Question
A sample of DNA contains 28% adenine. Using Chargaff’s rule, calculate the percentage of guanine, cytosine, and thymine in the sample.
Solution — Step by Step
Before reaching for a formula or a definition, let’s identify exactly what the question is asking. For exam strategy, the key is to pin down which concept or process is being tested. Underline the given data and the unknown in the question — this five-second habit prevents 90% of careless errors.
This problem sits inside the exam strategy chapter. The concept we need here is the core mechanism of exam strategy, so we’ll apply it directly rather than guessing. Writing down the relevant definition or formula first keeps us anchored and turns the problem into a simple substitution.
Now we map the given values onto the concept. Work systematically — one substitution at a time, units checked, intermediate values written down. Don’t skip steps in the rough work, because examiners give partial credit for correct method even if the final answer slips due to a small arithmetic mistake.
Does the result make biological sense? If we got a percentage above 100%, a negative concentration, or an organism with more chromosomes than humans, we’ve made a sign or unit error somewhere. Sanity-checking takes 10 seconds and has saved thousands of NEET ranks over the years.
Answer: A = T = 28%, G = C = 22%
Why This Works
Chargaff’s rule states that in double-stranded DNA, adenine equals thymine and guanine equals cytosine. The four bases must sum to 100%, so once we know A, we know T, and the remainder splits equally between G and C.
The beauty of exam strategy questions is that once we understand the underlying process, the numbers and terminology fall into place. We’re not memorising random facts — we’re applying a mental model of how the system actually behaves inside the living organism.
For NEET and CBSE, questions on exam strategy often repeat with slight variations in the given data. If we’ve solved one carefully, we’ve essentially solved the whole family of questions that could appear on this sub-topic. That’s the leverage of understanding over rote learning.
Alternative Method
If the direct approach feels heavy, we can also work backwards from the options (for MCQs) or use a quick dimensional check. For exam strategy questions, substituting each option into the original constraint often reveals the correct answer in under a minute — useful when time is running out in the final 20 minutes of the paper.
Another useful shortcut is to sketch a rough diagram of the process. Visualising exam strategy as a flow chart helps us spot which step the question is targeting, especially in assertion-reason and matching type questions.
Common mistake: Students often forget that and try to split the remaining 72% equally as 36% each. You must pair A with T first, then split the rest.
For exam strategy questions in NEET, practise 5 PYQs per sitting rather than 50 mixed questions. Depth beats breadth when the concept is still fresh. Review each solution the next day — spaced repetition is what moves facts from short-term to long-term memory.