because it undergoes antigenic drift at an extremely rapid

rate. This is the reason why a successful vaccine has not been developed to
prevent its spread.

Influenza viruses and HIV both have high mutation rates, with common
recombination and reassortment of genetic material.
• This results in an ever-changing population of viruses capable of surviving
previously affected immune systems and vaccines.

https://www.cdc.gov/flu/about/viruses/change.htm
A2.3.6 Rapid evolution in viruses

When treating diseases caused by rapidly evolving viruses such as influenza virus
and HIV, it is possible that viruses exist because they are resistant to the
treatment because of genetic variation. One form of the virus might be
controllable, but a variant with resistance to this treatment may survive. This
'selected' variant could then thrive.

The major problem we are currently facing when treating HIV and influenza
viruses is that we are not able to completely inhibit the growth of all of the
variants of the virus. The result is continuing occurrence of infection.

Because antibiotics are only effective in the treatment of living disease-causing

organisms, we cannot use them to stop a virus infection successfully.

• In some cases, the most effective treatment for a viral infection is to use
the organism’s own immune or defense system to control the virus.

• Another means of controlling virus infections in populations is isolation, they

effectively stop transmission with the causal agent
End of A2.3 Viruses (HL)