<span>It has to do with the size of each molecule. One is large (double ring) and the other is small (single ring). If 2 purines would pair, pyrimidines next to them wouldn't be able to bind with each other, and if two pyrimidines would bond, there wouldn't be enough space for 2 purines next to them to fit. By having them pair with each other, the spacing along the length of DNA or RNA is kept uniform. </span>
Epithelial cells line the cavities and surfaces of blood vessels and organs.
The correct answer is outer electrons. These are sometimes called 'valence electrons', and they are responsible for the element's properties and how it forms bonds. For example, elements in Column 2 are likely to form ionic bonds with elements in Column 16. This is because elements in Column 2 have 2 outer electrons, while elements in Column 16 have 6 outer electrons. If the element in Column 2 gives its 2 electrons to the element in Column 16, then the former will be stable because the energy level underneath will be complete, and the latter will be stable because it will have a complete outer energy level with 8 electrons.
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Key points. The differences in characteristics between individuals of the same species is called variation . Some variation is passed on from parents to offspring, via genes during reproduction. This is inherited variation. Some variation is the result of differences in the surroundings, or what an individual does.