“Various phenotypes and variations in different organisms are evolved due to the mechanism known as genetic polymorphism.”
DNA is known to us only since 1950, ‘chemically’ explained in 1953 by Watson and crick. Fortunately, tremendous research in genetics and genomics leads us to understand so many things related to DNA.
However, I think we don’t know enough about DNA.
Chemically, it is made up of the sugar, phosphate and nitrogenous bases and popularly referred to as the ‘molecule of life’.
DNA is a type of nucleic acid, present in the cell nucleus and functions as storing and transferring the biological information throughout the generations.
Means, it inherited genotypes!
The human genome project revealed that the genome- a compilation of all the DNA of a cell is made up of some functional DNA sequences and useless sequences.
The functional portion makes proteins, that is the obvious function of the DNA and popularly referred to as the “genes”. The rest is known to us as just DNA.
More than 90% of the genome is just junk, although recent discoveries suggest that junk DNA performs a function to regulate gene expression.
That is yet another fundamental proposed so recently covered in the ‘epigenetics’.
Structurally and chemically the DNA is the same in all organisms ranging from prokaryotes to the eukaryotes which are only made up of the three elements stated above, nothing different!
If we explain DNA more scientifically, it is a chain of nucleotides or more precisely, a polynucleotide chain. The nucleotide is made up of base, phosphate and sugar.
In every organism the sequence of bases is different and that makes them different.
It was popularly believed that all the organisms on the earth evolved from primitive prokaryotes, which indicated that over a period of time, the change in the genetic composition of our primitive ancestors guided the process of evolution.
So now it is proven that the process of evolution is governed by the DNA but how?
What happens in the DNA that created a new feature or new species? Is the process normal or induced by environmental interaction? In the present article we will try to explain to you how DNA governs evolution and the phenomenon known as ‘genetic polymorphism’ let it happen.
Genetic polymorphism and evolution:
First let me make it clear that the evolution of new species is not an overnight process, it takes thousands of years.
“Genetic polymorphism is defined as the change or alteration in the DNA sequence due to intrinsic or extrinsic factors that lead to change in the phenotype.”
It is also known as “mutation”. Mutation or genetic change or genetic polymorphism is one of the forces that lead to the evolution of various organisms on earth and the rest of the other are gene flow, genetic drift and change in allele frequency.
What is genetic polymorphism?
Change in the genotype of an organism causes change or alteration in the phenotype is popularly referred to as genetic polymorphism. If you want to learn more about genotype and phenotype, read this article: Genotype vs phenotype.
Error in the process of replication causes genetic polymorphism, which is one of the most common intrinsic factors.
Replication is a process of copying DNA, simply put. In the replication, the enzyme is known as DNA polymerase (which is also encoded by DNA!) synthesizes the entire DNA of the genome to make a genetic composition of new cells.
A new cell inherited an exactly same type of DNA, resultantly. But the DNA polymerase doesn’t work correctly all the time.
At the end of the replication, the process of polymerization slows down which adds mismatches to the growing strand, the process is known as end replication problem. But thanks to the DNA repair mechanism of our body, which is a type of SOS system for us that repairs mismatches immediately.
But….still… it is unfortunate that sometimes, some DNA can’t be repaired precisely. And the errored DNA inherited to the next cell. In the next replication cycle, this errored DNA is copied and transmitted to the new daughter cell.
That is how change occurs in the system. The entire mechanism is known as the mechanism of genetic polymorphism and the error is known as mutation.
Now the evolutionary force generated if the genetic polymorphism occurs in the germ cell. I will explain it later, after explaining the external forces that cause genetic polymorphism.
Studies during the last two decades explained that environmental factors have a great impact on gene expression. For instance, a melanin encoding gene expresses higher in the people living at higher altitudes and extreme temperature whilst expresses lower in the people living in cold.
The reason for that is that higher melanin coating protects our skin from harmful sun radiation which is not required in people living in colder environments.
Meaning that though the sequence of genes coding melanin doesn’t change, its expression in different organisms changes.
Alteration in the gene expression commonly known as the epigenetic changes are also a type of genetic polymorphism.
In epigenetic alterations, the sequence of DNA can’t be changed still the phenotype varies. On the other side, polymorphism in gene sequence viz insertion, deletion, duplication or base substitution leads to an altered phenotype.
The product of mutation is either helpful or harmful to us.
For example, the single base mutation in the HBB gene includes sickle cell trait invade malaria infection. And it is common in the people where malaria is common like the denser rainforest of Africa. It is actually helpful but if two recessive sickle traits transmit together in progeny, it causes sickle cell anemia, a kind of blood disorder.
Notably, when and how the sickle cell trait evolved is still a mystery for scientists.
How new variation or genetic polymorphism occurs?
So far we have discussed the mechanism of genetic polymorphism via either epigenetic alteration or gene mutation.
But the question immediately arises in mind that, in which condition it is heritable?
