The structure that is primarily composed of DNA and protein is called a chromosome. Chromosomes are thread-like structures found in the nucleus of eukaryotic cells and they play a crucial role in genetics.
To explain further, DNA (deoxyribonucleic acid) carries the genetic information necessary for the growth, development, functioning, and reproduction of all living organisms. Proteins, on the other hand, serve a myriad of functions including catalyzing biochemical reactions (as enzymes), providing structural support, and facilitating communication between cells.
In the context of chromosomes, the DNA is wrapped around proteins called histones, which helps to package the DNA into a compact, dense shape. This organization is essential, especially during cell division, as it ensures that DNA is accurately replicated and distributed to daughter cells. The combination of DNA and proteins in chromosomes is what allows for the efficient regulation of gene expression and the complex processes of inheritance.