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| Who Knew? |
Even some scientists, as usual, are less than aware of "Chargaff's Rule". A general description and discussion of Chargaff's Rule is:
"In the mid-1900s Erwin Chargaff discovered a crucial element of DNA structure. Chargaff knew that DNA was made up of four different bases: adenine, cytosine, guanine, and thymine. In his experiments he observed that the ratio of cytosine to guanine was one to one. And the ratio of adenine to thymine was also one to one. This means that the molecules of DNA he observed were made up of equal percents of cytosine and guanine, and there were also equal percents of adenine and thymine. This means that if a DNA molecule has four guanine nucleotides then it must also have four cytosine nucleotides, and in the same molecule if you have six adenine nucleotides then you will also have six thymines. This phenomenon can be explained by the complementary base pairing that gives DNA its double stranded structure. Each strand of DNA is made up of nucleotides. The two strands are connected together by hydrogen bonds linking the bases of complementary nucleotides. Guanine is complementary to cytosine and adenine is complementary to thymine. So, in this double-stranded molecule everywhere you see guanine you will find it attached by hydrogen bonds to cytosine. And everywhere you see thymine you will find it attached to an adenine. That is why Chargaff noted a one-to-one guanine-to-cytosine ratio and a one-to-one adenine to thymine ratio. In the DNA he studied genome diversity. Chargaff also discovered that how much of a genome is made up of each of these complementary pairs varies from species to species. One species may have more cytosines and guanines than another species. This discovery led to a better understanding of genetic diversity among different species. For example the human genome is about 40 percent guanine and cytosine and about 60 percent adenine and thymine while the yeast genome is about 38 percent guanine-cytosine and about 62 percent adenine-thymine so you can see that the amounts of each base vary from species to species. outro so there you have it. Chargaff's first rule states that in DNA the number of guanines will match the number of cytosines and the number of adenines will match the number of thymines, and his second rule states that the composition of DNA varies from species to species."
(transcript of 1st video below). Those one-to-one nature of base pairs tends to not show up in modern genetic sequence totals contained in public databases.
Today's appendices clearly show us that fact (01_homo sapien, 01_DNA, 02_DNA, 03_DNA, 04_DNA, 01_RNA, 02_RNA, 03_RNA, 04_RNA).
Closing Comments
There are some valid reasons why Chargaff's Rule does not apply to some genetic sequences.
The main one is that the Chargaff Rule applies to double stranded dsDNA, not single stranded ssDNA.
That does not rule out applying Chargaff's Rule to dsDNA viruses (Wikipedia; Difference between ssDNA and dsDNA).
So, as we peruse genetic sequences in public databases sampled in today's appendices, we can ponder why the A-T, A-U, and C-G counts don't match what the Chargaff Rule supposes (none of the relevant table values "obeyed" the Chargaff Rule).
According to NASA "citizen scientists help make important scientific discoveries" (Study Shows Citizen Scientists Are Crucial for Observing ...).
So, let's get to the bottom of this.
