Bookmarking
Encyclopedia
In genetics
and epigenetics
, 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
, throughout 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:
In some cases, 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 throughout mitosis
.
In the case of specific genes, for example, the stress-inducible hsp70
gene, bookmarking may also function as a mechanism for ensuring 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.
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
and epigenetics
Epigenetics
In biology, and specifically genetics, epigenetics is the study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA sequence – hence the name epi- -genetics...
, bookmarking is a biological phenomenon believed to function as an epigenetic mechanism for transmitting cellular memory
Cellular memory
Cellular memory can refer to:*A variation of body memory, the pseudoscientific hypothesis that memories can be stored in individual cells*A memory card used in cellphones...
of the pattern of gene expression
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
in a cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
, throughout mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
, to its daughter cells. This is vital for maintaining the phenotype
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
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
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
. In terms of mechanism, it is believed that:
- at some point prior to the onset of mitosisMitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
, the promoters of genes that exist in a transcriptionTranscription (genetics)Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
-competent state become "marked" in some way, - this "mark" persists both during and after mitosisMitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
, and - 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, 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
Histone
In biology, histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes. They are the chief protein components of chromatin, acting as spools around which DNA winds, and play a role in gene regulation...
modification or presence of histone variants that are characteristic of active genes, and which are believed to persist throughout mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
.
In the case of specific genes, for example, the stress-inducible hsp70
Hsp70
The 70 kilodalton heat shock proteins are a family of ubiquitously expressed heat shock proteins. Proteins with similar structure exist in virtually all living organisms...
gene, bookmarking may also function as a mechanism for ensuring that the gene can be transcribed early in G1 phase
G1 phase
The G1 phase is a period in the cell cycle during interphase, before the S phase. For many cells, this phase is the major period of cell growth during its lifespan. During this stage new organelles are being synthesized, so the cell requires both structural proteins and enzymes, resulting in great...
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.