12.2: Introduction

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Epigenetics is the study of heritable changes in genetic expression and phenotype that do not result from a sequence of DNA. Each cell, despite having an identical copy of the genome, is able to differentiate into a specialized type. There are many biological devices for accomplishing these including DNA methylation, histone modification, and various types of RNA.

DNA methylation is a binary code that is effectively equivalent to turning a gene ”on” or ”off”. However, often times a gene might need to be more highly expressed as opposed to just being turned on. For this, histones have tails that are subject to modification. The unique combination of these two elements on a stretch of DNA can be thought of as a barcode for cell type. Even more important is the method of their preservation during replication. In the case of DNA methylation, one appropriately methylated strand is allocated to each mother or daughter cell. By leaving one trail behind, the cell is able to fill in the gaps and appropriately methylate the other cell.

As the intermediary between DNA sequences and proteins, RNA is arguably the most versatile means of regulation. As such, they will be the focus of this chapter.

Did You Know?

Cell types can be determined by histone modification or DNA methylation (a binary code, which relies on a euchromatic and heterochromatic state). These histone modifications can be thought of as a type of epigenetic barcode that allows cell DNA to be scanned for types. Non-coding RNAs called Large Intergenic Non-Coding RNAs (lincRNAs) are heavily involved in this process.

A quick history of RNA:

1975: A lab testing relative levels of RNA and DNA in bull sperm discovers twice as much RNA as DNA.
1987: After automated sequencing developed, weird non-coding RNAs are first found.
1988: RNA is proved to be important for maintaining chromosome structures, via chromatin architecture
1990s: A large number of experiments start to research
2000s: Study shows Histone-methyltransferases depend on RNA, as RNAase causes the proteins to delocalize.
Transcription is a good proxy of what’s active in the cell and what will turn into protein. Microarrays led to the discovery of twice as many non-coding genes as coding genes initially; now we know the ratio is even far higher than this.