What structure in the nucleus of cells contains a chain of DNA?

The correct answer is the chromosome, which is a structure within the nucleus that contains a long chain of DNA. Chromosomes are composed of tightly coiled DNA strands, organized with proteins called histones. This highly organized structure allows for efficient packing and management of the genetic material during cell division and other cellular processes. Each chromosome can carry numerous genes, which are segments of DNA that encode for specific traits or functions within an organism.

The other options, while related to DNA and its organization, do not correctly represent the structure that contains the entire chain of DNA. A chromatid is one half of a duplicated chromosome, appearing during cell division. A gene refers to a specific sequence of DNA that codes for a protein or functional product, not the entire structure containing that sequence. Nucleosomes consist of a segment of DNA wound around histone proteins and are a fundamental part of chromatin, but alone they do not encompass the full DNA chain that makes up a chromosome. This makes the chromosome the most comprehensive definition in this context.