Question

Reflect and Apply One of the original structures proposed for DNA had all the phosphate groups positioned at the center of a long fiber. Give a reason why this proposal was rejected.

Short answer

The proposal was rejected because the negative charges of the phosphate groups would repel each other, destabilizing the structure.

Step-by-step solution

Understand the structure of DNA

DNA is composed of nucleotides, each containing a phosphate group, a deoxyribose sugar, and a nitrogenous base. These nucleotides form a double helix structure.

Analyze the proposed structure

The proposed structure suggests that all the phosphate groups are located at the center of the DNA fiber.

Consider the properties of phosphate groups

Phosphate groups are negatively charged, which means they would repel each other if they were all placed in the same central location.

Apply Coulomb's law

According to Coulomb's law, like charges repel each other. Placing all negatively charged phosphate groups at the center would create strong electrostatic repulsion, destabilizing the structure.

Conclude why the structure was rejected

The proposal was rejected because having all phosphate groups at the center would lead to electrostatic repulsion, making the molecule unstable and unable to form the stable double helix structure observed in DNA.

Key concepts

nucleotide composition

DNA, or deoxyribonucleic acid, is made up of smaller units called nucleotides. Each nucleotide consists of three main components: a phosphate group, a deoxyribose sugar, and a nitrogenous base.
The nitrogenous bases can be adenine (A), thymine (T), cytosine (C), or guanine (G). In the double helix structure, the nitrogenous bases pair up (A with T, and C with G) due to hydrogen bonding.
The backbone of DNA chains is formed by alternating phosphate groups and deoxyribose sugars, which are linked together by covalent bonds known as phosphodiester bonds.

electrostatic repulsion

Electrostatic repulsion occurs when particles with the same electrical charge repel each other. In the case of DNA, the phosphate groups carry a negative charge. When these groups are in close proximity, they repel each other due to their identical chages.
This phenomenon would make a structure with all phosphate groups at the center highly unstable due to the strong repulsive forces. The arrangement of the phosphate groups along the backbone of the helix helps to minimize this repulsion by spacing them apart.
This spatial arrangement contributes significantly to the overall stability of the DNA double helix.

Coulomb's law

Coulomb's law describes the force between two charged objects. It states that the force is directly proportional to the product of the charges and inversely proportional to the square of the distance between them. Mathematically, it can be expressed as follows:
\( F = k_e \frac{q_1 q_2}{r^2} \)
Here, \( F \) is the force, \( k_e \) is Coulomb's constant, \( q_1 \) and \( q_2 \) are the magnitudes of the charges, and \( r \) is the distance between them.
If all phosphate groups were concentrated in the center of the DNA molecule, the repulsive force (\( F \)) would be immense due to their negative charges, leading to destabilization of the structure. This is why nature arranges these groups along the backbone to keep the molecule stable.

double helix stability

The stability of the DNA double helix is a result of several interplaying factors. First, the complementary base pairing between adenine and thymine, and cytosine and guanine, ensures the helices are held together stably through hydrogen bonds.
Second, the helical shape of DNA allows the placement of phosphate groups along the exterior, reducing repulsive interactions (electrostatic repulsion).
Additionally, stacking interactions between adjacent base pairs (known as pi-pi interactions) contribute to the overall stability and rigidity of the double helix structure.
Proper spacing of phosphate groups and their incorporation into the backbone is a key factor in maintaining the integrity and function of DNA.

phosphate groups

Phosphate groups are a crucial part of the DNA structure, linking the deoxyribose sugars to form the backbone of the molecule. Each phosphate group carries a negative charge, which contributes to the overall negative charge of the DNA molecule.
In the proposed rejected structure where all phosphate groups were at the center, these negative charges would repel each other, according to Coulomb's law, causing extreme instability. This repulsive force would prevent the formation of a stable helical structure.
Instead, in the actual structure, phosphate groups are located along the outer edge of the double helix, spaced apart to minimize repulsion and ensure the molecule's stability and function. This tidy arrangement allows DNA to efficiently store genetic information in a stable form.