Before we talk about the Coronavirus, let’s begin with some basics of evolution.
Roughly 0.1–1 mutations per genome occur during every cycle of DNA replication. In an average human, about 52.8 billion new cells (except blood cells) form each day. Hence, if an average of 0.5 mutations per replication cycle remains unrepaired, it can cause half of the human genome to mutate in a single day.
This calculation is only to visualize the number of genetic variations that can take place in a human being. In reality, the proofreading activity DNA polymerase (99 percent accurately) corrects any replication errors keeping the genome intact and identical to the original.
Hence, variation in the genome is a rare phenomenon, but if it occurs, eventually contributes to evolutionary changes.
Apart from mutations, other phenomena might also give rise to genetic variations: — Random matings, Recombination of homologous chromosomes during meiosis.
While genetic variations explain the changes that take place in an organism with a mutated/ altered DNA sequence, epigenetics idealizes physical (phenotypical) changes that could occur in organisms with no change in DNA sequence.
This essentially explains the mechanism for the adaptation of organisms to certain environments or circumstances.
For an explanation, let's assume two identical mesophilic bacterial cells with similar DNA sequences are placed in two different environments — one in warm temperature and another in cold.
Assuming both cells survive.
The one in warm temperature switches on the genes that protect it from heat. It produces more heat-resistant proteins, grows a thinner membrane with each multiplication, and regulates cellular mechanisms for water retention to survive.
Similarly, the cell at cool temperature switches on the expression of cold-resisting proteins produces a thicker layer of lipid in the cell membrane and protects the freezing of the nucleus and organelles.
This way, both the cells adapt to the environment by only switching off and on certain genes in their DNA, while both their sequences remain the same. When the exposure of hot/cold is removed, the cells function as usual and in the entire process, the DNA doesn’t change.
This ability of the cell to function and regulate its genes as per the environment is best described by epigenetic mechanisms.
It occurs by methylation of DNA, histone modifications, or by silencing genes. These epigenetic modifications in the DNA sequences are flexible, reversible, and quickly responsive to the environment. In the end, they might or might not be inherited in their offspring.
Only if they are inherited, brings about evolutionary changes.
This is the reason that even if the tails of both parents of mice are cut, the offspring are born with full-grown tails.
Viruses contain either DNA or RNA as their genetic material. Viral RNA genomes have higher rates of mutation (one in every ten thousand base pairs) than DNA (one in a million base pairs).
The reason is — viral RNA polymerase does not have an efficient proofreading mechanism and allows a greater number of errors. This proofreading activity is far better in viral DNA polymerases, so makes fewer errors.
Another reason for the high mutation of the viral genome is their replication rate. When a virus enters inside a host cell, adapts and controls the machinery of the cell to produce thousands of virion copies. In contrast, cells of higher organisms divide into two (mitosis) and four (meiosis) at maximum. The production of a large number of viruses at once brings an unprecedented number of variations to the viral genome. During the release, these viruses also bag host proteins from the cytoplasm that might influence adaptations in certain environments by epigenesis.
The first strain of coronavirus causing the pandemic hopped into the human being from an unknown host. Just on the entrance, it evaded our defense mechanism penetrated inside the cell. The virus must have undergone the first set of epigenetic or genetic variations to adapt to the human host.
This explains how dozens of SARS-CoV-2 variants evolved within a short time period.
However, many of the mutations in the viral genome have no biological significance. Not all the random mutations and their traits prove beneficial to the virus, some might even weaken it. Among all the variants identified of the COVID-19 virus, the delta variant is determined to be the most virulent and infective among all the others.
With the unpredictability and randomness of the mutation in a viral genome, the chances of occurrence of a more infective and severe form of COVID-19 virus cannot be denied. However, the scientific community also shares some optimism. At one point, the SARS-CoV-2 virus will eventually subside when the viral genome gains stability or is eventually tired out of mutations. And when the rate of mutation slows down, the infectivity and prevalence of the virus will also eventually decrease.
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| Photo by Anne Nygård on Unsplash |
When Charles Darwin proposed the theory of evolution in his book Origin of the Species (1859), the scientific community criticized it on various grounds.
The most heated concern in the evolutionary theory was the lack of a concrete basis to explain it. Darwin appeared to have relied on the already disproved Jean B. Lamarck’s theory of Inheritance of Acquired Characters to conceptualize it. Lamarck in the early 1800s proposed various organs in the body appear and are lost depending on their special use, and these physical traits are acquired in the offspring — in his so-called theory.
