Genetics, Histone Code

Excerpt

Epigenetics is the study of making chemical modifications to DNA. Our DNA has a determined nucleotide sequence that cannot be changed. However, this genetic code can undergo chemical modification through epigenetic mechanisms. DNA strands wrap around proteins called histones, which are composed into structures called nucleosomes. There are four types of histones, named: H2A, H2B, H3, and H4. Octomers of two of each type of histone form nucleosomes. These nucleosomes are wrapped together in a spiral structure called a solenoid. Additional H1 proteins are associated with each nucleosome as links to maintain the overall chromatin structure. There are two states of chromatin: euchromatin, which is open and amenable to transcription, and heterochromatin, which is a compact DNA-protein structure that cannot be transcribed. Chemical modifications to these histones result in the conversion of DNA from its euchromatin state to its heterochromatin state and vice versa. The “histone code” is a hypothesis which states that DNA transcription is largely regulated by post-translational modifications to these histone proteins. Through these mechanisms, a person’s phenotype can change without changing their underlying genetic makeup, controlling gene expression.