I'm sorry if this is the wrong forum... DNA/RNA question!

[water]

I've always been curious about this, but I'm having trouble using Google or Wikipedia to find the answer.

Could someone explain to me why

1. ...all life uses the same DNA/RNA (with genes from one plant/animal having the same effect when put into another plant/animal)? Shouldn't some life using some other system, at least with different molecules (or different meanings of particular genes)?
2. ...how the cells "know" what to do and when/where to multiply (or which type of cell to become) based on the particular sequence of DNA/RNA? I understand that DNA/RNA essentially contains instructions, but instructions are meaningless unless something knows how to follow them. I know we understand somewhat what each "instruction" does... but why and how?

It's fascinating to me, but I can't find the answers!

[aquart]

1. Life on earth has one source.

That's the answer to number one. If we find something that runs otherwise, then we'll have two sources. Currently, same source.

For 2, that's a Nobel Prize question. I think they recently discovered some extra "stuff" that has the job of reading the instructions. But we haven't figured everything out with that at all. Did you see the PBS Nature thing on Dogs? When they started breeding for gentleness, they got black and white animals. Turns out there are all kinds of links in our genes that trigger whole sequences of reactions.

Really, you just gotta keep typing DNA and assorted words into google till you get your answer.

[water]

2. I clarified #2, it was conufsing.

Regarding #1, I suppose that since all live evolved from the same source, they would all use the same instruction set, but I don't understand why the instructions haven't evolved.

Thanks for the reply! :)

[ismnotwasm]

3. Ha!

How and why are being studied to this day. After the onset of HIV, interest in retroviruses, "RNA" lead to much research and new information. If you have a basic understanding of DNA, check out the history of "unraveling" the human DNA codex. Might help

[mike_c]

4. great questions!

aquart nailed #1-- although there is still considerable controversy, the ubiquity of the RNA/DNA mechanism of inheritance is strong evidence that there is a single common ancestor for all life on Earth. Google LUCA or Last Universal Common Ancestor for more information.

Answering #2 is a bit more complicated and has multiple parts. First, a couple of clarifications. When you ask that question you are specifically asking about multicellular organisms. That sounds like a no-brainer but it needs to be explicit. Also, remember that all of the body cells of any multicellular organism-- like you and me-- has exactly the same DNA, i.e. all the body cells contain a precise copy of the DNA brought together during fertilization. This bears repeating: the DNA in every cell is the same, whether that cell is a neuron in the brain or a fibroblast in the foot.

What distinguishes different cell types at the genetic level is differential gene expression. All your liver cells have the same DNA as all your neurons, but the liver cells and neurons express different parts of their genomes (and some parts that are the same, of course).

The process you're asking about is called differentiation. During embryogenesis undifferentiated cells-- stem cells-- divide to produce more and more undifferentiated cells, some of which undergo determination and differentiate. Determination is a cellular response to chemical signals that tell the cell what kind of cell to become. Those chemical signals can come from a variety of sources, ranging from molecules placed into different parts the egg cell cytoplasm by the mother to cell surface markers on adjacent cells that have already determined or differentiated. For example, the egg cell cytoplasm is a highly organized gel that contains chemical signal gradients indicating which is the front and which is the rear of the organism that develops, which side is the top and which is the bottom. As the egg cell divides, these chemical signals are divided unevenly, creating a ball of cells with definite polarities-- the internal chemical environment of each cell is slightly different from most other cells, depending on it's location in the ball of cells (or blastocyst). That positional information is crucial.

All the cells have identical DNA, but the specific genes that are expressed in any given cell depend upon chemical signals within and around the cell, and those depend at least partly on its location in the organism's body. Cells developing in what will become the liver respond to signals from other cells around them becoming liver cells, and to signals from other cells that tell them where they are in the developing organism. They begin to express liver cell genes and they turn off inappropriate genes that are operative in other cell types. Turning on the correct genes and inactivating the others is determination-- that cell has determined its developmental course. Expressing the correct parts of its genome so that it physically and functionally becomes the "right" type of cell for its location is differentiation, which is the result of turning genes on and off depending on which are needed for its eventual function.

That's the short version. Google topics like embryogenesis, cell differentiation, etc for more info.

[mike_c]

5. re-reading question #2-- you should also Google "gene regulation..."

"...gene expression," "gene transcription," and "mRNA translation." I'm not sure whether you were asking about the mechanism of differentiation or whether you wanted to know the mechanisms of differential gene expression. They're all part of the same process-- I just didn't get into gene regulation, transcription, and translation in #4.

[eppur_se_muova]

6. Short, superficial answers...

(on edit: We do have a Science Forum! Only the nerdiest hang out there. :))

1) Natural selection operated even before life existed. If there were alternative biochemistries, they would have had to compete with RNA/DNA. Evidently, any which existed have not survived. Or it may just be that whichever started first, won.

There may have been a slightly variant form of nucleic acids early on, known as pyranose RNA/DNA. This area is still being explored.

Google or Wiki "RNA world" for some interesting background, not *directly* connected to your questions:
http://en.wikipedia.org/wiki/RNA_world_hypothesis

2) Very partial answer, a lot of DNA/RNA behavior is "hardwired" into the structures of the molecules themselves. That is, they behave the way they do because of their structures, and their structures lead to behavior which results in their structure being duplicated -- a case of self-propagating structure. This assumes a supply of the necessary building blocks is in hand!

Certain components of RNA are known to be produced under abiotic (or prebiotic) conditions, such as would have existed on a primitive Earth (or maybe Titan, who knows). Simple sugars, for example, can be produced from formaldehyde, while hydrogen cyanide, ammonia and other precursors lead to the nucleoside bases themselves. Thus RNA incorporates what are probably the simplest, earliest building blocks that could lead to a self-propagating, information-bearing structure. So the way the machinery functions springs directly from the properties of atoms and molecules.

That only addresses your question at the most basic level, and says nothing about the intricate details, where I think you're asking about ontogeny (the development of organisms during reproduction). Unfortunately, the wiki articles on both ontogeny and developmental biology are little more than stubs.

Take the old-fashioned approach: read a book! I recommend Horace Freeland Judson's "The Eighth Day of Creation", which is a *history* of modern molecular biology, not a textbook on the subject, but a more enjoyable read for that reason. The discovery of the structure of DNA is something that's been discussed (and dramatized) endlessly, but reading this book will make you realize just how many new questions were raised once the structure was known. Solving the structure was just the beginning. Sir Francis Crick was heavily involved in working out many of the answers to these questions, and posed the "fundamental dogma" with which all molecular biologists are now familiar. (A very *thick* book, I know ...)