From Wikipedia,
the free encyclopedia
Bookmarking is a
biological phenomenon believed to
function as an epigenetic
mechanism for transmitting
cellular memory of the pattern of
gene expression in a
cell through
mitosis to its daughter cells.
This is vital for maintaining the
phenotype in a lineage of
cells so that, for example, liver
cells divide into liver cells and
not some other cell type. It is
characterized by non-compaction of
some
gene promoters during
mitosis. In terms of
mechanism, it is believed that a)
sometime prior to the onset of
mitosis the
promoters of genes that exist
in a transcription-competent state
become "marked" in some way, b)
that this "mark" persists through
mitosis, and c) that the
marking transmits gene expression
memory by preventing the mitotic
compaction of DNA at this locus,
or by facilitating reassembly of
transcription complexes on the
promoter, or both. In some cases
the bookmarking is mediated by
binding of specific factors to the
promoter prior to onset of
mitosis, but in other cases could
be mediated by patterns of histone
modification or presence of
histone variants that are
characteristic of active genes,
and which are believed to persist
through
mitosis. In the case of
specific genes such as the
stress-inducible hsp70 gene,
bookmarking may also function as a
mechanism for insuring that the
gene can be transcribed early in
G1 phase if a stress were to occur
at that time. If this gene
promoter were compacted it would
take time to de-compact in G1
during which time the cell would
be unable to transcribe this
cytoprotective gene, leaving it
vulnerable to stress-induced cell
death. In this case, bookmarking
appears to be important for cell
survival.
References
Sarge, K.D. and Park-Sarge,
O.K. (2005) Gene bookmarking:
keeping the pages open. Trends
Biochem. Sci. 30, 605-610.
John, S. and Workman, J.L.
(1998) Bookmarking genes for
activation in condensed mitotic
chromosomes. Bioessays 20,
275-279.
Xing et al. (2005) Mechanism of
hsp70i gene bookmarking. Science
307, 421-423.
Michelotti, E.F. et al. (1997)
Marking of active genes on mitotic
chromosomes. Nature 388, 895-899.
Christova, R. and Oelgeschlager,
T. (2002) Association of human
TFIID-promoter complexes with
silenced mitotic chromatin in
vivo. Nat. Cell. Biol. 4, 79-82.
Kouskouti, A. and Talianidis,
I. (2005) Histone modifications
defining active genes persist
after transcriptional and mitotic
inactivation. EMBO J. 24, 347-357.
Chow, C.M. et al. (2005)
Variant histone H3.3 marks
promoters of transcriptionally
active genes during mammalian cell
division. EMBO Rep. 6, 354-360.
