The DNA of chromosome 1 in the human genome has about 249 million base pairs, each with an average mass of about 650 Da, or 156 GDa total. Titin, one of the largest known proteins, has an atomic mass of between 3 and 3.7 megadaltons. The molecular masses of proteins, nucleic acids, and other large polymers are often expressed with the units kilodalton (kDa), equal to 1000 daltons, megadalton (MDa), one million daltons, etc. In general, the standard atomic weight of an element is the average weight of its atom as it occurs in nature, expressed in daltons. For example, an atom of helium has a mass of about 4 Da, and a molecule of acetylsalicylic acid (aspirin), C9H8O4, has a mass of about 180.16 Da. This unit is commonly used in physics and chemistry to express the mass of atomic-scale objects, such as atoms, molecules, and elementary particles. As of June 2019, the value recommended by the Committee on Data for Science and Technology (CODATA) is 1.66053906660(50)×10−27 kg, or approximately 1.66 yoctograms. Despite being an official abbreviation for a related obsolete unit and not widely used in the scientific literature, the abbreviation "amu" now often refers to the modern unit (Da or u) in many preparatory texts. It is defined precisely as 1/12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. The symbol Da is more widely used in most fields. As of 2019, the International System of Units (SI) lists the dalton, symbol Da, as a unit acceptable for use with the SI unit system and secondarily notes that the dalton (Da) and the unified atomic mass unit (u) are alternative names (and symbols) for the same unit. The current IUPAC endorsed definition is the unified atomic mass unit, denoted by the symbol u. Several definitions of this unit have been used, implying slightly different values. A mass of 1 Da is also referred to as the atomic mass constant and denoted by mu. It is approximately the mass of one nucleon (either a proton or neutron). The dalton or unified atomic mass unit (SI symbols: Da or u) is a unit of mass widely used in physics and chemistry.
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