A2.3 Viruses Flashcards
A2.3.1—Structural features common to viruses
Relatively few features are shared by all viruses: small, fixed size; nucleic acid (DNA or RNA) as genetic
material; a capsid made of protein; no cytoplasm; and few or no enzymes
Viruses are non-cellular. They infect cells and reproduce inside them. Unlike living organisms, which share features because of their common ancestor (LUCA). viruses probably have multiple origins, with any shared features due to convergent evolution.
* Nucleic acid as genetic material: all viruses have genes made of DNA or RNA and they use the universal genetic code. This is essential as their proteins are synthesized by the nucleic acid-to-polypeptide translation mechanisms of their host cell.
* Capsid made of protein: before viruses are released from their host cell, their genetic material is packed into a protein coat called the capsid. This is made of repeating protein subunits. Self-assembly of the repeating subunits of the capsid gives viruses a symmetrical structure that is strikingly different from
* Small size: most viruses are between 20 and 300 nanometres in diameter. This is smaller than almost all bacteria and much smaller than plant or animal cells.
Viruses must be smaller than their host cells so they can enter them. Viruses are also small because they lack cytoplasm and other structural features.
* Fixed size: viruses do not grow, so they do not increase in size. A virus is assembled inside a host cell, in a similar way to a car being assembled from components-both a virus and a car are their full size as soon as assembly is completed. Many viruses are composed of a fixed number of components, each with a fixed size, so this determines the overall size.
the shape of living cells.
* No cytoplasm and few or no enzymes: viruses rely on the metabolism of their host. The few enzymes produced by some viruses are required for replication of the virus’s genetic material, for infecting host cells, or for lysis (bursting host cells to release the new viruses.
A2.3.2—Diversity of structure in viruses
Students should understand that viruses are highly diverse in their shape and structure. Genetic material
may be RNA 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.
The diversity of viruses suggest that they have multiple
produced by some viruses are required for replication
of the virus’s genetic material, for infecting host cells,
or for lysis (bursting host cells to release the new
evolutionary origins.
1. Genetic diversity: no genes occur in all viruses.
The genetic material can be DNA or RNA and it can
be single- stranded or double-stranded. If DNA, the
molecule can be linear or circular. If RNA, the genes
can be ‘positive-sense’ and used directly as mRNA or
negative sense and need to be transcribed before
translation.
Example
Type
Enveloped?
Genetic
material
Features
Bacteriophage lambda
Bacteriophage-DNA viruses that use
prokaryotes as hosts
Non-enveloped
1 double-stranded DNA molecule with
32 genes
Can follow either a lytic cycle in which it bursts and kills the host or a lysogenic cycle with its DNA inserted into the host cell DNA, so it is passed on to daughter cells when the host divides E. coli-a gut bacterium Host
Structure
tail capsid (protein linear double
fibres coat of the virus) stranded DNA
tail tube through which DNA
is injected into the host
h e a d
2. Enveloped and non-enveloped viruses: some
viruses become enveloped in membrane during lysis,
with phospholipids from the plasma membrane of the
host cell and proteins, mostly glycoproteins, from the
virus itself. The membrane helps the enveloped virus to
make contact with a host cell and infect it. Other viruses
d o not b e c o m e enclosed in a membrane—they are non-
enveloped. Animal viruses are mostly enveloped. Plant
viruses and bacteriophages are mostly non-enveloped.
COVID-19
Corona virus-RNA viruses with
crown shape and animal hosts
Enveloped
1 single-stranded positive-
sense RNA molecule with 16
genes
Caused a pandemic starting
in 2020 that killed more than
6 million people. The disease
was zoonotic because the virus
spread to humans from another
species, probably a bat
Epithelium cells in the airways
spike
protein
system
envelope
proteins protein coats
RNA with
protein
coating
membrane
envelope
HIV
Retrovirus-viruses that convert
RNA to DNA after entry to host
Enveloped
2 copies of a single-stranded
positive-sense RNA molecule with
9 genes
The virus contains reverse
transcriptase which makes a
double-stranded copy of the viral
RNA genome, which is integrated
into the host cell’s chromosomes
T-helper cells in the human immune
-RNA
- protein
* bound
viral to RNA
enzymes
and lungs of humans
phospholipid
e n v e l o p e
A2.3.3—Lytic cycle of a virus
Students should appreciate that viruses rely on a host cell for energy supply, nutrition, protein synthesis
and other life functions. Use bacteriophage lambda as an example of the phases in a lytic cycle.
