Mutations are changes in the genetic material of an organism. From the ‘non-living’ entities, like viruses, to the warm-blooded mammals, the genome of every organism undergoes mutation. Change in genetic material is described as the change in the base pairs of DNA. Deoxyribonucleic acid (DNA) has three components: a deoxyribose sugar, phosphate backbone, and a nitrogenous base. These nitrogenous bases are the components that undergo mutations. There are four nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). The order or sequence of these molecules is the genetic blueprint of the entire organism. So, if the order of A, G, T, or C changes, the genetic information of the organism changes.
Structure of Deoxyribonucleic acid (DNA). Image credit: https://www.genome.gov/genetics-glossary/Deoxyribonucleic-Acid
The consequences of these changes’ ranges from benefits such as the adaptation of living organisms to different environment and variation present in an offspring to being harmless, even unnoticeable, to cause the most severe genetic disorders. A classic example of a severe genetic disorder is SCID (Severe combined immune deficiency). Mutations happen all the time in all organisms, even humans. All living organisms possess in-built systems of repair, the ones in eukaryotes being more advanced than that of prokaryotes. A typical example of a consequence of mutations in prokaryotes is antibiotic resistance. Every time humans discover a new antibiotic bacteria find new ways to fight against them through mutations. In the case of viruses, mutations occur very frequently but only sometimes bring significant changes to the virus. The mutation rate is much higher in RNA viruses. This is because RNA is more unstable than DNA and RNA virus do not have a built-in proofreading mechanism in their replication. [5]
Mutations in viruses can either cause variations in tulips or contribute to an endless pandemic. These are two extremes with one being pursued intentionally and the other being avoided at all costs.
So how does a mutation in virus impact a flower? Viruses are responsible for the beautiful patterns in the tulip flowers. Tulips are ornamental plants that belong to the genus Tulipa. They are vegetatively propagated under open conditions. The phenomenon of patterns arising in the flowers is called ‘tulip breaking syndrome’. This syndrome is characterised by irregular flame- and feather-like patterns in the flowers. The pattern, type of colour and the contrast in the breaking depends on the cultivar. The colour change intensifies as the flowering progresses. Nepovirus, Carlavirus, Cucumovirus, Potexvirus, Necrovirus, etc. infect certain species of tulips. This is an asset for trading and commerce. The public wants these ornamental plants, and many cultivars grow these tulips on demand. [1]
Normal tulips and ‘tulip breaking syndrome’. Image credit: https://amsterdamtulipmuseumonline.com/blogs/tulip-facts/broken-tulips-the-beautiful-curse
The second case is nothing unfamiliar. In December 2019, an epidemic broke out in China which eventually spread to the world and became a pandemic. Since then, there have been several mutations in SARS-CoV-2, the virus responsible for COVID-19. The actual structure of the virus hasn’t changed but the nitrogen bases in the virus have changed. That’s why it is called a variant. A variant is a viral genome that has one or more mutations [3]; a variety of the original virus.
As of November 29, 2021 [4], the many variants of SARS-CoV-2 are Alpha, Beta, Gamma and Delta and the most recent and the deadliest one yet, according to the WHO, Omicron. These are Variants of Concerns (VOCs). VOCs have increased the transmissibility of COVID-19, leading to an increase in virulence and a decrease in the effectiveness of available diagnostics, vaccines, and therapeutics. Variants of Interest (VOIs) are the variants with genetic changes that are predicted or known to affect virus characteristics such as transmissibility, disease severity, diagnostic or therapeutic escape. These include the Lambda and the Mu variants.
Transmissibility refers to the ability of a disease to pass on from one individual to another. An increase in this property means that the proteins and receptors present on the virus can bind better to the cells present in any organism, i.e the virus is not specific to a type of cell/host; this property is uncommon in viruses since they are generally highly specific. So, this increases the rate of infection in the organisms.
Mutations happen in both cases. Humans deliberately induce them in one and in the other, the goal is to wipe out any future mutations. Scientists are building models and running computer simulations on the proteins present on SARS-CoV-2 to prevent any future breakouts of variants.
So, mutations are both good and bad. But, in most cases, they are neutral. Eukaryotes have a well-developed repair mechanism in their cells to fix any mutations, yet many remain. These generally don’t affect the organism in any way. And so, many organisms become carriers of these mutations, passing them on to their offspring. When these organisms reproduce, the mutations (in sex cells) are passed on to the next generations. Over time, in one generation, the accumulation of mutations starts to act and thus, affect the organism severely. If the mutation is detrimental to the life of the offspring, it dies before reproduction, thus wiping that mutation out of the population. This phenomenon acts as the basis of the concept of natural selection.
Viruses have the widest range of mutation rates. This is because of the wide variety of genetic material in the viruses. According to the Baltimore system of classification, viruses are classified into 7 groups: positive-strand RNA (-E.g: hepatitis C virus, tobacco mosaic virus), negative-strand RNA (Ebola virus, rabies virus), double-stranded RNA viruses (rotaviruses, bursal disease virus), retrovirus (HIV), para-retrovirus (hepatitis B virus), single-stranded DNA virus (parvovirus), and double-stranded (herpes virus, pox virus). [2]
With all the different viral genomes present, mutations can happen in any way. Predicting mutations in these pathogenic viruses is a difficult task, but with the help of bioinformatics and computer modelling, scientists can predict which sequence of nitrogenous bases is more vulnerable to mutation than others. But predicting mutations in eukaryotes is even more difficult because other factors like repair mechanisms come into the picture.
Thus, in a nutshell, mutations are random. They are raw materials needed for variation in a population. Variation in any organism over generations leads to evolution. Hence, though they may be unfavourable sometimes, all living organisms and viruses need and undergo mutations.
As Richard Dawkins has said, “Mutation is random; natural selection is the opposite of random”.
References
[1] Genetic Diversity of Potyviruses Associated with Tulip Breaking Syndrome. János Ágoston,
Asztéria Almási, Katalin Salánki, and László Palkovics. Plants (Basel). 2020 Dec; 9(12): 1807
[2] Mechanisms of viral mutation. Rafael Sanjuán, Pilar Domingo-Calap (Cell Mol Life Sci. 2016; 73(23); 4433-4488.
[3] CDC (2021) SARS-CoV-2 variant classifications and definitions [Online]. Available at: https://www.cdc.gov/coronavirus/2019-ncov/variants/variant-info.html (Accessed: 03 December 2021)
[4] WHO (2021) Tracking SARS-CoV-2 variants [Online]. Available at: https://www.who.int/en/activities/tracking-SARS-CoV-2-variants/ (Accessed: 03 December 2021)
[5] History Of Vaccines (2018) Viruses and evolution [Online]. Available at: https://www.historyofvaccines.org/content/articles/viruses-and-evolution/ (Accessed: 03 December 2021)
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