A laboratory analysed the bases of four samples of nucleic acid…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A laboratory analysed the bases of four samples of nucleic acid. Each sample is given in per cent of its bases.
| Sample | A | T | U | G | C |
|---|---|---|---|---|---|
| 1 | 24 | 24 | 0 | 26 | 26 |
| 2 | 31 | 19 | 0 | 28 | 22 |
| 3 | 22 | 0 | 29 | 31 | 18 |
| 4 | 25 | 0 | 25 | 25 | 25 |
- Which sample is most likely a double-stranded DNA? Give the rule that you used.
- Which sample is most likely a single-stranded DNA?
- Which sample is an RNA that is almost certainly single stranded?
- Sample 4 is also an RNA. What can you say about its strands?
Need a reminder? The key ideaShow
The worked solution
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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.
Step 1. (a) Sample 1 has thymine, so it is DNA. A equals T ( and ) and G equals C ( and ), as Chargaff's rules ask for a double helix. It is double-stranded DNA.
Step 2. (b) Sample 2 has thymine, so it is DNA. But A is and T is , and G is and C is . The rules fail, so the DNA is single stranded.
Step 3. (c) Sample 3 has uracil, so it is RNA. A is and U is , and G is and C is . These are not equal, so the RNA is single stranded.
Step 4. (d) Sample 4 has uracil, so it is RNA, and A equals U and G equals C. This is what a double-stranded RNA would show, as in some viruses. But a single strand can equal these amounts by chance, so the data suggest double strands and do not prove them.
Answer: (a) sample 1; (b) sample 2; (c) sample 3; (d) it may be double stranded.
Check. Each row adds up to . The rules A T and G C are a test for a double strand, and a failure means a single strand.
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