Everything about Single Nucleotide Polymorphism totally explained
A
single nucleotide polymorphism (
SNP, pronounced
snip), is a
DNA sequence variation occurring when a single
nucleotide -
A,
T,
C, or
G - in the
genome (or other shared sequence) differs between members of a species (or between paired chromosomes in an individual). For example, two sequenced DNA fragments from different individuals, AAGC
CTA to AAGC
TTA, contain a difference in a single nucleotide. In this case we say that there are two
alleles : C and T. Almost all common SNPs have only two alleles. For a variation to be considered a SNP, it must occur in at least 1% of the population.
Within a population, SNPs can be assigned a
minor allele frequency - the lowest allele frequency at a
locus that's observed in a particular population. This is simply the lesser of the two allele frequencies for single nucleotide polymorphisms[1]. It is important to note that there are variations between human populations, so a SNP allele that's common in one geographical or ethnic group may be much rarer in another. In the past, single nucleotide polymorphisms with a minor allele frequency of less than or equal to 1% (or 0.5%, etc.) were given the title "SNP," an unwieldy definition. With the advent of modern
bioinformatics and a better understanding of evolution, this definition is no longer necessary.
Single nucleotide
polymorphisms may fall within coding sequences of
genes,
non-coding regions of genes, or in the
intergenic regions between genes. SNPs within a coding sequence won't necessarily change the
amino acid sequence of the
protein that's produced, due to
degeneracy of the genetic code. A SNP in which both forms lead to the same polypeptide sequence is termed
synonymous (sometimes called a
silent mutation) - if a different polypeptide sequence is produced they're
non-synonymous. SNPs that are not in protein-coding regions may still have consequences for
gene splicing,
transcription factor binding, or the sequence of
non-coding RNA.
Variations in the DNA sequences of humans can affect how humans develop
diseases and respond to
pathogens,
chemicals,
drugs,
vaccines, and other agents. However, their greatest importance in biomedical research is for comparing regions of the genome between
cohorts (such as with matched cohorts with and without a disease).
The study of single nucleotide polymorphisms is also important in crop and livestock breeding programs (see
genotyping). See
SNP genotyping for details on the various methods used to identify SNPs.
Further Information
Get more info on 'Single Nucleotide Polymorphism'.
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