• Question from:Karen

I'm sorry I had to write a new post I couldn't find a way to reply to the previous one.


Anyway, yes Graeme I am aware that it has always been taught that prokaryotes came before eukaryotes, I'm a biology major myself and was, like you, shocked.  I'm reading a book called Genome by Matt Ridley, and I came across this passage that said that it has been suggested by a couple of scientists from New Zealand  that the "tree of life" may actually be upside down. I read the passage on and on, and weirdly enough, the explanation made sense. 

It explained the RNA theory, which states that before DNA and proteins existed there was only RNA. Which makes sense, because in advanced animals such as ourselves, RNA plays a messenger role, it neither holds the genetic code as does the DNA , nor does it perform all of the body's activities like proteins, it is only the link between those two. So more or less, it has a smaller (i won't dare say minor) role than DNA and Proteins. And since evolution strives to the better, this confirms the theory that RNA evolved to DNA and proteins only to become less important.

It is much more acceptable , thus, to say that prokaryotes dropped the RNA (i'm sure you know they do not have RNA) rather than eukaryotes created it.

Why did they drop RNA? Most bacteria live in places with extreme conditions- very high temperatures.
So , in order for them to avoid the excess mutations that might occur, they dropped their RNA (and also them having a single DNA makes sense).

It is a theory. If i wasn't clear enough make sure you look it up.


And after you get over your disappointment, try answering my questions :)

Photo
  • Natural History Museum, London, UK
  • Posts: 113

Hi Karen, I will do my best!

I'm sorry I had to write a new post I couldn't find a way to reply to the previous one.

Yep, the site is designed so that only we 'experts' can reply to posts, even though it is essentially the code for normal forum sites underneath.

Anyway, yes Graeme I am aware that it has always been taught that prokaryotes came before eukaryotes, I'm a biology major myself and was, like you, shocked.  I'm reading a book called Genome by Matt Ridley, and I came across this passage that said that it has been suggested by a couple of scientists from New Zealand  that the "tree of life" may actually be upside down. I read the passage on and on, and weirdly enough, the explanation made sense.

It would be good if you could provide a citation and then I can look it up! I had thought you meant the paper by Rivera and Lake published in Nature in 2004 where they argue that genomes indicate eukaryotes evolved via the fusion of an archaeon and a eubacterium (or at least, that is how I understood it).

It explained the RNA theory, which states that before DNA and proteins existed there was only RNA. Which makes sense, because in advanced animals such as ourselves, RNA plays a messenger role, it neither holds the genetic code as does the DNA , nor does it perform all of the body's activities like proteins, it is only the link between those two. So more or less, it has a smaller (i won't dare say minor) role than DNA and Proteins. And since evolution strives to the better, this confirms the theory that RNA evolved to DNA and proteins only to become less important.

Yes, this is a hypothetical stage in the evolution of life known as the RNA world and appears in many models of the evolution of life. However, it is not proven (and probably can't be) nor can we really say that "evolution strives to the better." Natural selection may favour the 'fittest', but what qualifies can vary and not everything in life is adaptive. We don't really know why DNA became the only way organisms store genetic information, there are many hypothetical alternatives, but this could be a case of 'lock-in' such that once it happened (perhaps accidentally) alternatives became practically impossible, e.g. like the inefficient QWERTY keyboard.

It is much more acceptable , thus, to say that prokaryotes dropped the RNA (i'm sure you know they do not have RNA) rather than eukaryotes created it.

I'm not a molecular biologist, but I am trying to learn (my training is in geology and palaeobiology). However, from my reading they do have what is known as polycistronic messenger RNA, but this is unique to them. But still, this fits with RNA-first and I don't see a reason to dispute this.

Why did they drop RNA? Most bacteria live in places with extreme conditions- very high temperatures.
So , in order for them to avoid the excess mutations that might occur, they dropped their RNA (and also them having a single DNA makes sense).

It is a theory. If i wasn't clear enough make sure you look it up.

Well, I'm still confused. I will look it up if you post the citation.

And after you get over your disappointment, try answering my questions :)

It's my confusion and ignorance I need to get over!

Last edited by Graeme Lloyd (5th Oct 2008 17:25:38)

Photo
  • Institut Pasteur, Paris, France
  • Posts: 78

Just to clarify.
Bacteria did not drop RNA. The main mechanism DNA>RNA> Protein is the same in eucaryotes and procaryotes. The genetic code is (roughly) also the same. What is not found in bacteria is the complex RNA processing machinery which allow one gene to make several RNA and proteins (RNA splicing machinery) and some RNA based elements who are thought to be parasitic.
It is still speculation to say that bacteria dropped them, rather then eucaryotes creating them. It is probably a more parsimonious therefore attracting hypothesis, mainly because some parts of the RNA machinery of eucaryotes inspired the RNA world theory and  could be primitive mechanisms (some seem not even useful to the cell anymore). But we should be cautious with that idea.

Another thing. I don't think we could say "most bateria leave in extreme environment". However most bacteria depend on external sources of energy or metabolites, either methan or thermic energy or other life forms (through parasitism or scavenging). It really depends on what you mean by most bacteria, biomass or number of species?