Deoxyribonucleic acid (DNA) stores the hereditary information that dictates the structure and function of every cell. This information is accessed through a process where instructions are transcribed from DNA into a messenger RNA (mRNA) molecule. The mRNA is then used to construct proteins, the final functional products of life.
Decoding the Genetic Blueprint
The genetic information encoded within DNA is read in a precise, non-overlapping manner. The basic unit of this code is the codon, a sequence of three nucleotides. The entire set of these three-base codes specifies the twenty standard amino acids and stop signals, forming the genetic code.
The process begins with transcription, where the double-stranded DNA template is copied into a single-stranded messenger RNA (mRNA) molecule. During translation, cellular machinery reads the mRNA sequence. Each three-nucleotide codon on the mRNA is matched with a transfer RNA (tRNA) carrying the correct amino acid, assembling a chain that folds into a functional protein.
The genetic code is considered a universal language across most life forms, but it possesses redundancy, or degeneracy. With 64 possible three-base combinations and only 20 amino acids to code for, most amino acids are specified by more than one codon. This redundancy often occurs at the third position of the codon, meaning a change in the third base may not alter the resulting amino acid.
The four DNA bases—Adenine (A), Guanine (G), Cytosine (C), and Thymine (T)—are the alphabet of this code. In the mRNA molecule, Uracil (U) replaces Thymine (T). Understanding which DNA triplet corresponds to which amino acid requires mapping the DNA sequence to the mRNA codon.
The Codons for Aspartic Acid
The two DNA codons that specify the amino acid Aspartic Acid (Asp) are GAT and GAC. These sequences are found on the coding strand of the DNA. Since Thymine (T) is replaced by Uracil (U) in RNA, the corresponding mRNA codons for Aspartic Acid are GAU and GAC.
The fact that both GAT and GAC code for the same amino acid illustrates the redundancy of the genetic code. In this case, the third nucleotide of the codon can be either Thymine (T) or Cytosine (C) without changing the amino acid that is incorporated into the protein. This flexibility in the third position is a common pattern across the genetic code, often referred to as the wobble effect.
Functional Significance of Aspartic Acid
Aspartic Acid is classified as a non-essential amino acid, meaning the human body can synthesize it from other compounds. It is characterized as an acidic amino acid due to the presence of a second carboxylic acid group in its side chain. At physiological pH, this side chain typically loses a proton, resulting in a negative charge; this ionized form is known as Aspartate.
This negative charge is significant for protein structure, allowing Aspartic Acid to form ionic bonds, or salt bridges, with positively charged amino acids like Lysine or Arginine. These interactions stabilize the three-dimensional folding of proteins and maintain the active sites of many enzymes. Its acidic nature means it is often positioned on the surface of proteins where it can interact with the surrounding aqueous environment.
Beyond its role as a protein building block, Aspartic Acid is an active participant in metabolic processes. It plays a role in the urea cycle, which is the mechanism for eliminating toxic nitrogenous waste, such as ammonia, from the body. Furthermore, it serves as a precursor molecule for the synthesis of several other important biomolecules, including the amino acid Asparagine, and the purine and pyrimidine bases that are the building blocks of DNA and RNA. In the nervous system, Aspartic Acid also functions as an excitatory neurotransmitter.