If a section of DNA has no start codon it can’t be replicated and used as a gene product.
Thus it becomes junk, because there is no physical way for any information, if it is indeed in there to be expressed.
One way to look at it is from a mutagenesis/evolutionary standpoint. And this was how junk DNA was explained to me…
Mutations are generated randomly along your genome, the more non-coding DNA you have, you are left with a smaller percentage of coding % vs entire genome length. So each individual mutation has a smaller and smaller chance of damaging an important gene. Which further enchances, in a relative aspect, the effeciency of MMR. The junk DNA is retained because it provides additional fitness from resistance to mutations.
Junk DNA could contain long lost genes from creatures we evolved from, copies of genes we need that somehow got jumbled up during replication.
Some junk DNA is usefull, the telomeres at the end of each chromosome protect us from aggrigate DNA loss due to DNA polymerase’s function. Then again I’m not exactly sure telmoeres are considered junk DNA, they are non coding DNA…
Personally, I highly doubt junk DNA is anything useful. It’s not as if we’re likely to paw through junk DNA and find the hidden gene to make us immortal.
The human body is HIGHLY sensitive to errors in gene expression. The most severe mutation in gene expression that we can survive is Downs Syndrome, and that’s a duplication of chromosome 20-something… 21? Compare this to plants, strawberries can be induced to produce up to 25+ copies of their DNA. The result is larger, sweeter tastier strawberries. Bannanas we buy in the stores are triploids, which makes the seeds almost nonexistant and generates a huge flavorful fruit.
CRorex, I realize that this “junk” DNA isn’t expressed and doesn’t code for anything. I, along with every single professor I’ve had, call this non-coding DNA. Why we have it is still a mystery to scientists, but no one dismisses it as just “crap” because we’re obviously still using a lot of energy replicating our entire genomes and passing it on to our daughter cells.
I’m fully aware there are no genes in non-coding DNA, which is why it is given that name. If it was just junk however, I’m sure we would have lost it a long time ago. Our mitochondrial DNA, for example, has almost all coding DNA. If our chromosomal DNA could do the same, it would have years and years ago.
I think dismissing all of Junk DNA as garbage is folly. some of it has known functions, and some has unknown functions. Sure, a lot of it is crap (IIRC from Genetics lab ~5% of our genome is crap spat out by a retro virus thousands of years ago) but the crap serves the useful function of taking mutation bullets for us that could happen in coding regions, FUBARing up proteins and causing hosts of problems for the poor unfortunate. Sure it may cut down on our “evolution power” or whatever, but it is preferable to living where all my junk DNA has been streamlined out.
BTW, Down’s is 21, which more 21 means more superoxide dismutase enzyme, which is bad in large amounts. IIRC, you can have four 21s and still be running around. Patau is trisomy 13 and Edwards is trisomy 18, both have life expectancies of a few weeks.
You also can have multiple X and Y chromosomes (or lack one X). In school they listed all the way up to someone who was XXXXY. Not having an X will kill you.
Repetitive elements make up 43% of the assembled human genome. This includes LINEs, SINEs, Alu repeats, and transposons (from the Nature issue on the human genome). These are almost all retroviral in origin.
Only three quarters of the genome was able to be assembled using the whole genome shotgun approach with contig spanning and the like. We can safely assume therefore that the remaining one quarter of DNA is almost entirely repetitive, either in telomeres, centromeres, or large spans of repetitive DNA. So this means that around 56% of our genome is repetitive. This 56% percent does not include nonrepetitive noncoding DNA around genes. Only 1.1-1.4% of the sequenced regions actually codes for proteins.
Tars Tarkas is right – it certainly serves some purpose, even if it is only a secondary one like cutting down the mutation rate in really important genes. But most of it is parasitic in origin and does absolutely nothing, to the best that we can tell. I wouldn’t hold out too much hope of it doing anything – we can tell that there is very little evolutionary functional constraint on these regions and therefore at least on a nucleotide-based level, it is doing jacksquat.
I’m going to preface this by saying that what I know about genetics could fit on the back of a postage stamp and still leave room for my comprehensive plan to turn around the US economy, but we still have appendicies and pinky toes, which are also useless, right? Wouldn’t we’d only lose that stuff if there were a measurable increase in survivability by losing it? If it doesn’t affect our survival one way or the other, there’d never be any impetus for it to be bred out of the species.
Hey, you kids wanna keep it down with the DNA babble? There’s people trying to crack jokes in here.
We’ve had a Harry Potter one-off, a couple of space robots riffs, a Simpsons line or two, and a metric asston of Ivory soap gags. C’mon, please, don’t ruin it with knowledge.
Regular bathing is good for your parrot’s feathers and general health, and most of them enjoy it. They do not, however normally use Ivory soap, or any other brand of soap. If your parrot should get into something really oily or dirty, so that you had to use soap to get him clean, my WAG is that something unperfumed, like Ivory, would be best, although such a situation would be quite serious, and you really should consult your veterinarian.
Just a little tidbit of detail I thought I’d share, then we can get back to cracking jokes.
When we talk about an increase in fitness in evolution, it has been shown that advantages leading to as little as 1 extra progeny per 10,000 is significant enough for strong selection (over many generations). Standard mutational analysis does not allow us to identify changes producing such a little increase in fitness. Comparative genomics, though, can show us exactly the regions which are functionally constrained, and therefore do not change. So when we talk about junk DNA, we mean that it is not functionally constrained, and therefore has far less than a 1 in 10,000 improvement in fitness.
And please, when flying the Soapy Chimp ™ Airlines, be sure to spell your attendant’s name correctly.