Why is the proportion of Adenine and Guanine approximately equal to that of Thymine and Cytosine in the table?

Question

I got a table in a bio paper:

Type of Nucleic Acid Proportion of organic bases
(percentage)
A G C T/U
Squirrel DNA 29 21 22 28
Shark DNA 28 21 21 30
Human mRNA 40 15 30 15

a) Explain why the proportion of Adenine and Guanine is approximately equal to that of Thymine and Cytosine in both pieces of DNA

b) Explain why a similer equality does not exsit between the proportion of A+G and the proportion of U+C in the RNA.

c) Despite being very different organisms, the squirrel and the shark have simelar proportions of the four different bases in their DNA. Explain how this is possible.

I am completely lost...
Please Please please help if you can! I'm really struggling!

Answer 1

The first one is easy: A always pairs with T, and C always pairs with G. (almost always, but let's not worry about mutation yet)

So, since this is the case, then your proportion of A will equal your proportion of T, and your C will be proportionate to G. In the final analysis, the percentage of A/C and G/T in a particular organism's DNA will be approximately the same (because the DNA differences between members of a species are so small as to not make a difference in this calculation)

The reason that this isn't true in RNA is because RNA has been stripped of non-coding regions (introns) that change the overall makeup of the sequence -- removing different portions, often depending on which gene product is desired (a single mRNA can be spliced to form many final products that code for different proteins, or subsets of a type of protein); therefore, the A/G C/U proportion will be sometimes radically altered.

As for C), the thing about this is more of an evolutionary type question. Despite the fact that sharks are physiologically (or phenotypically) very different from squirrels, their DNA is not all that much different in a purely analytical sense -- it does not take a large change in the coding sequences to make something look (and act) totally different from it's progenitor. I would guess that this might indicate a common ancestor, but honestly -- it might also just be coincidence, as there is a limit to how much those ratios can change (the GC content is also critical to the ability of DNA to separate for duplication, so the amount this can decrease/increase does have a limit)

Oh, also: I just noted that your table shows mRNA for two (shark and squirrel), and DNA for the third. Sneaky, and from my point of view, somewhat unfair, since human DNA is about 30 percent A/T and 20 percent C/G (which looks like your numbers up there, doesn't it?).

IMHO, this is a rather rotten assignment.

Answer 2

a. Adenine always matches with thymine in DNA, so the amount of adenine and thymine should be the same. It's kind of like left and right shoes. If we all throw our shoes in a pile, there should be the same number of left shoes and right shoes. The left matches the right. Adenine matches thymine. Cytosine matches guanine.

b. RNA only matches one side of the DNA. So the gene might say AAAAACCGGT. Even though the other side of the DNA says TTTTTGGCCA, making the amounts even in DNA, RNA is only copied from the side of the DNA that is the gene. It doesn't copy both sides of the DNA. So the amount doesn't have to be equal. In the AAAAACCGGT example, the mRNA will be UUUUUGGCCA. There are 3 A+G, but there are 7 U+C. It's right, but they don't have to match.

c. Very different organisms have more similarities in their DNA than they have differences. Our genes are more alike than they are different.

Answer 3

Explain Proportion

Answer 4

Well C might have to do with as all coming from the same life source when life began on our planet. You could expand upon that. Or you could be really smug and walk up to your teacher and say "Because God said so."