A stretch of double-stranded DNA has 2000 base pairs, and 30% of its bases are G and C…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A stretch of double-stranded DNA has base pairs, and of its bases are G and C. An enzyme copies both strands completely, and so makes a new double helix. The building blocks are dATP, dTTP, dGTP and dCTP.
- How many nucleotides are needed in all?
- How many dATP are needed?
- How many dTTP are needed?
- How many dGTP and how many dCTP are needed?
Need a reminder? The key ideaShow
A DNA polymerase adds nucleotides only to a 3′ end, so a new strand grows 5′ to 3′. The leading strand is made in one piece. The lagging strand is made as Okazaki fragments, which DNA ligase joins. An origin starts two forks, and the copying takes .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. base pairs, G + C.
Plan. A new double helix has the same make-up as the old one. Find each base as a share of all the nucleotides.
Step 1. (a) Two new strands, nucleotides.
Step 2. (b) and (c). A + T is of . A equals T, so each is . dATP and dTTP: each.
Step 3. (d) G + C is of . G equals C, so each is . dGTP and dCTP: each.
Answer: ; ; ; ; .
Check. .
Watch out. The building blocks are the triphosphates, and each one loses two phosphates on joining the strand.
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