What is different about the structure of active genes versus inactive genes?

The basic difference between active genes and inactive genes is that active genes are expressed whereas inactive genes are not expressed. This is due to the DNA methylation of the sequences. For DNA replication or copying it must be accessible to the enzymes and the enzymes can access euchromatin and cannot access heterochromatin. The euchromatin is loosely packed form of DNA while heterochromatin is tightly packed form of the DNA. Therefore, the heterochromatin contains the active form of inactive form of genes or the genes are said to be silenced. The DNA bound around the histone proteins are heterochromatins are inaccessible to the enzymes as the histone proteins are in methylated form. The methylation of the histone proteins keeps the genes inactive.