DNA Encodes Heritable Information
A contrarian embodiment to the notion that science and religion are in conflict, the "Father of Genetics" was none other than Gregor Mendel, an Augustinian friar. He famously conducted experiments using pea plants and, in the process, deduced the basic patterns of inheritance. He referred to these heritable units as "elements"; today, we call them "genes." Amazingly, Mendel didn't even know DNA existed, and Charles Darwin knew about neither DNA nor the discoveries of Mendel.
It wasn't until 1952 that scientists determined that DNA was the molecule responsible for transmitting heritable information. An experiment conducted by Alfred Hershey and Martha Chase, using viruses with radioactively labeled sulfur or phosphorus to infect bacteria, rather convincingly demonstrated that this was the case. Then, in 1953, James Watson and Francis Crick, with substantial input from Rosalind Franklin, shattered the biological world with their double helix model of DNA structure.
From there, it was determined that the "letters" (A, C, G, T) of the DNA sequence encoded information. In groups of three (e.g., ACG, GAA, CCT, etc.), these nucleotides coded for amino acids, the building blocks of protein. Collectively, every possible combination of three letters is known as the "genetic code." (See diagram. Note that every T is replaced with U in RNA.) Eventually, the central dogma of molecular biology emerged: (1) DNA is the master blueprint and is responsible for inheritance; (2) DNA is transcribed into RNA, which acts as a messenger, conveying this vital information; and (3) RNA is translated into proteins, which provide structural and enzymatic functions for the cell.
Today, it is known that DNA sequences alone are insufficient to explain all the behaviors observed at the cellular level. Alterations to the DNA which do not affect the sequence of letters -- known as epigenetic changes -- are under intense investigation. It is currently unclear to what extent epigenetics is responsible for heritable traits.
Source: Galileo's Finger: The Ten Great Ideas of Science by Peter Atkins