The Human Genome Project has revealed that there are probably about 20,000-25,000 'haploid' protein-coding genes. The completed human sequence can now identify their locations. But only about 1.5% of the genome codes for proteins, while the rest consists of non-coding RNA genes, regulatory sequences, introns, and noncoding DNA (once known as "junk DNA").
The human DNA is composed of 23 pairs (total of 46) of molecules. Each molecule is a long string of nucleotides held together by two backbones that spiral around each other. Each such molecule is called a chromosome. When looked at physically as a jell-type mass inside the cell nucleus, the DNA molecules are collectively called the chromatin. When looked at from a functional point of view, the DNA molecules are collectively called the genome.
The histone protein complexes bind DNA molecules into coils that shrink the volume the DNA occupies and enable it to fit into the cell nucleus. The histones, along with other helper proteins, coil the DNA in a way that is organized and prevents the long DNA strings from knotting and breaking. During cell replication, the entire DNA for each chromosome is tightly coiled into large clumps that make the chromosomes very visible. When not in the process of replicating, the chromatin loosens, exposing the individual DNA lengths and genes ready to accommodate their intended functions. Enzymes and other proteins control the coiling and exposing of the DNA string.