Numerical/Problem-solving
1. TITLE
How to solve Chargaff’s Rule base percentage problems in NEET — Step-by-step
2. PROBLEM TYPE INTRO
This numerical belongs to Molecular Genetics. It tests your understanding of the double-helical structure of DNA and base-pairing rules. You will need the formula $A=T$ and $G=C$, along with the concept that total DNA composition equals 100%. A typical mistake is forgetting that purines ($A+G$) always equal pyrimidines ($T+C$).
3. MCQ
Problem:
In a double-stranded DNA segment, if the percentage of Cytosine (C) is observed to be 18%, what will be the percentage of Adenine (A) in that segment?
Options:
A) 18%
B) 36%
C) 32%
D) 64%
4. STEP-BY-STEP SOLUTION
SOLUTION
Given:
- Percentage of Cytosine ($C$) = $18\%$
- Nature of nucleic acid = Double-stranded DNA
To Find:
- Percentage of Adenine ($A$)
Formula:
According to Chargaff’s Rule for double-stranded DNA:
1. $A = T$ and $G = C$,
2. $A + T + G + C = 100\%$
Use this formula whenever you are given the percentage of one nitrogenous base and asked to find another in double-stranded DNA.
Step 1: Calculate the percentage of Guanine (G)
Since Guanine pairs with Cytosine ($G \equiv C$), their percentages are equal.
Step 2: Calculate the total percentage of G + C
Step 3: Calculate the remaining percentage for A + T
Subtract the G + C content from the total percentage ($100\%$).
Step 4: Calculate the percentage of Adenine (A)
Since Adenine pairs with Thymine ($A = T$), they divide the remaining percentage equally.
Answer: 32% (Option C)
5. WHERE STUDENTS GO WRONG
List 2-3 common mistakes:
- ❌ Mistake 1: Assuming $A = C$. Students often randomly equate the unknown base to the known base without checking pairing rules ($A$ pairs with $T$, not $C$).
- ❌ Mistake 2: Stopping at Step 3. Students calculate the remaining percentage ($64\%$) and mark that as the answer, forgetting to divide by 2 for the specific base.
- ❌ Mistake 3: Applying this rule to single-stranded DNA (ssDNA) or RNA, where these fixed ratios ($A=T$) do not apply,.
6. QUICK FORMULA REFERENCE
Formulas Used:
Formula 1 — $A = T$ and $G = C$ — Use for any double-stranded DNA base pairing calculation.
Formula 2 — $(A + T) + (G + C) = 100\%$ — Use to find missing pairs when one pair is known.
Formula 3 — $Purines (A+G) = Pyrimidines (T+C) = 50\%$ — Use for checking validity or quick checks.
7. SIMILAR PROBLEM HINT
"If NEET changes the values, the method stays the same. Practice with: If Cytosine is 20%, calculate Adenine." (Hint: This is a direct exercise from the NCERT textbook).
8. NCERT REFERENCE
Based on: NCERT Class 12, Chapter 5 (Molecular Basis of Inheritance), Section 5.1.1 and Exercise Q.2,.
9. CTA
More solved numericals → Genetics and Evolution Complete Guide
Practice problems → Mock exams