When the total pressure is considered, as in chemical analysis and in thermodynamics it is often referred to as psia or absolute pressure of the substance or of the fluid which would then include the atmospheric pressure. It is important to include the subscript g or a to define which pressure is being noted. Psi can also be the difference between two pressures, be they gage or absolute. Psi is also used in materials engineering referring to the unit stress caused when a load is imposed on a structure or on a part.
Adenine is one of the two purine nucleobases (the other being guanine) used in forming nucleotides of the nucleic acids. In DNA, adenine binds to thymine via two hydrogen bonds to assist in stabilizing the nucleic acid structures. In RNA, which is used for protein synthesis, adenine binds to uracil.
Answer: Hydrogen bonding, which holds the two DNA strands together, determines which nucleotides will form complementary base-pairs. Adenine is able to form two hydrogen bonds and so it lines up with thymine, which is also able to form two hydrogen bonds. Cytosine, which is a smaller nucleotide like thymine, does not base-pair with adenine because their hydrogen-bonding atoms would not line up correctly. Likewise, guanine is able to form three hydrogen bonds and so it lines up with cytosine. which is also able to form three hydrogen bonds.