Tars Tarkas, apparently that has been found to no longer be the case. The paternal mitochondria are in the little midsection right behind the head, and at least the head and this part of the tail completely enter the egg. It was previously thought that all of the paternal mitochondria were destroyed, but now this has been proven untrue. Here is a brief article from Nature about the implications of paternal mtDNA inheritance and possible recombination.
Hey, it was news to me, too!
choosybeggar used the phrase “junk DNA”. From the non-biologist’s perspective, this concept was explained to me as genes that have no function.
How is it known that a gene has no function? Are such genes absolutely and certainly non-functional, or is there a difference of opinion in the biological community? Were they ever functional? Might they become functional? If not, why are they there?
choosybeggar used the phrase “junk DNA”. From the non-biologist’s perspective, this concept was explained to me as genes that have no function.
How is it known that a gene has no function? Are such genes absolutely and certainly non-functional, or is there a difference of opinion in the biological community? Were they ever functional? Might they become functional? If not, why are they there?
choosybeggar used the phrase “junk DNA” to describe DNA that isn’t used.
How do we know that some part of DNA isn’t being used? Is this a conclusive opinion held by all the biological community? Was this DNA ever used? Could it be used in the future? If not, why is it there?
Thank you so much, SDMB server. (I did refresh the page to see whether the post had gone through… sigh)
Now that I’ve completed my first triple post, I feel bound to amplify on the original questions.
Are there mutations that cause a working gene to become non-functional? What about vice versa?
—How is it known that a gene has no function?—
Essentially because they do not ever get used for code for proteins in the organism in question.
—Are such genes absolutely and certainly non-functional, or is there a difference of opinion in the biological community? Were they ever functional? Might they become functional? If not, why are they there?—
One major theory as to “why they are there” is actually quite startling. It actually turns the question into: why are WE here? And the answer to THAT is “to carry them around.” In other words, the “purpose” of all this extra DNA (of which there is actualy much much more of in our genome than there is actual “human”) is simply to reproduce itself, and we happen to be a good ride.
Ooogh, Apos. You made it better, but worse.
What does “used for code for proteins” mean? How does the biological community know what’s used? The only analogy I know of assessing which parts of computer code can’t ever be accessed – there’s no way to positively identify every one of those in a complex program…
As for “we’re here to carry them around”… uh… er … these unused DNA bits are dormant? So … how could they have anything resembling a ‘purpose’? They sound like baggage. Should we just genetically splice them out, and save DNA? Let’s apply appropriate technology to basics, and cut down on cell inefficiencies…
They shut down school for the weekend. A little tropical storm gets them all in a bunch. Well, I suppose I don’t blame them after what happened last June in these parts. We lost millions of dollars after a tropical storm flooded our medical center.
partly_warmer:
It is a tricky question. “junk DNA” comes in a number of flavors, not all of them even look like they can code for something. Here is a brief list of them:
-pseudogenes – things that look like genes (containing things like enhancer and promoter sequences to tell where and when a gene should be made, many of them are even transcribed into messenger RNA, the intermediate in DNA->protein process) but don’t code for proteins. This is usually due to mutations in the protein-coding region that encode early stops. Much of it is degenerated to the point that it doesn’t even produce mRNA. We sometimes find genes which work in other mammals but have devolved in us – the gene encoding the synthesis enzyme for vitamin C in primates is one such example.
Pseudogenes, if I infer correctly, is to what Apos is alluding. It is to what your quote is relevant. Ergh grammar is not a strong suit with me. We know they don’t code for protein because we know the general rules of protein coding and they break them. Sometimes we can’t even detect the precursors, the message (although this can be a little tricky in a regulated gene).
-viral repeats – a large percentage, IIRC a majority, of our genome is made up of what appear to be repeats of viral genomes. These are usually retroviruses (RNA viruses which require a DNA intermediate, like HIV but they don’t cause disease) which have stuck themselves into our genome, and as Apos said, are mostly along for the ride. Although there is some evidence that some of them may encode proteins, and may have physiologic function (do a search for a Nature paper on syncytin).
-heterochromatin. This is the name we give to structural non-coding sections of the genome which do things like ensure chromosome integrity and sorting at mitosis and meiosis. Most of these are vast repeats, which ensure only structural properties. These areas are unable to be sequenced using our current technology. They may have a role in large-scale gene regulation, though.
All of these things may have function, so the term “junk DNA” is not really used in scientific circles. There are plenty of people working on all of these categories. There is plenty to be learned from it. Much of it may actually account for some of the reason we are human and not hominid.
Collounsbury:
I didn’t see that in the paper. Granted I didn’t read it that carefully. I did see some stuff on evidence for a selective sweep (as a prevalence of rare alleles), but I didn’t see a date. Eh, its not worth really bothering about. IMHO it is a crappy paper. No insult if you know the people, but it was another example of a small finding spun hard with a tenuous hypothesis to make an Am J Human Genetics paper into a Nature letter. I see this stuff all the time. Kudos to the group that did the work, as their spin succeeded, but I think that the actual science here is quite suspect.
When is said the “science was suspect” what I really should have said is the “science is lean.” The actual experiments were conducted without evident fault.
One commonly cited mutation is the ability to digest milk as an adult. Not everyone today has it yet, and it had to have occured since the domestication of milk animals (last 10,000 years or so).
