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Medical Forum / General / General / January 2006definition of gene, of chromosome, and difference between one species to another
Do we have a clear definition of gene and chromosome? I remember the definition of gene used to hinge on the idea that one gene produces one protein. What I want to know, as this question arose in sci.math thread, is whether we have a good idea of what differences in the DNA coding of one species is to its nearest relative species. We are the same as the ape in that we share 99 percent of the DNA code. So does that mean one species is different from another in just one gene? I seem to remember the debate about Neanderthal man whether he was our species or a different species. And that the test was whether there was a difference of 3 genes in mitochrondrial DNA. So in the A,C,T,G of DNA, what is the smallest difference between two species? Is it say a difference in 10,000 letters? Archimedes Plutonium www.iw.net/~a_plutonium whole entire Universe is just one big atom where dots of the electron-dot-cloud are galaxies >Do we have a clear definition of gene and chromosome? I remember the >definition of gene used to hinge on the idea that one gene produces one >protein. Chromosome is simple enough, for the most part. It is one physical thing -- we have 23 pairs of them (ignoring subtleties of sex chromosomes). But chromosomes per se are not particularly interesting, so that this is the easy one doesn't help much. One gene-one protein is a great idea -- and once a conceptual breakthrough -- but it is also a simplification. And no clear meaning has emerged, though the concept is fine to molecular biologists. Fortunately, neither has anything in particular to do with your question below (or in any meaningful sense with most real world questions that come up). >What I want to know, as this question arose in sci.math thread, is >whether we have a good idea of what differences in the DNA coding of >one species is to its nearest relative species. We are the same as the >ape in that we share 99 percent of the DNA code. So does that mean one >species is different from another in just one gene? No idea how that last sentence follows from the preceding one. >I seem to remember the debate about Neanderthal man whether he was our >species or a different species. And that the test was whether there was >a difference of 3 genes in mitochrondrial DNA. Aside from issues of how good the data for that really are, the question is somewhat silly. That is, there are no particularly clear criteria for how to define species based on such evidence. (In fact, in the absence of direct observation of the ability to inter-breed, the meaning of species is debated.) So the data may be interesting, but the answer is inevitably something of a guess -- and not really very important anyway. >So in the A,C,T,G of DNA, what is the smallest difference between two >species? Is it say a difference in 10,000 letters? It is 1% of about 3 billion, since we have 3 billion bases. Note it is a statistical number, the biol implications of which remain to be determined. That is, alone one cannot conclude anything in particular from the number. Further, do not try to compare any data on mitochondrial genes with nuclear genes. Rates of evolution are very different. bob > >Do we have a clear definition of gene and chromosome? I remember the > >definition of gene used to hinge on the idea that one gene produces one [quoted text clipped - 44 lines] > > bob --- quoting www.ornl.gov/sci/techresources/Human_Genome/home identify all the approximately 20,000-25,000 genes in human DNA, determine the sequences of the 3 billion chemical base pairs that make up human DNA, --- end quoting www.ornl.gov/sci/techresources/Human_Genome/home Bob, tell me whether the base pairs form distinct genes, or whether some base pairs have roles in numerous genes. Whether 100,000 base pairs have a role in only one gene or whether base pairs are not limited to one gene but have a function over many different genes? And a question about species. Can there exist a species A and a species B, who are identical except for the sex chromosomes. In other words, the species determinant is all found on the sex chromosomes. I would guess not because I think the major difference between human and ape are the bone and muscle morphology of throwing and I would guess that the sex chromosomes would not be the determinant of bone and muscle morphology. Archimedes Plutonium www.iw.net/~a_plutonium whole entire Universe is just one big atom where dots of the electron-dot-cloud are galaxies Hi, for your question concerning chromosomes and genes, let's say that chromosomes are physical entities composed of DNA (genes and regions that are not functionnaly knowned, if we try to make it simple) and the surrounding proteins that maintain the structure of chromosome (histones...). The gene itself can be define as a DNA sequence coding for a protein. this is the orthodox molecular biologist definition, but those last years, it has been shown that DNA could be transcribed to RNA that are not translated to protein and they can act as regulators (such as siRNA... have a look at some recent papers on that topic, it's pretty interesting!). Concerning the definition of species, that's the funniest part: Noone agree with the others. In bacteriology, the species is define at the threshold of 70% of identity between two genomes, whereas form human and chmpanzee (eukaryotes) you have less than 2% of diffrence. So this is a tricky point. The old fashion definition of a species is to say (for animals) that two animals are from the same species if they can mate and give a fertile new generation (unlike donkeys and horses that give a sterile mulet!). But here also we can fine some failures in the definition with the famous example of the sea bird Larus sp. There are populations all around the seas and they can cross with their closest neighbour but cannot with the further ones. Here where is the species limit??? So, as evolution is a continuous system, species are separating from their ancester all the time and there are also some nice concepts you should reed about. Speciation, whetever it is sympatric or allopatric is really an interesting topic. I just hope I could at least partially answer some of your questions and give you some new ways to explore, if you are interested in biology... Greetings, Ben. |
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