How do cells know what sort of cell to grow into? Skin or hair or liver or bone?
Answer: This one I actually DO KNOW!!! I am not making this one up!! I remember being told about it in sea urchins, and the same basic principle applies across all organisms... Anyway, what happens is that the egg cell gets fertilized, and then that cell divides and divides again. So you've got four cells, and they are sort of tall-thin cells, imagine they are in a square, *grin* like four beer cans. Now, if you take any one of these four cells, you get a whole sea urchin grow out of it.
Next, the cells divide again, across the middle, so you have two four-packs, one on top of the other. Any one of these cells will not be able to grow in to a whole sea urchin...I can't remember which one is which, but depending on if you take a cell from the top four or the bottom four, you end up with either a squishy mess of cells or a sea urchin 'larval form' that develops no further.
What's happened is that gravity has had an effect. Now, I know things are supposed to fall at the same rate, regardless of their weight, but let's be honest - we know in the real world that doesn't happen...If the leaning tower of Pisa were in a vacuum, maybe! If I stood at the top of the tower and dropped a cannon ball and a feather and a bit of paper, you might be able to have a guess at the order of landing - you'd certainly be sure it wouldn't be simultaneous. This is what's happened in the cells. The bottom cells are full of the heavier bits, and the tops cells have the lighter bits. The different proteins between the cells makes them different. We know this is a gravitational effect, as if you put the cells in a thing that tips them up and down so gravity can't work in a consistent direction, you just get the squishy mess of cells again.
So, OK, we have two different sorts of cells...what next? Well, the different 'factors' (generally proteins) in the different sort of cells enable the switching on or off of different genes. As the cells continue to divide, the division of these 'factors' can continue, and also the switching on or off of different genes (which basically determine the production of proteins) can provide or supress new factors. Some of these factors will enable or disable the reception of signals from surrounding cells, which can switch on or off other processes, so you get more and more distinct and more and more specialised, until you end up with skin or liver or bone or hair, whatever you wanted.
There's two processes - cell determination, when the cell says 'I am going to be part of the leg' and then cell differentiation, when the cell say 'I am going to be part of the leg bone'. The determination almost always comes before the differentiation.
I know that's a bit wooly but it's all I can remember this morning ... for a head-spinning example of muscle differentiation, see: http://www.acs.ucalgary.ca/~browder/muscle_determ-diffn.html.
Thanks Maizie.
How do they get the colours in marbles?
Answer: Marbles are made out of glass. They take rods or strips of different colors of glass and lay them together, then heat them till they begin to melt, so they fuse together. Sometimes they twist it while it's still soft, to make the colors swirl. then they cut it into chunks, heat them again, and roll them into spheres. Thanks Ace.
Why do helium balloons inside a care move forward when it accelerates, and backward when it brakes?
Answer: Air pressure. When the car accellerates, the air mostly moves to the back of the car so the helium baloon goes forword. Same in reverse for braking. Thanks lots of people.
