Chargaff's Rules - Video Tutorials & Practice Problems
On a tight schedule?
Get a 10 bullets summary of the topic
1
concept
Chargaff's Rules
Video duration:
4m
Play a video:
Video transcript
In this video we're going to introduce Chargaff's rules. And so way back in the early 19 fifties, a scientist named Erwin Chargaff made 2 very important discoveries related to DNA. And we've got these 2 very important discoveries number down below, number 1 and number 2. And so the first very important discovery that Chargaff made was that DNA base composition actually varies amongst different species and so different species of organisms will have different DNA composition and different percentages of, bases, nitrogenous bases, adenines, thymines, cytosines, and guanosines, a's, t's, c's, and g's. Now the second very important discovery that Chargaff made was that for each of these different species that he looked at, he noticed that the percentage of As and T bases or adenine and thymine nitrogenous bases were roughly equal to each other as were the percentages of Cs and Gs or Gs and Cs. And so today, we know that the percentage of a's and t bases are roughly equal because we know that in the structure of DNA, As are complementary base pairing with Ts on opposite strands. And, the percentage of Gs and Cs are gonna be roughly equal again because g's will base pair with c's on opposite strands. However, back in this time in the early 19 fifties, the structure of DNA was unknown. They did not know the structure of DNA. And so this important information here was critical because it helped to reveal the structure of DNA later. And so if we take a look at our example down below, we're showing you a little snippet of some of Chargaff's data. And so what you'll notice is that we've got this table here of the DNA composition in different species. And we've got the species over here on the left hand side and then the percent composition of the DNA in terms of the percentage of adenines, thiamines, guanosine, and cytosine or a's, t's, g's, and c's in each of the species. And so what you'll notice is that, Homo sapiens, humans, they have a very specific percentage of, a composition of DNA, and that varies from, the Escherichia coli, which has a different percentage of, bases, a different composition of bases. And so the very first, discovery that Chargaff made was just that, hey, different species are going to vary in their DNA base composition. And then the second thing that he noticed again is that it doesn't matter which species you look at, the percentage of a's and t's are roughly equal and the percentage of g's and c's are roughly equal. And so again, if we take a look at humans, Homo sapiens or humans, the percentage of a is approximately equal to the percentage of t, so they're roughly in the 31% ballpark. And the percentage of g's is roughly equal to the percentage of c's. They're roughly in the 19 ish, 18 18 ish percent ballpark range. And it, again, it doesn't matter which species you look at. Again, percentage of a's and t's are gonna be roughly equal, and the percentage of g's and c's are gonna be roughly equal. And so, that is really what this second important discovery is referring to. Now the reason that these percentages are not exactly the same is because of limitations in Chargaff's techniques that he was using. And so the reason why the a's is not exactly equal to t's, again, has to do with just some limitations in these techniques. But this was very, very important information to help reveal the structure of DNA. And so this here concludes our introduction to Chargaff's rules, these two discoveries here, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.
2
Problem
Problem
Cytosine (C) makes up 42% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be thymine (T)?
A
8%
B
16%
C
21%
D
60%
3
Problem
Problem
Thymine (T) makes up 28% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be guanine (G)?