DNA is composed of two strands which twist around each other to form the famous double-helix. The “single guanine (G)” always pairs with another guanine; likewise, cytosine (C) always pairs with another cytosine; and adenine (A) always pairs with another adenine. There are also thymines (TGs), but they only pair one way – C/g and A/t. So, if you know what base pairs with what, and the structure of DNA, then you can actually determine how long a segment of DNA is.
Therefore, the largest segment of dna means that this is how much you can expect to find out about a person’s genetic makeup based on their hereditary information: every gene in your body follows this pattern, so it should be possible to read off what genes exist and
The first step to breaking down this problem is figuring out exactly what we’re looking at when we say largest segment. One way it could be interpreted is that we’re talking about the longest continuous stretch of DNA that has all adenines (A) or cytosines (C) or guanines (G). However, there’s also the possibility that we mean “largest” as in “most bases.” But which should we use?
Let’s do some thinking on this one…
First, it’s important to remember that DNA is made up of long chains of four different nucleotides. The two strands are held together by hydrogen bonds, which occur between the nitrogenous base’s in adjacent nucleotides. Every time you see an A in one strand, there has to be a T opposite to it (because A always pairs with T, and C always pairs with G). Let’s also think back to our knowledge of structure; when we’re talking about single helices, they’re either right-handed or left-handed. That means that if this article were intended for humans (which I’m not saying it is), then all helixes would have to go 5′-A/C-(3′)3′.
Now, think about it. If we were going to have a DNA strand that was mostly A’s and C’s, there would have to be a lot of T’s mixed in. The more T’s the strand has, the less useful information would be for genetic coding purposes. This is because as far as protein synthesis goes, it doesn’t really care if you use say an amino acid glycine or another one like alanine – just as long as you’re using something from your 20 amino acids to build off of. So then our question becomes “which does this person most resemble?” Let’s see what forces might control which nucleotide gets used by saying that this individual most resembles either someone with more As than Cs OR someone with more Cs than As.
The first thing we have to figure out is how many bases the individual has (which would then tell us how big of a strand we’re dealing with). Now, because each person’s DNA contains unique information, it’s possible that the number of bases in an individual might not be evenly divisible by 3 -. However, my experience tells me that this is not the case. So then what’s up with
It might be hard to tell at first, but there are actually three sets of consecutive As in the sequence. The most important thing here is finding out how many triplets are contained in the strand overall. Let’s say for simplicity’s sake that there are 24 triplets total, and what we’re trying to figure out is how many As there are. This problem becomes pretty simple if we think back to our knowledge of hydrogen bonding. Remember that you can’t have a stretch longer than three nucleotides go consecutively without violating the rules of hydrogen bonding. So then this would mean that our strand is either composed of triplets of A’s, T’s, C’s or G’s, or it has to have one half that. Now here comes the part where we determine if this person most resembles someone with more As than Cs or
The situation for this scenario gets a little complicated because – as you may know – there are four nucleotides, and not three. With this in mind, we’d have to figure out which one of the four could possibly fit into the triplets we’ve described so far. However, I think it’s pretty safe to say that G doesn’t belong there – for reasons stated earlier. That means our strand must be all A’s and T’s, and possibly one half G’s. So then which would this person resemble?
Let’s think about it: if we had more triplets of A than C, the overall strand would have to be larger in size (because there are two additional nucleotides). However, because our strand is composed of alternating As and Ts, it’s possible that the strand is smaller in size (no need for all those Gs). So then this person most
describe their ancestry as having more T than A, and therefore most resemble someone with a lot of Cs.
This hypothesis makes sense when you think about it: our “ancestor” would be most like someone that we already know.