The structure in the nucleus composed of large amounts of DNA is called chromatin.

Here's a breakdown:

* DNA (deoxyribonucleic acid) is the genetic material that contains the instructions for building and maintaining an organism.

* Chromatin is a complex of DNA and proteins (primarily histones) that packages and organizes DNA within the nucleus.

* Chromosomes are condensed forms of chromatin that are visible during cell division.

So, in essence, chromatin is the "organized" form of DNA in the nucleus, allowing it to fit within the nucleus and to be accessed and used during processes like gene expression and replication.