Question

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 centralized phosphate model was rejected because the strong repulsive forces between negatively charged phosphate groups would destabilize the structure.

Step-by-step solution

- Review the Structure of DNA

Understand that DNA is composed of nucleotides, each containing a phosphate group, a sugar, and a nitrogenous base. The phosphates and sugars form the backbone of the DNA structure.

- Position of Phosphate Groups

Consider that in the proposed model, all phosphate groups would be positioned at the center of the DNA strand forming a dense cluster.

- Charge of Phosphate Groups

Recognize that phosphate groups are negatively charged. Therefore, having all of them concentrated in one area would result in strong repulsive forces due to their like charges.

- Repulsive Force Impact

Understand that these repulsive forces would make the structure unstable, as the phosphates would repel each other strongly, preventing the DNA from holding its shape.

- DNA Stability Requirement

Conclude that for DNA to maintain its stable double-helix structure, the negatively charged phosphate groups need to be spaced apart, running along the outside of the helix rather than clustered in the center.

Final Reason

Explain that the proposed centralized phosphate structure was rejected because the resulting strong electrostatic repulsion would destabilize the DNA molecule, making it a less plausible model.

Key concepts

Phosphate Groups

DNA, or deoxyribonucleic acid, consists of units called nucleotides. Each nucleotide has three main components: a nitrogenous base, a sugar (deoxyribose), and a phosphate group.
The phosphate group is critical as it forms part of the DNA's backbone.
This backbone is responsible for the structural integrity of the DNA molecule.

• Phosphate groups in DNA are negatively charged.
• They link the nucleotides together through phosphodiester bonds.

These groups are positioned outside the double helix, ensuring that the structure remains stable and the negatively charged phosphates are spaced apart.

Electrostatic Repulsion

Phosphate groups carry negative charges, and like charges repel each other, a principle called electrostatic repulsion.
If all phosphate groups were to be positioned in the center of the DNA strand, as in the rejected model, the repulsive forces would be immense.
This is because:

• Each negative charge would push away from the others.
• This would create strong repulsive forces around the central core.

Such repulsion would prevent the DNA from maintaining a stable configuration, making it highly unstable.
Hence, positioning phosphate groups at the center of the DNA strand would lead to excessive electrostatic repulsion.

Double-Helix Stability

The double-helix structure of DNA is essential for its function and stability.
This unique structure allows for several key properties:

• Efficient storage of genetic information.
• Protection against mutations.
• Consistency during cell replication.

For the double-helix to be stable, the phosphate groups need to be spaced out along the exterior of the helix.
This spacing minimizes repulsion between the negatively charged phosphates, allowing the DNA to maintain its shape and integrity.
If the phosphates were clustered in the center, the resulting instability would interfere with these crucial functions.

DNA Backbone

The backbone of the DNA molecule is made up of alternating sugar and phosphate groups.
This structure provides the support needed for the DNA's double-helix configuration:

• Sugar-phosphate linkage holds the strands together.
• Protects the nitrogenous bases that store genetic code.

Maintaining proper spacing between phosphate groups on the backbone ensures that repulsive forces are minimized.
This balanced arrangement enables the DNA to retain its stability and functionality. Ensuring that the phosphate groups are positioned on the outside of the helix is crucial for the DNA's overall structure and function.
It allows the molecule to be dynamically flexible while maintaining its overall integrity.