Assertion (A) : For sequencing, the total DNA from a cell is isolated and converted into random fragments.

Reason (R) : DNA is a very long polymer.

The correct option is: "Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A)."

Step-by-step Explanation:

1. Understanding Assertion (A):
In genomic sequencing (such as whole genome sequencing), the complete DNA of an organism is isolated from the cell. Because existing sequencing technologies cannot read an entire chromosome or very long DNA strands in a single continuous run, the isolated genomic DNA must be broken down into smaller, random fragments. These fragments are then sequenced, and the overlapping regions are aligned using computer programs to reconstruct the complete sequence. Thus, Assertion (A) is correct/true.

2. Understanding Reason (R):
DNA is a double-stranded macromolecule consisting of millions to billions of base pairs, making it an extremely long polymer. For instance, the human haploid genome consists of approximately $3\times {10}^9$ base pairs. Attempting to sequence such long strands directly as a single piece is technically impossible with current technologies due to limitations in read lengths and the physical fragility of long DNA molecules, which tend to shear during isolation. Therefore, Reason (R) is correct/true.

3. Analyzing the Link between Assertion and Reason:
Why is the total DNA converted into random fragments? It is converted precisely because DNA is an extremely long polymer that cannot be sequenced as a single piece. The vast length of the DNA polymer directly necessitates fragmentation into manageable sizes for successful sequencing. Thus, Reason (R) is the correct explanation for Assertion (A).