The term that defines a change in the DNA structure of a cell which affects the instructions for making particular proteins is mutation.

Here's why:

* DNA Structure: DNA contains the genetic code that instructs cells how to build proteins.

* Protein Synthesis: Proteins are essential for various cellular functions.

* Mutation: A mutation is a permanent alteration in the DNA sequence. This change can alter the instructions for protein synthesis, leading to a change in the protein's structure, function, or even complete absence.

Mutations can have a wide range of effects, from being silent (no noticeable change) to causing serious diseases. They are a primary source of genetic variation and can drive evolution.