DNA replication in early Drosophila embryos occurs about every 5 minutes. The Drosophila genome contains approximately 1.8×10⁸ base pairs. Eukaryotic DNA polymerases synthesize DNA at a rate of approximately 40 nucleotides per second. Approximately how many origins of replication are required for this rate of replication?

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Hi, everyone. Welcome back. Let's look at our next problem. It says The genome of a eukaryotic organism has approximately 3.2 times 10 to the 8th base pairs. And the polymerase synthesize the DNA at a rate of 40 nucleotides per second. Calculate the amount of nucleotides produced by each origin. If DNA replication occurs every four minutes in this organism, Your answer choices are a 9600B 19, C 30, ND 61,440. So let's look at our problem thinking about what information we need and what we have. We may be given some extraneous information. So we want to focus on the information we need to calculate what we're asked for. So the problem says to calculate the amount of nucleotides produced by each origin. Well, the information we have that would relate to that is we have a rate of nucleotide per second at which DNA is being synthesized by DNA polymerase. So we're producing 40 nucleotides per second. We need to know the number of nucleotides produced. So that means we need a time value in here over how much time is this being done? Well, our problem tells us that DNA replication occurs every four minutes. So the time Will equal four minutes. So let's convert that to seconds. So we know we have 60 seconds. So we'll say multiply it by 60 seconds per minute. So that's going to give us 240 seconds as our time. Now, there's one other little wrinkle in here. If we envision an origin of repli replication, we need to remember that replication is bidirectional, that two DNA strands are being synthesized at the same time because you have two DNA strands. So we have our rate nucleotides per second, we have our time. But we need to multiply that by two because we have replication happening on two strands at once. So to calculate the number of nucleotides that will equal 40 nucleotides per second times 240 seconds times two. So the units of seconds cancel out And we're left with 19, nucleotides produced by each origin. When we go over to our answer choices, that's choice B and we notice that the number, the size of the genome in base pairs is not relevant to our question. We don't need that amount. So it's just a good reminder that sometimes when we get a problem like this, we don't need every value that they give us to calculate our answer. So it's always important to think through what information do you need what information do you have? So again, calculate the amount of nucleotides produced by each origin. If DNA replication occurs every four minutes in this organism And that will be B 19,200. See you in the next video.

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Key Concepts

DNA Replication

Eukaryotic DNA Polymerases

Origins of Replication