Which of the following statement is wrong about mitochondrial DNA and nuclear DNA ?

Question

Accepted Answer

The correct option is 3: They differ in type of bases.

This is the statement that is **wrong** about mitochondrial DNA (mtDNA) and nuclear DNA (nDNA), and thus it is the correct answer to the question. Both nDNA, which holds the vast majority of the cell's genetic information in the nucleus, and mtDNA, which is located in the energy-producing mitochondria, are fundamentally composed of the same molecular structure. Their genetic code is universally written using the four identical nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). The unique chemical pairing rule—A always binds with T, and G always binds with C—applies to the double-helix structure of all true DNA, regardless of its location in the cell. If any other base, such as Uracil (U), were present, the molecule would either be RNA or highly damaged DNA, but their basic *type* of building blocks is identical. Therefore, the assertion that the two forms of DNA differ in the fundamental type of bases is incorrect.

The incorrect option is 1: They differ in G-C content.
This is a **correct** statement, meaning it is an actual difference between the two DNA types and therefore an incorrect choice for the answer. The G-C content refers to the percentage of Guanine and Cytosine bases within the total DNA molecule. Mitochondrial DNA is known to be particularly rich in Adenine-Thymine (A-T) pairs, and consequently, it has a lower overall G-C content compared to the nuclear DNA of the same organism. This variation in the proportional composition of the bases is a characteristic molecular difference between the two genomes, reflecting their independent evolutionary origins and replication mechanisms.

The incorrect option is 2: They differ in their buoyant density value.
This is also a **correct** statement, an actual difference between nDNA and mtDNA. The buoyant density of a DNA molecule is a physical property that correlates directly with its G-C content. A Guanine-Cytosine base pair is slightly heavier or denser than an Adenine-Thymine base pair because of the atomic weight difference between the bases. Since we know that mitochondrial DNA and nuclear DNA have different G-C contents (as explained for Option 1), they will inherently possess different buoyant densities. This principle is utilized in laboratory techniques like Cesium Chloride density gradient centrifugation, which allows scientists to physically separate the heavier nuclear DNA from the lighter mitochondrial DNA.

The incorrect option is 4: They differ in melting temperature.
This is a **correct** statement, meaning nDNA and mtDNA have different melting temperatures, and thus this option is incorrect as the final answer. The melting temperature ($\text{T}\_{\text{m}}$) is the temperature required to break the hydrogen bonds holding the two complementary DNA strands together. G-C pairs form three hydrogen bonds, while A-T pairs only form two. As a result, DNA with a higher percentage of G-C pairs (like nuclear DNA) is more stable and requires a higher temperature to "melt" or denature than DNA with a lower G-C content (like mitochondrial DNA). Therefore, because the G-C ratios are different, their melting temperatures are also different.