Thread: Unique Polypeptides

I was just thinking about how everyone's DNA is different. So that must mean that the RNA it makes is slightly different in one way or another. So I was wondering whether there could be polypeptides that are unique to a single individual or a family. If there is, it could be used in security and identification purposes. So, my question is: If everyone's DNA is unique to the individual, could there be a polypeptide that only that particular individual or family has?

Dear quantumwarrior,

Generally, if you were to take a sample of individuals off of the street wherever you live right now, it would - in theory, at least - be possible to extract a sample of their DNA and have it sequenced, before aligning all of the sequences together. The alignment would reveal vast swathes of homology between all of the individuals in coding regions of the DNA. This is simply because (presumably) all humans require certain proteins, for example haemoglobin, and once Evolution has stumbled upon a successful way of solving a problem (e.g. Oxygen transport); then it is more likely that the gene encoding that protein will be conserved, as opposed to the alternative scenario - to have Evolution 'tinker' with random mutations to 'invent' a new solution.

So, yes, people's DNA is different. But in coding regions, there is generally a high degree of homology between individuals. So, it's more like the whole DNA molecule is collectively different, between individuals. The people will tend to have genes encoding the same type of protein, but they may possess slightly different forms of that gene.

It's a bit like if you imagine that the DNA consists of coloured blocks (ROYGBIV) all strung together in a line; each coloured block being one gene, and the whole genome contains just 10 genes. If you found a string of blocks at the scene of a crime, you would not be able to identify the criminal on the basis of any one of those coloured blocks - because more than one person is likely to share that colour at that position on the string. A much better way of identifying the criminal, would be to look at the whole sequence, the whole string - rather than any one block/ gene/ protein.

It should be noted that single nucleotide polymorphisms (SNPs) are frequently found between different inidividuals in coding regions - and also that random mutations can introduce changes in the sequence - however I still think that it would be preferable to look at the whole sequence, rather than trying to identify one absolutely unique protein.

Also, the so-called "junk" DNA regions may be a good source of variation between individuals.

Interesting question.

Tridimity

Originally Posted by quantumwarrior

I was just thinking about how everyone's DNA is different. So that must mean that the RNA it makes is slightly different in one way or another. So I was wondering whether there could be polypeptides that are unique to a single individual or a family. If there is, it could be used in security and identification purposes. So, my question is: If everyone's DNA is unique to the individual, could there be a polypeptide that only that particular individual or family has?

Going by conventional thinking on the issue, no. There are bound to be many people capable of making one or more entirely novel gene products- but not everyone. Everyone's genome is unique, with the possible exception of clones or identical twins, but that does not mean that each person has at least one unique gene. It means that the combination of genes is unique.

Reality may be somewhat messier than this though, and the extent to which this is true will probably be revealed as sequencing becomes cheaper and faster. I suspect that what we'll find is that SNPs are present in most individuals on the order of hundreds, though SNPs do not necessarily fall within a coding regions and when they do, it does not necessarily result in a a change to the gene product.