PYQ + trends
1. TITLE
NEET 2023 Q35: Coding Strand Sequence Traps — Full solution + Why they asked this
2. THE ACTUAL NEET QUESTION
NEET 2023 — Question
Which one of the following is the sequence on corresponding coding strand, if the sequence on mRNA formed is as follows 5'AUCGAUCGAUCGAUCGAUCGAUCG AUCG 3'?
A) 3' UAGCUAGCUAGCUAGCUAGCUAGCUAGC 5'
B) 5' ATCGATCGATCGATCGATCGATCGATCG 3'
C) 3' ATCGATCGATCGATCGATCGATCGATCG 5'
D) 5' UAGCUAGCUAGCUAGCUAGCUAGCUAGC 3'
3. OFFICIAL ANSWER
NEET Answer Key: (B)
4. COMPLETE SOLUTION
For Biology:
- Concept explanation: In a transcription unit, the DNA strand with 3' $\rightarrow$ 5' polarity acts as the template strand for RNA synthesis. The other strand, which has 5' $\rightarrow$ 3' polarity, is called the coding strand. Because of the rules of complementarity, the sequence of the coding strand is identical to the transcribed mRNA, with one crucial exception: DNA contains Thymine (T) whereas RNA contains Uracil (U). Therefore, to find the coding strand from an mRNA sequence, you simply maintain the exact same 5' $\rightarrow$ 3' polarity and replace all Uracil (U) bases with Thymine (T).
- NCERT reference: Class 12 Biology, Chapter 5 (Molecular Basis of Inheritance), Section: Transcription Unit.
- Why other options are wrong:
- Options A and D contain Uracil (U). A coding strand is a DNA strand, so it must contain Thymine (T), not Uracil.
- Option C has the correct base replacement (T instead of U) but incorrect polarity (3' $\rightarrow$ 5'). The coding strand must match the mRNA's 5' $\rightarrow$ 3' polarity.
5. WHY NEET ASKED THIS
Explain the examiner's intent:
- What concept were they testing? They were testing the student's understanding of the transcription unit, specifically the structural relationship and polarity differences between the template strand, coding strand, and the resulting mRNA.
- What's the common mistake they expected students to make? Examiners expected students to hastily calculate the complementary sequence instead of the identical sequence, confusing the template strand for the coding strand. Another major mistake is ignoring the polarity (5' to 3').
- Which option was the "trap"? Option C is the biggest trap. A student might correctly change U to T but overlook the fact that Option C starts with 3' instead of 5'.
6. TREND ANALYSIS
PATTERN ALERT: This specific transcription sequence concept appears almost every year, alternating between asking for the mRNA, coding strand, or template strand.
| Year | How they asked | Twist |
| 2024 | Given the template strand (3' to 5'), asked to find the mRNA product. | Required applying base pairing rules (A-U, C-G, T-A) and reversing polarity to 5' → 3'. |
| 2023 | Given the mRNA, asked for the coding strand. | | 2023 | Given the mRNA, asked for the coding strand. | |
| 2019 | Given both template and coding strands, asked for mRNA. | Provided extra information to confuse students. Easiest path was just taking the coding strand and swapping T with U. |
| 2018 | Given the coding strand, asked for the transcribed mRNA. | A direct question testing the simple T to U replacement rule. |
Prediction for NEET 2026:
Expect them to give an mRNA sequence and ask for the sequence of the template strand or the tRNA anticodon, with reversed polarity options (e.g., 5' $\rightarrow$ 3' instead of 3' $\rightarrow$ 5') to trap students who forget that the template strand runs antiparallel to the mRNA.
7. SIMILAR QUESTIONS TO PRACTICE
"If you understood this, you can solve:"
- NEET 2024 Q17: Finding mRNA from template strand
- NEET 2019 Q68: Finding mRNA from a given transcription unit
- NEET 2018 Q73: Finding mRNA from coding strand
- NEET 2022 Q41: Finding tRNA anticodon sequence from mRNA codon
8. QUICK REVISION FOR THIS TYPE
For Transcription Sequence questions, always:
[First check] What is given and what is asked? Identify if you are dealing with the Coding Strand, Template Strand, mRNA, or tRNA anticodon. Look closely at the 5' and 3' ends.
[Then apply] The Golden Rules:
1. Coding Strand $\leftrightarrow$ mRNA: Sequences are exactly the SAME, keeping the same 5' $\rightarrow$ 3' polarity. Just swap Thymine (T) for Uracil (U).
2. Template Strand $\leftrightarrow$ mRNA/Coding Strand: Sequences are COMPLEMENTARY and ANTIPARALLEL. Swap A with U/T, C with G, and reverse the polarity (3' $\rightarrow$ 5' becomes 5' $\rightarrow$ 3').
[Watch out for] Polarity traps! Even if the base sequence is correct, picking a 3' $\rightarrow$ 5' option for a coding strand or mRNA will cost you 5 marks. Also, never pick an option containing 'U' when asked for a DNA strand!
9. NCERT BASE
This question tests: NCERT Class 12, Chapter 5 (Molecular Basis of Inheritance), Transcription Unit (Page references may vary, look for the 'Transcription' section).
10. CTA
All PYQs from this chapter → Genetics and Evolution Guide
Practice similar questions → Mock exams