Human genome comprises of about 3 billion bases. There are about 10 million single nucleotide polymorphisms. The DNA sequences will carry genetic code as the information regulator for carrying out genetic information from parents to the offspring.
We can sort out the difference between animal or a plant or a Human as they are looking phenotypically different, which means that there are variations in their genomes in each of the organisms.
Any change in the sequence of the genetic material results in mutation or change in the genetic information, which in turn brings out a change in the phenotypic expression and also in the biological functions.
This change in sequence of a nucleotide or in genetic material is commonly known as mutation.
When we found a frequency of mutation occurring greater than 2 percentage in the population then is known as DNA polymorphism.
Hence DNA polymorphism can be defined as the DNA which exists in more than one form, having a frequency of greater than 2 percentage.
DNA Polymorphism Features
DNA polymorphism is referred to the different forms of DNA sequences within individual or among populations.
Polymorphism at a DNA level involves a wide range of variations from single base pair to many base pair alterations and also in repeated sequences.
DNA polymorphisms are considered as endless. These DNA polymorphisms are produced by changes in the sequence of the nucleotides or in the length of the base pairs. Or in length of the fragments.
Nucleotide DNA Polymorphisms
Variations in the genomes at a DNA level may be present in many forms including the single nucleotide polymorphisms, variable number of tandem repeats in mini and micro satellites, transposable elements, structural alterations and variations in the copy number. These variations occur either in a nucleus or in the mitochondria.
There are two major sources for these variations, one is mutations which results as a chance processes or the ones which have been induced by external agents like radiations or other environmental factors.
The second is the recombination which is formed by inheritance from parents followed by the offspring.
Generally, the genomes of the humans can be divided into different parts based on the functional elements as coding and the non-coding regions.
The coding regions mostly contains the DNA sequences which determines the primary amino acid sequences.
On the other hand, non-coding DNA consists of the sequences of the DNA which does not have any functions.
Such sequences exists either as a single copy or in multiple copies which are known as repetitive DNA.
These regions which does not code for proteins tend to have more polymorphisms.
Types of DNA Polymorphism
DNA polymorphisms can be classified into various types as follows;
1. Single Nucleotide DNA Polymorphisms
When a change takes place in a single base, it leads to High density natural sequence variations in the genome of an individual.
Single Nucleotide Polymorphisms mostly occur when there is an error such as deletion or insertion or substitutions of the bases of the nucleotides.
SNPs are considered as a prominent source in variation in the human genome which serves as an excellent genetic marker.
Whereas some regions of the genomes are richer in single nucleotide polymorphisms while others are in a normal condition.
SNPs are mostly present in the non-coding regions of the genomes and they have no direct impact on the phenotype of the particular, but their role remains active and might causes different consequences at the phenotypic level.
2. Insertion or Deletion Polymorphisms
This is one of the types of variation in a DNA; in which the specific nucleotide sequences of the various lengths ranging from one to more than 100 base pairs are deleted or inserted. These variations are spread widely across the genomes.
3. Polymorphic Repetitive Sequences
DNA repeats are often classified as interspersed repeats or tandem repeats, which comprises over two third of the human genome.
Whereas interspersed repeats are found dispersed across the genomes as intergenic and it includes pseudo genes and transposons.
On the other hand, Tandem repeats are found adjacent to each other and involves only a few copies of base pairs.
Tandem repeats are often found in Telomeres and centromeres. Tandem repeats are arranged in a head to tail manner depending upon the size of each repeat unit.
Whereas satellite repeats can be divided into microsatellites and minisatellites. Where, microsatellites have a repetitive sequence which is longer than hundred base pairs and they are also larger than the tandem repeats, located on one or many chromosomes.
Minisatellites are stretches of the DNA and they are characterised by normal length patterns which has ten to hundred base pairs, but usually less than 50 base pairs are only present.
Macrosatellites are also known as short tandem repeats, because the repat units are less than 10 base pairs.
4. Structural and Copy Number Variations
These are another type of frequent genome variability. Structural and copy number variations are denoted as CNVs, Which involves segments of DNA less than one kilo bases, copy number polymorphism is also known as duplication or deletion and it involves less than one kilo base of DNA.
Where as the intermediate sized structural variant and a structural variant involves more than eight to forty kilo bases.
This is usually referred as CNVs or balanced structural rearrangement, also known as inversion.
DNA Polymorphism Citations
- DNA polymorphisms in human population studies: a review. Ann Hum Biol . May-Jun 1987;14(3):203-17.
- Pathogenic mutations and non-pathogenic DNA polymorphisms in the most common neurodegenerative disorders. Folia Neuropathol . 2007;45(4):164-9.
- DnaSP v5: a software for comprehensive analysis of DNA polymorphism data. Bioinformatics . 2009 Jun 1;25(11):1451-2.
- A DNA polymorphism discovery resource for research on human genetic variation. Genome Res . 1998 Dec;8(12):1229-31.
- Mitochondrial DNA polymorphisms and extraversion. Am J Med Genet B Neuropsychiatr Genet . 2004 Jul 1;128B(1):76-9.
- Colorful DNA polymorphisms in humans. Semin Cell Dev Biol . Jun-Jul 2013;24(6-7):562-75.
- New application for DNA polymorphism. N Engl J Med . 1987 Feb 19;316(8):478-80.
- DNA polymorphism of schistosomes and their snail hosts. Mem Inst Oswaldo Cruz . Mar-Apr 1995;90(2):211-3.
- The application of capillary electrophoresis for DNA polymorphism analysis. Methods Mol Biol . 2001;162:3-26.
- DNA polymorphism and the study of disease associations. Hum Genet . 1988 Apr;78(4):299-312.