Abstract:
Pancreatic deoxyribonuclease preferentially digests active genes
during all phases of the cell cycle including mitosis. Recently, a DNAse
I-directed in ~ nick translation technique has been used to demonstrate
differences in the DNAse I sensitivity of euchromatic and heterochromatic
regions of mitotic chromosomes. This ill ~ technique has been used in
this study to ask whether facultative heterochromatin of the inactive X
chromosome can be distinguished from the active X chromosome in mouse and
human tissues. In addition to this, in ~ nick translation has been used
to distinguish constitutive heterochromatin in mouse and human mitotic
chromosomes. Based on relative levels of DNAse I sensitivity, the inactive
X chromosome could not be distinguished from the active X chromosome in
either mouse or human tissues but regions of constitutive heterochromatin
could be distinguished by their relative DNAse I insensitivity. The use of
!D situ nick translation was also applied to tissue sections of 7.5 day
mouse embryos to ask whether differing levels of DNAse I sensitivity could
be detected between different tissue types. Differences in DNAse I
sensitivities were detected in three tissues examined; embryonic ectoderm,
an embryo-derived tissue, and two extraembryonic tissues, extraembryonic
ectoderm and ectoplacental cone. Embryonic ectoderm and extraembryonic
ectoderm nuclei possessed comparable levels of DNAse I sensitivity while
ectoplacental cone was significantly less DNAse I sensitive. This suggests
that tissue-specific mechanisms such as chromatin structure may be involved
in the regulation of gene activity in certain tissue types. This may also
shed some light on possible tissue specific mechanisms regulating X
chromosome activity in the developing mouse embryo.
