Epigenetics

The DNA in cells carries the blueprints, or genes, that code for the proteins necessary for cellular structure and function via the processes of transcription and translation. The double-stranded DNA is divided into several large, discrete components within the cell called chromosomes, and distinct chromosomes carry specific genes. An entire set of chromosomes is called a genome. Epigenetics refers to the study of changes in the genome of an organism that do not alter the actual sequence of nucleotides in that organism's DNA, but do modify the DNA structure in other ways, by adding other chemical groups or by binding regulatory proteins to the DNA backbone. An epigenetic change affects the phenotype, the appearance, physiological functioning, or physical construction of that organism, rather than the genotype, the actual DNA sequence of the gene, the genes found on each chromosome, and the number of chromosomes. Thus, epigenetic changes are distinct from genetic mutations, which alter the DNA sequence of nucleotides.

Despite not affecting the gene itself, epigenetic changes are heritable, meaning that they can be passed on from one cellular generation to the next as cells divide and replicate. Furthermore, although it was previously believed that epigenetic changes could not be passed from one organism to another via the sexual reproduction of multicellular organisms, new evidence indicates that epigenetic changes to gametes, or sexual reproduction cells, can also remain stable from one generation to the next. Epigenetic changes are less permanent than alterations to the genetic code, however, and can often be reversed, thus allowing cells more flexibility in adapting to environmental conditions than genetic mutations alone would allow.

There are many cellular processes that lead to epigenetic changes. Some of these involve differential gene expression at the cellular level. One of these is differential ribonucleic acid (RNA) splicing, whereby the nascent messenger RNA (mRNA) transcript coded from the same gene in two different types of cells is patched together differentially to lead to two different types of protein products. Another involves the preferential transcription of certain genes in certain cells via the use of transcription factors, molecular ‘tags’ that encourage RNA polymerase to copy some genes more often than others. ‘Maternal effect’ epigenetic changes can occur during fetal gestation, as the embryo inherits RNA and transcription factors from the maternal oocyte during the process of fertilization. Unlike genetic mutations, however, epigenetic changes do not generally affect the sequencing of the amino acids that make up the normal protein found in a specific cell.

Gene expression is also controlled at the chromatin level, and other epigenetic processes alter the physical structure (although not the nucleotide sequence) of DNA such that some genes are more readily transcribed and translated than others. Histone modification, the process of altering the proteins that bind and compact strands of DNA chromatin into chromosomes, can alter gene expression. Methylation of histone proteins will inactivate a gene, while acetylation of histones will make a gene more readily available to the cellular machinery that transcribes DNA. In human females, one copy of the X chromosome is often randomly inactivated via methylation.

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