How are homologous chromosomes defined?

Why is the DNA in the nucleus loose and not well defined into chromosomes?

• Answer:

  In the interphase nucleus the chromatin exists in two forms; loosely coiled form called euchromatin and tightly coiled form called heterochromatin. These two types become much more coiled to form the chromosome at the time of cell division. Now come to the point. When DNA is a bit loosely colied, that is, in the euchromatin, its bases are available for transcription. On the other hand, when it is in the form of heterochromatin, the bases are not accessible for reading by RNA polymerase, hence transcription is not possible. As majority of genes must be transcribed in the nucleus, so the chromosomes must be decondensed to appear in the form of chromatin, and that too in the form of euchromatin. If the DNA remained organized in the form of chromosomes, genes would remain inactive in the sense that they would not be acessible for transcription.