of promoters (regulatory regions), exons (which code for amino acids), and introns (which are spliced out of the mRNA before protein translation begins). Genes sometimes overlap, so the same piece of DNA can contain more than one gene. Genome: The total DNA of a given cell or species. The human genome was sequenced by the Human Genome Project, completed in 2003. Heritability: The extent to which variation in a trait is determined by genes, usually expressed as a percentage. Most conditions are controlled by more than one gene and are affected by environmental factors, thus have a heritability score below 100%. Histone: A class of protein that binds closely to DNA, helping to pack it into the nucleosome structures that form the basic unit of chromatin. Histone variants: Non-standard versions of histone proteins, which have specialized functions and bind to DNA only under certain conditions, such as when the DNA is damaged and needs to be repaired. Imprinting: The epigenetic phenomenon by which some genes are transcribed from only the maternal, or only the paternal, chromosome, in some or all cells. Induced pluripotent stem cell (iPS cell): A stem cell that has been artificially created from a fully differentiated mature cell in a laboratory. Long non-coding RNA (lncRNA): An RNA strand at least 200 bases long that doesn’t code for a protein. Among various functions, lncRNAs act as guides to connect proteins to each other and to specific locations in the genome; determine which epigenetic modifications should be added to which pieces of DNA; and mop up miRNAs so that they can’t bind to mRNAs. Meiosis: The specialized type of cell division that creates egg and sperm cells. Meiosis consists of DNA replication followed by two rounds of cell division, creating four cells that each contain half the amount of genetic material as the original cell. Genetic recombination between paired chromosomes occurs before the first cell division, shuffling the genes to give each egg or sperm cell a unique genome. Messenger RNA (mRNA): An RNA strand that codes for a protein. A molecular cap and a string of A bases help the translation machinery convert the sequence of mRNA codons into the corresponding amino acid sequence. Methyl group: A small molecule with the chemical formula CH3 and a neutral charge, which can be attached to DNA, histones and other proteins (a process known as methylation), changing the molecule’s shape and thus affecting its functions. DNA methylation silences gene transcription. MicroRNA (miRNA): A 19–24 base RNA strand that can bind to complementary sequences within mRNAs and that prevents protein translation by either physically blocking the translation machinery from accessing the mRNA or by triggering the destruction of the mRNA strand. Mitosis: Standard type of cell division, consisting of DNA replication followed by a single cell division, creating two cells that each contain the same amount of genetic material as the original cell. No genetic recombination occurs. Molecule: A discrete chemical structure formed by a combination of atoms of the same or different types. Nucleosome: The basic unit of chromatin, consisting of DNA coiled around 8 histone proteins. Adjacent nucleosomes are linked by a DNA sequence bound by a single protein. Nucleus: A compartment inside most cells, surrounded by a membrane and containing the cell’s chromosomes plus certain proteins and RNAs. Different regions within the nucleus specialize in different functions, such as gene transcription.