POLYNUCLEOTIDES:

Interestingly, different sequences of nucleotides of the same combination, may make up two or three distinct Amino Acids (e.g., GUG = Valine, GGU = Glycine, UGG = Tryptophan).  

Because three nucleotides are involved, another name for a Codon is a “polynucleotide”.  The one below is known as ACG and is one of the possible nucleotide sequences that make up the essential Amino Acid called Threonine, abbreviated “Thr” or just plain “T”.  

AMINO  ACIDS:

Each distinct Amino Acid, once again, is comprised in turn of one or more possible Codons.  However, three particular Codons, i.e. UAA, UAG and UGA do not constitute a given Amino Acid, and are instead known as “Stop” Codons) because their position in the chain indicates the end of that particular code.  As aforesaid, bear in mind that with the exception of two of the essential Amino Acids, Tryptophan (UGG) and Methionine (AUG), all Amino Acids occurring in protein may be expressed by as many as four or six (usually two) of the possible different combinations of nucleotides.  In other words, more than one Codon may represent the same Amino Acid.  (Refer to the table on page 3.)

To make matters perhaps more confusing, each Amino Acid’s name also may be referred to by either a three-letter abbreviation (such as Thr for Threonine, above distinct from the corresponding Codon), or a one-letter shorthand form; but both of these systems may appear to be arbitrary or subjective at best. (Further examples: “Asn” or “N” stand for Asparagine, while “Arg” or “R” stand for Arginine.  “A” or “Ala” represent Alanine and “Asp” or “D” stand for Aspartate).  These other three-letter designations used to abbreviate particular Amino Acid names are written partially in lower case so as not to confuse them with recognized Codons (always written in UPPER CASE).  Notice also that most of the letters of the alphabet used in such abbreviations do not exist in any of the Codons set forth in the table on the previous page.

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