The correct order of steps is (2) C → A → D → B → E.

Understanding each step

• C. Pulsed field gel electrophoresis

  • Used early to separate very large DNA fragments or entire chromosomes and to generate a physical map framework.

• A. Partial digestion with restriction enzymes

  • Genomic DNA is partially digested (not cut at every site) to give overlapping large fragments.

• D. Cloning in cosmids/YACs/BACs

  • These large fragments are cloned into large‑insert vectors to create an ordered clone library.

• B. Assembly of contigs

  • Overlaps between clones are analyzed to order them into contiguous sets (contigs) that map along the chromosome.

• E. Sub‑cloning and sequencing

  • Each large clone is then sub‑cloned into smaller fragments and sequenced; sequences are assembled using the contig map as a scaffold.

Putting that in order:

$$\text{C}\to \text{A}\to \text{D}\to \text{B}\to \text{E}$$

Why other options are incorrect

1. C–B–D–E–A – Assembles contigs (B) before the large‑insert clones (D) exist; also puts partial digestion (A) last.

2. E–A–B–D–C – Starts with sequencing (E) before any mapping or cloning; order is reversed relative to the logical workflow.

3. A–C–B–D–E – Uses pulsed‑field gels after partial digestion and before cloning, and assembles contigs (B) before a proper large‑clone library (D) is built.

Therefore, the correct sequence of steps in map-based sequencing is C → A → D → B → E (option 2).