I have mentioned previously that the pleasure for me in maintaining this blog is that some previously unnoticed (by me at least) plant or insect, once researched, takes on a whole new aspect. So it is with today's posting. A scattering of facts about the genetics of an inauspicious weed may seem obscure to some, but for me, a weed on my lawn catches my eye with a new interest these days. Photos 1,2 and 3 shows said weed. It pops up frequently in my garden.
For the unskilled amateur botanist (=me!) identifying yellow flowered weeds isn't altogether trivial as the guide books contain a long lists of yellow-flowered ragworts, fleabanes, marigolds, colts-foots, dandelions, sowthistles, cats'-ears and hawkbits. After a little work however I'm fairly confident my plant is none of these and is instead Smooth Hawks-beard (Crepis capillaris). A characteristic feature of the Hawks-beards are 'involucre bracts' (=the little leaves around the base of the flower head - best seem in photo 2) organised in distinct inner and outer rows.
Surprisingly, although it is a common UK weed, I've been able to find almost no detailed online information on Smooth Hawksbeard. One exception is the bioimages site that lists a handful of fungal rusts (including some Puccinia species - see my post here) and a gall fly known to parasitise Hawksbeard. The other exception was an online paper by Oud et.al. [1] that describes the chromosomes of C. capillaris. With my comments of the opening paragraph above in mind, its this I'll discuss.As most people know, the DNA inside cells is packaged into structure called chromosomes. When new cells are needed by a body, the existing cells set about creating copies of themselves by dividing into two, in a process known as meitosis. (Cells destined to become specialist structures such as sperm or eggs do something slightly different called meiosis, but never mind that here). To create two cells from one its clearly necessary to replicate the DNA. To do this the chromosomes perform a complex little 'dance' inside the cell in which they double in number (in a process called interphase), pair up and line up along of the middle of the cell ('prophase' and 'metaphase' respectively) and finally split apart ('anaphase') as the cell separates into two ('telophase').
I got the hang of this terminology recently using my colouring crayons! (I've a copy of the very clever - 'The Botany Colouring Book', Young - in which you learn by doing). You can find any number of explanations on the web however (here for example).
It's perfectly possible to watch the whole miraculous 'chromosomal dance' down a hobbyists microscope. The classic place to look is at the growing tip of a young plant root where new cells are being feverishly created to grow the root. Indeed, I've tried looking for it myself a few times. I've yet to succeed in getting any really good results, but rest assured when I do there'll be a posting...
I can't resist a small digression at this point to mention microtubules. When it comes to pulling chromosomes apart (i.e. during anaphase), cells do this by strapping tiny cables (microtubules) to chromosomes in a process akin to hauling logs out a log pile by pulling on ropes. These microtubules are tiny (around 25nm across where a 'nm' is a millionanth of a millimetre) and hollow. So tiny are they that some physicists (notably the famous blackhole physicist Roger Penrose) have even speculated that inside microtubules, reality ought to be dominated by the small and weird world of quantum physics (cats being both alive-and-dead; particles being in two places at once - that sort of thing) and that signals propagating inside microtubules in the brain might have something to do with the mystery of human consciousness (you can listen to Penrose give a lecture on it here). Anyway, this is a highly contentious claim and a long way from today's discussion. To return to more certain issues:
In their paper above Oud et.al. set out to study the three-dimensional arrangement of chromosomes inside replicating cells during prophase. It turns out if you want to study chromosome arrangements inside a cell without being mired in complexity, you can do worse than make use of Smooth Hawks-beard since it has a mere 6 chromosomes (strictly one should write '2n=6' - see my previous explanation here). Compare that with humans with (2n=) 46, or some ferns with around a thousand! (I have no idea why there is such variation in nature. Generally, there is no relationship between the number of chromosomes and the complexity of an organism. Anyone?).
Trying to image the three-dimensional arrangement of tiny objects is not simple when you remember that looking down a conventional microscope all you see is a flat, two-dimensional view of an object. To achieve a 3D visualisation of chromosomes Oud et.al. used a special type of microscope known as a confocal scanning laser microscope. As anyone who has a normal microscope knows, images suffer from a limited 'depth of field': only a portion of your object appears in focus. Other parts of an object, at different depths, appear blurred. Often this is a nuisance, but confocal microscopes cleverly use it to advantage. They physically block out light from anywhere other than the one extremely thin section of a sample that happens to be in focus. The advantage of this may not be immediately clear, but the point is that by slowly varying the point of focus, one can build up a stack of images where each image contains only light coming from that single, selected slice through the sample. By taking a bunch of such images from different depths, and stacking them all together (using a computer) one arrives at a three dimensional image.
Using this method Oud et.al. arrived at the wonderful picture of the three dimensional arrangement of chromosomes in a Smooth Hawks-beard root cell in their paper. Of course, their study was not about making 'pretty pictures'. They were interested in adding to biologists' long-standing interest in knowing how chromosomes are arranged in space inside the nuclei of cells; In particular whether the arrangement is random or not. Their conclusion (as I understand it) was that although a range of differing arrangements were observed across cells, overall the chromosomes seemed to arrange themselves in space in a finite number of non-random ways.
So there we have it! A week ago, for me, an unknown weed. Now, a named plant with a intriguing inner life. Bye for now.
Footnote:
I have begun to think that my habit of normally giving only a link to articles isn't the best, as articles may disappear online in future. From now on therefore, I'm going to make an effort to selectively include proper references at the end of postings. To that end:
[1] Oud JL, Mans A, Brakenhoff GJ, van Der Voort HT, van Spronsen EA, Nanninga N. Three-dimensional chromosome arrangement of Crepis capillaris in mitotic prophase and anaphase as studied by confocal scanning laser microscopy. J Cell Sci. 1989 Mar;92:329–339
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