Why not just a ladder shape?
No benefit, just the way the molecules fit together. It’s like the hexagon of the benzene ring: it falls out of the geometry of chemistry.
More can fit within the same length of chromosome. Think of a ladder vs a spiral stair case. They might be the same height, but the spiral stair case has more steps. Think about if you could twist the ladder, it would get shorter. So, more material could be fit into the same length of ladder if it were in a double helix rather than straight.
Human DNA is around 6 feet long when stretched out (think about that for awhile).
Does that mean un-helixed? If so then the helix coils must be considerably tighter than most text book illustrations.
Take a look.
this may sound dumb, but aside from size, wouldnt the spring coil make it more durable as well?
Ok maybe that is a dumb idea, but it would give more flexibility to survive physical damage
By assuming a coiled shape the DNA molecule assumes a double helix which produces a more compact shape. This limits the physical access of any of a class of mutagens that insert themselves between the rungs of the DNA ladder. These mutagens are particularly potent as they tend to cause frameshift mutations which tend to be more toxic than single nucleotide changes.
Trinopus got it in one. That’s just how the molecules line up.
The two helices are held together by hydrogen bonds. The bonds are weaker than covalent bonds. They allow the two strands to cling together, but “unzip” easily when necessary, then re-anneal later.
If you were to take a DNA double helix and untwist it so it formed a straight ladder, it would come apart because the base pairs would no longer be facing one another properly.
That’s less than 2 inches on average per chromosome.
Still, two inches is far too long to unravel inside a cell, some of which are very tiny. Only a small part of the DNA molecule can be uncoiled at a time for transcription or replication. And this all has to be done with various special enzymes.
The complexity of all this almost makes one want to believe in Intelligent Design! :eek:
I think the OP already understands that there are good reasons why the topology of the genetic material is an orderly double-stranded molecular “ladder” with repeating units on each strand, I don’t think I need to go into that.
The answer to the question is that in a three-dimensional world, any repeating molecular “ladder” like this will by default form into something rotating. If the molecular bonds connecting the repeating segments on the backbone of the “ladder” are anything other than dead straight at 180-degrees, each repeat unit will add a little bit more twist, and some kind of progressively rotating 3-d structure will result*. And few bond angles in organic chemistry are dead straight.
There is no question to answer. We would only have to ask “why” if the converse were true, if DNA that exists in a three-dimensional world were an unnatural weird pseudo-two-dimensional non-rotating dead-straight ladder.
[*Something non-rotating could also result if the repeating units were alternately arranged forwards and backwards, so that the backbone zig-zagged back and forth; but there’s obviously a good reason why DNA bases all point in the same direction in the chain.]
Thank you both, for explaining this phenomena, less the ladder hyperbole of more (intended) information per unit length.
This is correct. Twist up a long rubber band, then twist up the twist. Then twist is up again. You get the picture.
I’d like to hear some actual evidence for the notion that the helix is a design feature of any kind. For the helix to be an evolutionary adaption for compacting DNA, there would need to have been some ancient variation upon which evolution could act, with and without the helix. In other words, another candidate molecule for the genetic material, that had all the other key properties of DNA, but not the helical structure.
In any event, as for the idea that the helix is an evolutionary adaption for compacting DNA, prima facie it doesn’t hold water:
(1) The cell and the DNA are 3-dimensional objects. You don’t gain any volume advantage from a helix. It’s shorter, but also thicker.
(2) Even if a ~2X shortening of a strand were achieved without any thickening, this factor of around 2 is negligible compared to the many orders of magnitude of additional packaging that needs to be done to fit several feet of eukaryotic DNA inside a nucleus.
(3) What evidence (or even plausible argument) is there that is more difficult to compact a hypothetical non-twisting ladder structure?
Of course, a huge amount of molecular biology is designed around various aspects of the helix structure, including compacting mechanisms, but that does not speak to whether the helix itself was ever an evolutionary adaptation.
It seems to me that a helix is just what results naturally from any necessary “ladder” polymer. Now, the compactness and stabilitiy of the particular helix of DNA and the bases used in life, that is much more plausibly a design feature.
add to the above -
(4) If compacting DNA by a supplementary factor of around 2 were important, one might expect to observe selection acting to actively remove some of the large amount of junk and shorten genomes. I’m not aware of any evidence for this.
To get a straight-line ladder molecule without twisting, doesn’t the angle between two relevant bonds have to be exactly 90°? And if by chance the molecule were “unlucky” enough to get that exact angle, it would be under attack from other molecules. Folding into a denser 3-D shape provides stability.
The skeleton fork fern has a genome almost 80 times that of humans, about 500 feet linearly altogether instead of 6 feet. It doesn’t seem slowed down by this huge waste.
I like the way you think … great questions all … I have no answers but the questions are magnificent …
I’m guessing the Alpha Helix evolved first … it’s plausible this structure could carry genetic information … extremely unlikely over 6,000 years but the odds improve somewhat if we have several billion years to work with … and the point that not all life of Earth uses DNA, some simple bacteria uses RNA …
Evolution can be caste as a “trial-and-error” operation … maybe DNA/RNA just by accidentally fit the needs best and first … so alien species might not use such at all …
Great cite/site.
The chirality of the helix may also be relevant - the building blocks of DNA are chiral (the ribose units) no matter what shape you assembled it into, but the helix itself is chiral - it has a right hand twist under usual circumstances. This must be of fundamental importance when considering DNA interactions with other macromolecules (proteins such as transcription factors, other DNA helices etc) which are all chiral themselves.
A ladder shape is not chiral (not that DNA could form such a flat structure anyhow, as noted), so wouldn’t have this structural (and then functional) dimension.
No, a “flat” molecular ladder can certainly be chiral. You are correct that something with the gross morphology of a helix (a piece of pasta, say) is chiral, whereas something with the gross morphology of a ladder (i.e. an actual ladder!) is not chiral. But here we are considering complex molecular structures arranged overall in those shapes. All you need is a minimum of one chiral center somewhere in the molecular structure.