A Single Nucleotide Polymorphism is a change of a nucleotide at a single base-pair location on DNA. Created using Inkscape v0.45.1. (Photo credit: Wikipedia)
OK, I don’t like it either, but scientist use so many big words and then they use so many acronyms and often think that the rest of the world uses them also. Not only does the rest of the world have little clue about what most of them talk about, they certainly don’t know/understand/recognize the acronyms and terms. There is said it. Let angry scientist yell at me…won’t stop it from being true.
So, today I thought I might demystify (if possible) a term we use all the time and in this case, an acronym also. SNP. SNPs stand for Single Nucleotide Polymorphisms. What? Yes, they are a single change at the nucleotide level (one of the four letter codes that represent the entirety of our DNA, an A,C, T or G) of the DNA. You might have heard about a mutation, which can also be a result of a change at the single nucleotide at the DNA level also. Let me explain.
We have over 3 billion nucleotides that in essence make up the entirety of our DNA makeup. You and I are different, so we express that DNA differently, but essentially we all have quite a similar 3 billion sequence code. The DNA is where our functional proteins come from, although the steps to get from a piece of DNA to a protein is quite complex. You get your DNA from your mom and dad (a combination of them) as your parents did from their parents. So, you see, the DNA is going to be different in all of us as the mix we get from our parents is unique. But, in general all of us who get blue eyes (let’s keep blue simple..but we know there might be many shades of blue) essentially have the same blue pigment at the DNA level and thus code for a blue protein (pigment). Now, let’s say there is a random mutation that changes one nucleotide (again the building blocks of the DNA itself) in the blue gene. That mutation might result in no color being made, or a different color, or even the same color if the mutation is in an unimportant part of the gene. The point is that this mutation is random. And it might not even occur in the cells of the eye or not even in the cells that will be given to your offspring. It means that that mutation would only be passed down to future generations if it was in the sex cells (sperm or eggs). If not, than it is random and a single incident mutation.
A SNP on the other hand is handed down. It is a single nucleotide change that sticks in the chromosome (the way the DNA is bundled together….the structural unit of DNA) that is passed down through subsequent generations. So, a SNP is a change that is maintained at the genomic level. Have you ever heard that you can not simply give your organ to someone else, as their body may very well reject it. Well, that is due to the presence of SNPs in the genes of the body that are known as transplant antigens. So, your mom and dad gave you a mix of their own SNPs in their transplant antigens and you will give your future offspring a unique mix as well. However, overall in a population, a certain number of SNPs will be present. In other population, let’s say Asia, another set of SNPS are more frequent. A SNPs is not a random mutation. It is a difference in the nucleotide level (a single one) in the population due to mixing of that population over time.
So for transplants (giving someone your liver for example) you can only give the organ to a person who has the exact same set of polymorphisms in your transplant antigens. If you have different ones that the recipient, then they will likely reject your liver (or rather their own immune system will try to remove it).
SPS are found all over the genome. The one’s I was referring to above are functional, they actually have a known consequence. There are many other SNPS that lie in regions where there is no known function (for example that do not code for protein). However, if you read my post last week, you will note that those regions may not be junk at all and may be functional…in ways we don’t fully understand yet.
In summary, SNPs are single nucleotide changes that are in the DNA that are acquired by hereditary properties and exist in groups of people in a population. Some may be functional and some non functional. They are not mutations in that they are not random and must be inherited. Over millions of years, some SNPS can be lost, but usually the whole set of SNPs in the genome are thought to be kept in the population (as long as the population does not die-off of course)!
I hope that was helpful and the next time you see SNP will you feel a tad bit more enlightened.