EasyBiologyHigh School (11–12)Heredity and Evolution › Molecular Basis of InheritanceNumericalAPA-LevelIB
One strand of a DNA reads 5′-CCGATCCGTA-3′ (from the 5′ end to the 3′ end)…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
One strand of a DNA reads 5′-CCGATCCGTA-3′ (from the 5′ end to the 3′ end).
- Write the partner strand from its 5′ end to its 3′ end.
- What is the per cent of G and C in this strand?
- How many hydrogen bonds hold the two strands of this piece together?
Need a reminder? The key ideaShow
In a double-stranded DNA, A equals T and G equals C, so the purines equal the pyrimidines. A pairs with T by 2 hydrogen bonds. G pairs with C by 3 hydrogen bonds. The two strands run in opposite directions. A single strand, or an RNA with uracil, does not follow the rule.
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. The strand 5′-CCGATCCGTA-3′ has 10 bases.
Plan. Pair each base, then reverse the order to read the partner from its 5′ end. Count the pairs of each kind.
Step 1. (a) Pair each base: C goes with G, C with G, G with C, A with T, T with A, C with G, C with G, G with C, T with A, A with T. Written along the strand, from its 5′ end, the partner is 3′-GGCTAGGCAT-5′. Turned round, to read from its 5′ end, it is 5′-TACGGATCGG-3′.
Step 2. (b) The strand has 4 C and 2 G, which is 6 bases of 10. That is .
Step 3. (c) There are 6 G-C pairs and 4 A-T pairs. Bonds: .
Answer: 5′-TACGGATCGG-3′; ; bonds.
Check. The G-C content of the partner strand is also , because every G pairs with a C.
Watch out. If you do not turn the partner round, you write it 3′ to 5′ and the answer is marked wrong in a test that asks for 5′ to 3′.
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