-
Asked by randomperson12345 to Anna on 30 Jun 2012.
-
Angharad Davies answered on 30 Jun 2012:
Bugs like bacteria and amoeba are small because they only have one cell. So they can never get very big, because they don’t have enough DNA to get big.
There is a kind of amoeba called a ‘slime mold’ which can get very big (like a big sheet of paper or bigger) which is huge for a single cell. It’s like a flat bag of jelly and can move around. It can do that because it has lots and lots of nuclei containing DNA. A professor I worked with had a scientist friend who had one of these and kept it in a drawer, and kept it alive on porridge!
In the deep sea a kind of single-celled amoeba has been discovered that is up to 10cm across, which is also absolutely gigantic for a single cell.
-
Comments
Jonathan commented on :
Humans can only be our size because we have specialized transport systems for blood and air. Without those most of our cells would be too far from the surface for the chemicals to move around.
Lots of things differ for animals of different sizes. Body shapes that work at one size would fail at another. Elephants need much thicker legs than antelopes. Whales can be much bigger than other mammals because their weight is supported by water over a big area rather than just through four feet.
Interestingly the biggest multicellular organisms in completely different groups are quite similar in overall size: whales, sharks, trees and underground fungi.
I’m not sure how closely this relates to Angharad’s answer about DNA.
Anna commented on :
Hi again! The DNA problem is relevant to bugs which only have one cell, like bacteria. They can only contain enough DNA to make a small amount of protein at once, so can’t grow big.
For animals with lots of cells, each cell has its own DNA so there is no problem there. But insects, for example (another type of ‘bug’) cannot get very big because of the way they breathe, through tubes on the sides of their bodies, which are too small to allow enough oxygen in for a bigger sized creature.
Jonathan commented on :
Anna’s taken us back to DNA. As well as each of our cells having its own DNA it has DNA from other living things! Our cells have mitochondria inside them, and mitochondria have their own DNA which is separate and different from the DNA in the cell’s nucleus. At the moment we think that the mitochondria are the descendants of bacteria that got inside our ancestors’ cells. We couldn’t live without them now.
The DNA in your nuclei comes from your mother and father, but the DNA in your mitochondria only comes from your mother.
It’s possible that other parts of your cells have also descended from other organisms, but we don’t know. We probably will know quite soon.