How does a base pair weigh approx. 650 Da (which is two paired nucleotides) but an amino acid (3 nucleotides) weighs only approx. 110 Da ?
closed as off-topic by David, WYSIWYG, theforestecologist♦ Apr 9 at 2:01
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In short: base pairs are not composed of amino acids but nucleotides. Base pairs are parts of the DNA, while chains of amino acids forms proteins.
Certain parts of the DNA (called genes) are transcribed to messenger RNAs (mRNA) which are sequentially translated to amino acids and the chains become proteins. 3 bases (called a codon) code for 1 amino acid.
Yours is a question based on a wrong premise.
First, it is very important to note that an amino acid is not made by sticking together 3 nucleotides. However, in the process of translation, the mRNA sequence is read by groups of three nucleotides, named codons. For example, 5'-AAA-3 is a codon that anneals with the 5'-UUU-3' anticodon. So where is this anticodon?
Unlike DNA, which is double-stranded, mRNA is single stranded. Other single-stranded RNAs called tRNA contain these anticodons. In the previous case, the 5'-UUU-3' tRNA always contains the amino acid lysine on its 3' end. This is because there are special enzymes called aminoacyl-tRNA transferases that ensure that a particular tRNA always links to a particular amino acid.
Long story short, the ribosome allows the tRNAs to orientate themselves so that the amino acids they carry react and form a peptide chain. The incoming tRNAs take this growing chain of amino acids, until a special "tRNA" stops the process and the amino acid chain is "set free". Obviously, this chain once complete is not bound to any nucleotide, so once you calculate the average molecular weight of a proteinogenic amino acid you don't have to take them into account.