Genetic polymorphism is a random and spontaneous process, we don’t have control over it! Nature itself decides which genotype is to alter and which to not!
However, the process is kind of a “trial and error” that might be worthless initially but help in the long run.
It’s totally random!
If genetic polymorphism occurs in the somatic cells, it remains restricted to some of the cells, area or tissue of the body and is temporary.
This means, it may be helpful to the organism for some time but can’t be inherited because it doesn’t occur in the germ cells.
Contrary, if genetic polymorphism occurs in germ cells such as sperm or egg, it is definitely inherited to offspring. Germ cells take part in the process of fertilization where the sperm and egg cells make an embryo.
That is how the change or genetic polymorphism passes to the offspring of an organism, if it is helpful it settles in the genome to express in consecutive generations.
Nonetheless, how the polymorphism inherited between the species is not known to us.
Genetic polymorphism through mutation or epigenetic alteration is the common theory for evolution, yet another genetic factor that was involved in the evolution process in the transposable elements.
The transposable elements are the mobile or moving genetic sequences that can move from one location to another into the genome of an organism. The concept of the transposition and transposable elements were discovered by Barbara McClintock from her experimentation of Maize plant.
She stated that the transposition of some genetic sequences in the genome of maize creates different colors of maize kernels.
The transposons work magically, which not only insert at a new location or gene but also transport some DNA sequences taken from the previous location. It actually works like a vehicle to transfer DNA sequences from one location to another.
If it is inserted into the active gene, gene function may either be altered or suppressed. If it is inserted into the non-coding region, the expression of the gene is altered.
Meaning that by either way it actually changes the gene function, ultimately a protein product got changed. However, the studied effects of transposons are more deleterious.
Transposons may jump into regulatory elements of a gene, non-coding intervening sequences, gene insect, exon of a gene or promoter.
If you want to learn more about how the transposons function, you can read this article of ours.
- Transposons in eukaryotes.
- Transposons: A Jumping Entity and a Foe with Benefits.
- Replicative Transposition of DNA transposons and Retrotransposons.
Besides genetic polymorphism, other popular genetic factors are the gene flow, genetic drift and allelic frequency.
Genetic drift- a type of rapid and random change in the smaller population causes evolution. Genetic drift alters the genotypic frequencies.
When some individual migrates from one population to another, it may also produce a new combination of genotypes and change the genotypic frequency, known as the gene flow.
When the frequency of a particular allele is changed, it causes a change in the population phenotype.
Phylogenetic tree and genetic polymorphism:
The process of evolution and the change or alteration in the DNA can be mapped by constructing the phylogenetic tree.
It is a diagrammatic representation showing how different organisms are related. The phylogenetic tree is the best thing to describe or show how two species or organisms are different or related.
The phylogenetic tree is constructed based on the data of the genetic polymorphism of an organism. Means, the closely situated species or organisms on the tree are more related and have less genetic polymorphism and vice versa.
See the image
The phylogenetic tree indicates that the relation between the organisms is inversely proportional to the genetic polymorphism.
To construct the phylogenetic tree various organisms are first selected and DNA isolation is performed.
A common marker that is not variable in terms of sequence-structure is selected and amplified in the Polymerase chain reaction.
All the amplified samples are sequenced using the DNA sequencer followed as sequence alignment and bioinformatic analysis.
Based on the similarities and differences between the sequence a tree is constructed by the computational software. We will discuss the process of the phylogenetic tree and phylogenetic analysis in some other article.
Simply put, the phylogenetic tree is the representation of the genetic polymorphism of different organisms, all organisms of a species, organisms of all the species or entire life pool on the earth.
Related article: What is Point mutation?
Genetic polymorphism and survival:
Surviving on earth is the primary goal of any organism! Our genome is a change a sensitive organic matter that alters its composition even under the influence of tiny environmental fluctuation.
But trust me what changes nature produces in our genome is only for our survival or for a life to sustain on earth.
Let me take an example, what cancer is! Someone is consuming harmful things that are not at all right for the human body like tobacco and it causes cancer. Nature isolates the person by creating some sudden unusual genetic polymorphism and literally tells us that, “isolate him he is harmful to the entire population”.
That is nature, it might be harmful to the entire population in the future so at safe side nature eliminates it by producing lethal polymorphism. There are two possibilities of it, may either eliminate the person (he may die) or produce resistance.
In either way, the change (a genetic polymorphism) is helpful to the entire population. That is how the mechanism of survival through genetic polymorphism works!
Epigenetic changes also work in the same manner for survival. It changes gene expression as per the environment and helps the organism to survive.
Conclusively, the role of genetic polymorphism either mutation or epigenetic alteration drives evolution. Still, scientists have less data on how evolution actually occurred. Also, it is yet not clear how genes are inherited between different species.
For maintaining the basic function of a cell several gene sequences remained unchanged since evolution that is the highly conserved DNA domains. Furthermore, less published data are available on how conserved sequence DNA is preserved and not influenced by the genetic polymorphism.