Decades later, when DNA and the genetic basis of living organisms were discovered, Darwin’s theory of evolution found its actual validity. The genetic basis of life explained how it was the ultimate source of all genetic alterations in the living systems. Those genetic variations, most of which were inherited in the offspring formed the solid underlying mechanism of evolutionary changes.
Genetic Variations: How it occurs?
The code of life or the DNA is a sequence of nucleotides presented as hereditary material in humans and all other organisms including some viruses. Cellular mechanisms transcribe and translate it to build the entire organism. The DNA sequence is a unique pattern of the four nucleotides namely: Purines: Adenine (A) and Guanine (G) and Pyrimidines: Cytosine (C) and Thymine (T).
This sequence of DNA is identical within all the cells of an organism. During cell division, a series of complex biochemical mechanisms replicate it into an exact copy.
Image by OpenClipart-Vectors from Pixabay
Cell division begins with the synthesis of new DNA, for which an enzyme called DNA polymerase joins the nucleotides to the elongating DNA chain based on a template DNA strand. However, a few slips are possible. These errors in replication which are not successfully repaired by the polymerase during the proofreading process appear as alterations/ mutations in the DNA sequence.
This calculation is only to visualize the number of genetic variations that can take place in a human being. In reality, the proofreading activity DNA polymerase (99 percent accurately) corrects any replication errors keeping the genome intact and identical to the original.
Hence, variation in the genome is a rare phenomenon, but if it occurs, eventually contributes to evolutionary changes.
Apart from mutations, other phenomena might also give rise to genetic variations: — Random matings, Recombination of homologous chromosomes during meiosis.
Epigenetic Mechanisms of Adaptation and Evolution
This essentially explains the mechanism for the adaptation of organisms to certain environments or circumstances.
For an explanation, let's assume two identical mesophilic bacterial cells with similar DNA sequences are placed in two different environments — one in warm temperature and another in cold.
Assuming both cells survive.
The one in warm temperature switches on the genes that protect it from heat. It produces more heat-resistant proteins, grows a thinner membrane with each multiplication, and regulates cellular mechanisms for water retention to survive.
Similarly, the cell at cool temperature switches on the expression of cold-resisting proteins produces a thicker layer of lipid in the cell membrane and protects the freezing of the nucleus and organelles.
This way, both the cells adapt to the environment by only switching off and on certain genes in their DNA, while both their sequences remain the same. When the exposure of hot/cold is removed, the cells function as usual and in the entire process, the DNA doesn’t change.
This ability of the cell to function and regulate its genes as per the environment is best described by epigenetic mechanisms.
It occurs by methylation of DNA, histone modifications, or by silencing genes. These epigenetic modifications in the DNA sequences are flexible, reversible, and quickly responsive to the environment. In the end, they might or might not be inherited in their offspring.
Only if they are inherited, brings about evolutionary changes.
This is the reason that even if the tails of both parents of mice are cut, the offspring are born with full-grown tails.
Virus and Evolution
The reason is — viral RNA polymerase does not have an efficient proofreading mechanism and allows a greater number of errors. This proofreading activity is far better in viral DNA polymerases, so makes fewer errors.
Another reason for the high mutation of the viral genome is their replication rate. When a virus enters inside a host cell, adapts and controls the machinery of the cell to produce thousands of virion copies. In contrast, cells of higher organisms divide into two (mitosis) and four (meiosis) at maximum. The production of a large number of viruses at once brings an unprecedented number of variations to the viral genome. During the release, these viruses also bag host proteins from the cytoplasm that might influence adaptations in certain environments by epigenesis.
Rapid Evolution of Coronavirus SARS-CoV-2
The COVID-19 virus (SARS-CoV-2) has an RNA genome with about 29,811 base pairs. Even if one mutation occurred per 10,000 base pairs during the first replication cycle, it will result in at least three mutations in the first cycle of replication. Hence, the virus must have been evolving since the first day of infection.
However, many of the mutations in the viral genome have no biological significance. Not all the random mutations and their traits prove beneficial to the virus, some might even weaken it. Among all the variants identified of the COVID-19 virus, the delta variant is determined to be the most virulent and infective among all the others.
With the unpredictability and randomness of the mutation in a viral genome, the chances of occurrence of a more infective and severe form of COVID-19 virus cannot be denied. However, the scientific community also shares some optimism. At one point, the SARS-CoV-2 virus will eventually subside when the viral genome gains stability or is eventually tired out of mutations. And when the rate of mutation slows down, the infectivity and prevalence of the virus will also eventually decrease.
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