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Development before birth, including gametogenesis, embryogenesis, and fetal development, is the process of body development from the gametes are formed to eventually combine into a zygote to when the fully developed organism exits the uterus. Epigenetic processes are vital to fetal development due to the need to differentiate from a single cell to a variety of cell types that are arranged in such a way to produce cohesive tissues, organs, and systems. Epigenetic modifications such as methylation of CpGs (a dinucleotide composed of a 2'-deoxycytosine and a 2' deoxyguanosine) and histone tail modifications allow activation or repression of certain genes within a cell, in order to create cell memory either in favor of using a gene or not using a gene. These modifications can either originate from the parental DNA, or can be added to the gene by various proteins and can contribute to differentiation. Processes that alter the epigenetic profile of a gene include production of activating or repressing protein complexes, usage of non-coding RNAs to guide proteins capable of modification, and the proliferation of a signal by having protein complexes attract either another protein complex or more DNA in order to modify other locations in the gene. ==Epigenetic Definitions== Gene expression refers to the transcription of a gene but the RNA produced does not necessarily have to encode a protein product. Transcription may produce so called noncoding RNA products such as tRNA and regulatory RNA. Repression may refer to the decrease in transcription of a gene or inhibition of a protein. Proteins are often inhibited by binding the active site or causing a conformational change so that the active site can no longer bind. By making these alterations, proteins, like transcription factors, may bind DNA less or some protein may be inhibited so that it becomes a block in a signaling cascade and certain genes will then not be induced to be expressed. Repression can occur pre- or post-transcriptionally. Methylating the DNA or the modifying the histones that the DNA wraps around is one example that commonly leads to repression. Pre-transcriptional repression can also occur by altering the proteins that allow transcription to occur, namely the polymerase complex. Proteins can sit on the DNA strand and serve as a kind of block to polymerase proteins, halting them from transcribing. Post-transcriptional repression generally refers to the degradation of the RNA product or binding the RNA with proteins so that it cannot be translated or carry out its function. DNA methylation in humans and most other mammals refers to the methylation of a CpG. Methylation of these cytosines are common in DNA, and in sufficient numbers can prevent proteins from attaching to the DNA by obscuring the domain binding site's matching DNA to the protein. Regions in which cytosines prior to guanines are clustered and highly unmethylated are called CpG islands, and often serve as promoters, or transcription start sites. Histone modifications are modifications made to the amino acid residues in the tails of the histones that either restrict the histone's ability to bind to DNA or boost the histone's ability to bind to DNA. Histone modifications also act as sites for proteins to attach, which then further alter the gene's expression. Two common histone modifications are acetylation and methylation. Acetylation is when a protein adds an acetyl group to a lysine in a histone tail in order to restrict the ability of the histone to bind to DNA. This acetylation is commonly found on lysine 9 of histone 3, notated as H3K9ac. This results in the DNA being more open to transcription, due to the decreased binding to the histone. Methylation, meanwhile, is when a protein adds a methyl group to a lysine in a histone tail, although more than one methyl group can be added at a time. Two sites for histone methylation are common in current studies: trimethylation of lysine 4 on histone 3 (H3K4me3), which causes activation, and trimethylation of lysine 27 on histone 3, which causes repression (H3K27me3). Cis acting elements refer to mechanisms that act on the same chromosome they come from, usually either in the same region from which they were produced or a region very close to this origin region. For example, a long non-coding RNA that is produced at one location silences a the same or a different location on the same chromosome. Trans acting elements, however, are gene products from one location that act on a different chromosome, either the other in a chromosomal pair, or on a different chromosome from a separate chromosome pair. An example of this is a long non coding RNA from Hox gene C silences Hox gene D on a different chromosome, from a different chromosomal pair. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Epigenetics of human development」の詳細全文を読む スポンサード リンク
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