—As for “we’re here to carry them around”… uh… er … these unused DNA bits are dormant? So … how could they have anything resembling a ‘purpose’?—
I don’t mean that in any sort of intentional sense. They simply were successful because they hitched a ride with a successful genome. The “purpose” of all genes, in the theory I’m describing, is simply to reproduce themselves as much as possible (those that don’t have this “purpose” obviously aren’t going to be around in great numbers as time goes on), and being in our genome is one of many successful “tricks” for doing so.
—They sound like baggage. Should we just genetically splice them out, and save DNA?—
It may or may not be possible to do so. The main factor is their additions to the length of our genome. When the DNA is “read,” oftentimes position matters. It’s like (and this is a VERY loose analougy) when you have a BASIC program (every line of program code is numbered) and you delete a few lines of code that are ordinarily never called: but the effect of this on the program is severe, because it destroys the “goto line such and such” references by changing what usefull lines of code are at which line.
You’d think nature would have beaten us to it. Why should a cell waste energy and resources supporting the duplication of DNA that in no way contributes to the cell’s functioning? If nothing else, one would think that pure selection would favor cells that weed out useless DNA. Which is one argument that so-called “junk” DNA might be more useful than currently realized.
—Why should a cell waste energy and resources supporting the duplication of DNA that in no way contributes to the cell’s functioning?—
Again, the real question might instead be “why does the genome waste resources by carrying around the code for a slack-jawed hairy biped?” And the answer would be: it’s a good strategy for ensuring the survival and propagation of those genes.
—If nothing else, one would think that pure selection would favor cells that weed out useless DNA.—
Not necessarily: if selection put it there in the first place (or, rather, put us here on top of it).
The thing is, the DNA isn’t there for the cell’s sake–none of it is. The cell–the entire organism, really, but things get very much more debateable the more complex the organism gets–is there for the DNA’s sake.
The cell “wastes” energy and resources because a cell is a DNA-protection-and-duplication factory. As long as a particular chunk of DNA doesn’t shoot itself in the foot by damaging the cell or larger critter, it’ll get duplicated because that’s what life does.
Life is only efficient and streamlined accidetnally–its nature is prodigal. A good thing, too.
Yeah I’d like to stress that most of life isn’t streamlined. Most processes in the cell are not streamlined for efficiency – the respiratory cycle could be more efficient, much of neurotransmission is wasteful, DNA synthesis, etc. etc.
We can get into a whole debate about the purpose of this padding DNA. We could discuss intron size over evolution, we could discuss why bacteria have little or no introns while large complicated animals with long life spans have huge introns. Not much of it is known, much of it is speculation.
Needless to say, the Alu repeats and retroviral sequences and heterochromatin is responsible in some part for genome stability, for large scale gene regulation over wide ranges of chromosomes, and perhaps other housekeeping purposes. It also may act as some kind of shield for radiation/UV mutagens – it gives more targets where mutations don’t hurt. I wouldn’t say that this stuff is useless, that it serves no purpose and is only along for the ride. Certainly it is not crucial, and only necessary in a large-scale, structural kind of way. But it is necessary.
Isn’t the wide-spread ablity to digest milk into adulthood a fairly recent mutation? As described in this GQ Thread
That may be true - I have no idea how recent that mutation is - but I don’t think it qualifies as the sort of mutation the OP is looking for. I’m lactose intolerant myself, as are enough other people that the marketing of Lactaid, lactose-free milk, etc., is profitable. There are plenty of mutations that are not shared by the entire human species - for example, sickle-cell trait.
If I understand correctly, the OP is looking for a single mutation shared by the entire species, like the opposable thumb or a certain size brain cavity. Or the mutation involving the ability to develop a spoken language, as mentioned earlier (and seems to be a good candidate).
I misunderstood the OP meaning. I took it as “what is the most recent mutation in humans that’s fairly widespread” (as opposed to rare genetic disorders) when he apparently meant “what mutation was the ‘crowning touch’ of the current human genotype?”
Do you mean to say that we are “slack-jawed hairy bipeds”, or that we just have the latent DNA for them?
I assume the latter. You’re saying that “junk DNA” might have been used in the past, and might again be used in the future, should mutations follow a particular path? If that’s the case, then what’s “junk DNA” for one human being may be non-junk in another. True?
Coupla points:
1) Selection operates on phenotypes, not genotypes.
This is ultimately the reason behind “junk DNA”. When a mutation occurs in a functioning gene which renders it inoperable, whatever effect on the organism’s phentoype the gene had is no longer available for selection to act upon. Because these sequences, through the loss of any phenotypic expression they once had, are no longer “visible” to selection, strings of base pairs which do not constitute functional genes build up over time within a DNA strand. It is highly unlikey that such strands will ever become functioning genes again in the future.
2) A single mutation will not necessarily translate to a single, or, indeed, any, change in phenotype.
3) Humans (i.e., Homo sapiens sapiens) lack a formal taxonomic definition and diagnosis (Linnaeus’ full description for our species was simply, “Homo nosce Te ipsum”; loosely translated as, “Man, know thyself”).
These two points, taken together, render the question in the OP essentially unanswerable. Because of point #3, it is difficult, at best, to tell when our ancestors “officially” became Human. Because of point #2, even if we could tell definitively “Human” from “non-Human”, all we can point to are specific phenotypic traits which set us apart from our ancestors and relatives (such traits are known as autapomorphies). However, there is no guarantee that there is but one such trait, nor is there a guarantee that said trait is the sole product of a single gene.