12.  In general, the ground state of a bacterial gene is “on.” Thus, transcription initiation is prevented or reduced if the binding of RNA polymerase is blocked. In contrast, the ground state of eukaryotes is “off.” Thus, the transcriptional machinery (including RNA polymerase II and associated general transcription factors) cannot bind to the promoter in the absence of other regulatory proteins.

16.  Among the mutations that might prevent a strain of yeast from switching mating type would be mutations in the HO and HMRa genes. The HO gene encodes an endonuclease that cuts the DNA to initiate switching and the HMRa locus contains the “cassette” of unexpressed genetic information for the MATa mating type.

19.  The term epigenetic inheritance is used to describe heritable alterations in which the DNA sequence itself is not changed. It can be defined operationally as the inheritance of chromatin states from one cell generation to the next. Genomic imprinting, X-chromosome inactivation, and position-effect variegation are several such examples.

23.  The inheritance of chromatin structure is thought to be responsible for the inheritance of epigenetic information. This inheritance is due to the inheritance of the histone code and may also include the inheritance of DNA methylation patterns.

36.  A gene not expressed owing to alteration of its DNA sequence will never be expressed and will be inherited from generation to generation. An epigenetically inactivated gene may still be regulated. Chromatin structure can change in the course of the cell cycle; for example, when transcription factors modify the histone code.

38.  Chromatin structure greatly affects gene expression. Transgenes inserted into regions of euchromatin would more likely be capable of expression than those inserted into regions of heterochromatin.