Viruses - What are examples of DNA and RNA viruses and how do they differ?
Are you sometimes confused and overwhelmed by the large number of virus types?
Let’s try and get around this by grouping viruses according to their genome and understand their
corresponding characteristics. During transmission, the virus particle contains
either DNA or RNA as genetic material and is surrounded by a protein shell, called a capsid,
and sometimes an envelope. Once the virion penetrates the host cell, the viral
genome is unpacked, resulting in the hijacking of the host cell's transcription and translation
machinery. The interaction between the host and virus depends on the nature of the viral
genome. Therefore, it helps to distinguish viruses according to their genome.
The virus classification system based on genome structure is the Baltimore classification.
It was developed in the early 1970s by the Nobel laureate David Baltimore.
According to this classification system, viruses are separated into seven groups.
Let’s start with a relatively simple case: a viral genome similar to the genetic material of the host,
double-stranded DNA, or in short dsDNA. Many viruses belong to this first group, including
Adenoviruses, Herpesviruses, Poxviruses, Polyomaviruses, and Papillomaviruses.
Most viruses in this group are usually required to deliver their genome into the host nucleus, where it can
be transcribed by the host’s infrastructure. It should be noted that the viral DNA stays
as a separate module and doesn’t fuse with host cell DNA.
Let’s use another simple example for RNA viruses, namely a virus with a positive sense,
in short plus, single-stranded, in short, ‘ss’ genome. The viral genome can remain
in the host cell’s cytoplasm and either functions as messenger RNA and is translated
into protein or is transformed into messenger RNA. RNA viruses require their own RNA polymerase
to replicate their genomes, as the host cell is unable to replicate the RNA template and
doesn’t possess a suitable enzyme. Positive-sense single-stranded RNA viruses belong to group
4 of the Baltimore classification system. Notable examples include the rhinoviruses,
the primary cause of the common cold, as well as hepatitis A and C viruses.
Viral genomes are diverse in size and structure and can consist of positive sense, single-stranded
DNA, as found in parvoviruses, classified as group 2 viruses. As host cells can’t
use single-stranded DNA for replication or transcription, these single-stranded viruses
also transport proteins required for converting their DNA into double-stranded DNA. This is
also the case for the hepatitis B virus genome, which is circular and contains gaps. These
gaps are due to the incomplete synthesis of the positive strand. The positive strand gaps
need to be filled in accordance to the intact negative strand, before DNA processing in
the host cell nucleus. Hepatitis B virus belongs to group seven of the Baltimore classification
system, which was only added to the system at a later stage as the genomic nature of
the virus was unknown at the time. Viral RNA is comparably diverse and can also
be double-stranded. A notable example are the rotaviruses, which fall under group 3
of the Baltimore classification system. Yet, viral RNA can also be single-stranded with
a negative sense, as found in influenza, Ebola, and rabies viruses.
Negative-sense viral RNA must first be transformed into positive-sense, single-stranded RNA before
translation into proteins by the host. Interestingly, some positive-sense, single-stranded RNA viruses,
such as poliovirus and Zika virus, go through a stage in which their genome is transformed
into a negative-sense RNA before being transformed back to messenger RNA.
A very special ability can be observed in the retroviruses, which include human immunodeficiency
virus, in short HIV. The name retrovirus indicates that its positive-sense, single-stranded RNA
genome is converted into DNA. This process is mediated by an enzyme present in the virus
termed reverse transcriptase. The viral DNA is permanently integrated into the DNA of
the host cell, which can be transcribed upon gene activation. Retroviruses belong to group
6 of the Baltimore classification system.
So, what are some advantages of having a DNA or RNA genome?
DNA genomes are generally larger than RNA genomes, meaning that the information
storage capacity is greater. This is beneficial as viruses with a large genome are usually
less dependent on their host. On the other hand, DNA replication is much slower than
RNA replication. But most importantly, DNA viruses are confronted
with many elaborate repair mechanisms in the host cell, because host DNA is intended for
the long-term storage of genetic information. In contrast, the host cells RNA is a short-lived
molecule and consequently more likely to be degraded rather than repaired upon damage.
Therefore, there are no elaborate repair mechanisms in cells enabling RNA viruses to mutate at
a higher rate than DNA viruses. The transcription error rate in DNA is approximately one error
in each 10⁷ nucleotides, whereas in RNA the rate is one error in each 10³ to 10⁴ nucleotides.
High mutation rates are clearly advantageous
in viruses, as can be seen for the influenza viruses, which mutate so quickly that the
vaccine offers limited or no protection in the given season. This is also reflected in
the eradication of smallpox. Eradication was only possible by extensive vaccination of
potential human hosts combined with the viruses’ inability to evade the sudden emerging immunity
through mutation. In the next episode, we’ll be taking a detailed
look at the different viral life cycles of DNA and RNA viruses.