NEET 2023 Q33: Nucleosome Charge Statements — Full solution + Why they asked this

PYQ + trends

1. TITLE

NEET 2023 Q33: Nucleosome Charge Statements — Full solution + Why they asked this

2. THE ACTUAL NEET QUESTION

NEET 2023 — Question
Given below are two statements:
Statement I: In prokaryotes, the positively charged DNA is held with some negatively charged proteins in a region called nucleoid.
Statement II: In eukaryotes, the negatively charged DNA is wrapped around the positively charged histone octamer to form nucleosome.
In the light of the above statements, choose the correct answer from the options given below:
A) Both Statement I and Statement II are false.
B) Statement I is correct but Statement II is false.
C) Statement I is incorrect but Statement II is true.
D) Both Statement I and Statement II are true.

3. OFFICIAL ANSWER

NEET Answer Key: (C)

4. COMPLETE SOLUTION

Concept Explanation:
* Prokaryotic DNA Packaging: DNA is universally a negatively charged molecule due to the presence of phosphate groups in its backbone. In prokaryotes (like E. coli), this negatively charged DNA is held together with some positively charged proteins in a specific region termed the nucleoid. Therefore, Statement I has the charges completely reversed, making it incorrect.
* Eukaryotic DNA Packaging: In eukaryotes, the packaging is more complex. The negatively charged DNA is wrapped around a core of positively charged basic proteins called a histone octamer. This DNA-protein complex forms a structure known as a nucleosome. Because this accurately describes the charge distribution, Statement II is true.

NCERT Reference:
This concept is directly lifted from the "Packaging of DNA Helix" section. Histones acquire their positive charge due to an abundance of basic amino acid residues, specifically lysine and arginine.

Why other options are wrong:
Options A, B, and D are incorrect because they fail to identify that Statement I hallucinates the electrical charges of DNA and its associated proteins, while Statement II is factually perfectly aligned with NCERT.

5. WHY NEET ASKED THIS

Explain the examiner's intent:
* What concept were they testing? The examiner was testing the fundamental biochemical properties of DNA (acidic/negative) and packaging proteins (basic/positive) across both prokaryotic and eukaryotic organisms.
* What's the common mistake they expected students to make? Students often read questions in a hurry and might just see "prokaryotes", "DNA", "proteins", and "nucleoid" in Statement I, assuming it is a correct definition without paying close attention to the specific words "positively charged" and "negatively charged".
* Which option was the "trap"? Option D (Both statements are true) is the ultimate trap for students who skim-read Statement I and fail to notice the swapped charges.

6. TREND ANALYSIS

Prediction for NEET 2025:
Expect an Assertion-Reason question linking the negative charge of DNA (due to phosphate groups) to the specific positive charge of the histone octamer (due to lysine and arginine residues), or a numerical twist asking about the exact length/base pairs wrapped in a single nucleosome (200 bp).

7. SIMILAR QUESTIONS TO PRACTICE

"If you understood this, you can solve:"
* NEET 2022 Q45: Read the following statements (focusing on histone octamer wrapped by negatively charged DNA and rich in lysine/arginine).
* NEET 2021 Q59: Which one of the following statements about Histones is wrong? (Testing the acidic vs. basic nature of histones).
* [Topic] with [variation]: Calculate the number of nucleosomes in a mammalian diploid cell given the total base pairs (using the 200 bp per nucleosome rule).

8. QUICK REVISION FOR THIS TYPE

For DNA Packaging questions, always:

• First check: The assigned charge to DNA. It must always be negative/acidic, regardless of whether it is in a prokaryote or eukaryote.
• Then apply: The matching packaging proteins must be positive/basic to neutralize the DNA. In eukaryotes, these are Histones (rich in Lysine and Arginine).
• Watch out for: Absolute statements swapping the charges, or questions trying to label histones as "acidic".

9. NCERT BASE

This question tests: NCERT Class 12, Chapter 5 (Molecular Basis of Inheritance), "Packaging of DNA Helix".

10. CTA

All PYQs from this chapter → Genetics and Evolution Guide
Practice similar questions → Mock exams