Similar to the cytoplasm of a cell, the nucleus contains nucleoplasm or nuclear sap. The nucleoplasm is one of the types of protoplasm, and it is enveloped by the nuclear membrane or nuclear envelope. The nucleoplasm is a highly viscous liquid that surrounds the chromosomes and nucleoli. Many substances such as nucleotides (necessary for purposes such as the replication of DNA) and enzymes (which direct activities that take place in the nucleus) are dissolved in the nucleoplasm. A network of fibers known as the nuclear matrix can also be found in the nucleoplasm. The soluble, liquid portion of the nucleoplasm is called the nuclear hyaloplasm.