Bacteriophage lambda binds to its host, Escherichia coli using proteins at the tip of its tail. It then injects its DNA into the host cell through the tubular tail. The viral DNA has single-stranded ends, which link by base pairing to convert the molecule from a linear to a circular form. Like all viruses, lamoda relies on the host cell for almost all life functions including providing energy and nutrition.
If the virus follows the lytic cycle, it reproduces inside the host cell and then bursts it, releasing the new viruses, which can then infect other host cells.
DNA entry
Attachment to a host cell using tail fibres
via tail and pores in plasma membrane
Lysis (bursting) to release the new viruses
Lytic cycle of bacteriophage lambda
DNA
replication (about
100 copies)
Assembly
of new viruses with DNA inside a protein coat|
Synthesis of viral proteins using mRNA transcribed from viral DNA
A.2.3.4—Lysogenic cycle of a virus
Use bacteriophage lambda as an example.
The lysogenic cycle is an alternative to the lytic cycle
for b a c t e r i o p h a g e lambda and other viruses. The virus
attaches to a host cell and injects its DNA (as in the
lytic cycle) but instead of replication, the virus’s DNA
becomes integrated into the host cell’s DNA molecule.
It stays there undetected and inactive. Each time the
host replicates its DNA, prior to cell division, it also
replicates the viral DNA, so all daughter cells inherit the
viral DNA but do not produce viral proteins.
4. The lysogenic cycle
Integration
of viral DNA
into the circular
bacterial
c h r o m o s o m e
A t t a c h m e n t
and
DNA entry
Lysogenic
cycle of
bacteriophage l a m b d a
The viral DNA is known as a prophage while it remains
integrated in the bacterial DNA. The virus is temperate
in this state as it d o e s not kill its host and causes
minimal harm. It is lysogenic because it could change
to the lytic state and then cause lysis. The stimulus for
this can come from inside or outside the bacterial cell.
A2.3.5—Evidence for several origins of viruses from other organisms
The diversity of viruses suggests several possible origins. Viruses share an extreme form of obligate
parasitism as a mode of existence, so the structural features that they have in common could be regarded
as convergent evolution. The genetic code is shared between viruses and living organisms.
- Viruses are obligate parasites. They need a host
cell in which to replicate. This suggests that cells
evolved before viruses. - Viruses (which are not regarded as truly living) use
the same universal genetic code as living organisms.
This suggests that viruses evolved from cells. - Viruses are extremely diverse in structure and
genetic constitution. This suggests multiple origins
from living cells, rather than all viruses evolving from
o n e c o m m o n viral a n c e s t o r. The similarities b e t w e e n
viruses would therefore be due to convergent - Viruses could have evolved in a series of steps
by taking and modifying cell components. This
hypothesis fits with the occurrence of virus-like
components in some cells. - Viruses could have evolved from cells in a series of
steps by loss of cell components and of more and
more life functions, including respiration and protein
synthesis. This fits with the observation that both
viruses and bacteria show variation in complexity and self-relience
A2.3.6—Rapid evolution in viruses
Include reasons for very rapid rates of evolution in some viruses. Use two examples of rapid evolution:
evolution of influenza viruses and of HIV. Consider the consequences for treating diseases caused by
rapidly evolving viruses.
Viruses can show extremely rapid rates of evolution.
There are three main reasons:
1. Very short generation times of <1 hour in the lytic cycle
2. High mutation rates, especially in RNA viruses
3. Intense natural selection due to host organisms evolving
defences such as antibodies for destroying viruses.
Influenza is caused by a rapidly evolving RNA virus. It
uses RNA replicase to replicate its genetic material. This
enzyme does not proofread or correct errors, leading to
a high mutation rate. The flu virus has eight separate RNA
molecules. If a host cell is invaded by two different strains
of the virus, a new strain can be formed with combination
of RNA from the two strains. Transmission of flu between
humans and other species also triggers evolution.
Because of the rapid evolution of the flu virus, annual
vaccinations are needed to give immunity to new strains.
HIV has the highest known mutation rate of any virus. It
is a retrovirus that uses reverse transcriptase to convert
its single-stranded RNA genome to DNA. This enzyme
does not proofread or correct errors. Mutations are
also caused by cytidine deaminase, an enzyme made
by the host that converts cytosine to uracil. Even within
a person infected by one strain of HIV, mutations will
produce many genetically different strains, helping
the virus to evade the immune system and become
resistant to antiretroviral drugs. HIV infection is
therefore almost always chronic rather than curable.
