In genetics Genetics , a discipline of biology, is the science of heredity and variation in living organisms. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding. However, the modern science of genetics, which seeks to understand the process of and epigenetics In biology, the term epigenetics refers to changes in phenotype or gene expression caused by mechanisms other than changes in the underlying DNA sequence, hence the name epi- (Greek: over; above) -genetics. These changes may remain through cell divisions for the remainder of the cell's life and may also last for multiple generations. However,, bookmarking is a biological phenomenon believed to function as an epigenetic mechanism for transmitting cellular memory Cellular memory is the hypothesis that such things as memories, habits, interests, and tastes may somehow be stored in all the cells of human bodies, not only in the brain of the pattern of 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 rRNA genes or tRNA genes, the product is a functional RNA in a cell The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. Some organisms, such as most bacteria, are unicellular . Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion or 1014, throughout mitosis Mitosis is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing roughly equal shares of these cellular components, to its daughter cells. This is vital for maintaining the phenotype A phenotype is any observable characteristic or trait of an organism: such as its morphology, development, biochemical or physiological properties, or behavior. Phenotypes result from the expression of an organism's genes as well as the influence of environmental factors and possible interactions between the two. The genotype of an organism is the 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 In genetics, a promoter is a region of DNA that facilitates the transcription of a particular gene. Promoters are typically located near the genes they regulate, on the same strand and upstream during mitosis Mitosis is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing roughly equal shares of these cellular components. In terms of mechanism, it is believed that:
- at some point prior to the onset of mitosis Mitosis is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing roughly equal shares of these cellular components, the promoters In genetics, a promoter is a region of DNA that facilitates the transcription of a particular gene. Promoters are typically located near the genes they regulate, on the same strand and upstream of genes that exist in a transcription Transcription is the synthesis of RNA under the direction of DNA. RNA synthesis, or transcription, is the process of transcribing DNA nucleotide sequence information into RNA sequence information. Both nucleic acid sequences use complementary language, and the information is simply transcribed, or copied, from one molecule to the other. DNA-competent state become "marked" in some way,
- that this "mark" persists both during and after mitosis Mitosis is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing roughly equal shares of these cellular components,
- and 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 In genetics, a promoter is a region of DNA that facilitates the transcription of a particular gene. Promoters are typically located near the genes they regulate, on the same strand and upstream, 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 In biology, histones are the chief protein components of chromatin. They act as spools around which DNA winds, and they play a role in gene regulation. Without histones, the unwound DNA in chromosomes would be very long. For example, each human cell has about 1.8 meters of DNA, but wound on the histones it has about 90 millimeters of chromatin, modification or presence of histone variants that are characteristic of active genes, and which are believed to persist throughout mitosis Mitosis is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing roughly equal shares of these cellular components.
In the case of specific genes, for example, the stress-inducible 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. The Hsp70s are an important part of the cell's machinery for protein folding, and help to protect cells from stress gene, bookmarking may also function as a mechanism for ensuring that the gene can be transcribed early in G1 phase The G1 phase is a period in the cell cycle during interphase, after cytokinesis and 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 amount of protein synthesis 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.
Categories: Genetics | Epigenetics |
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