If the coding part of an mrna molecule is 1800 nucleotides (bases) in length, this molecule will contain codons and code for a polypeptide that is amino acids long.

If the coding part of an mRNA molecule is 1800 nucleotides (bases) in length, this molecule will contain 600 codons and code for a polypeptide that is 599 amino acids long .

A codon is a sequence of three DNA or RNA nucleotides.  Each codon corresponds to a single amino acid (or stop signal). Because of that, in this example, we are going to have 600 codons (1800/3= 600).

At the end of every mRNA is a stop codon. Stop codon means that translation is over and it does not translate into any amino acid. Because of that, in this example, we are going to have a polypeptide that is 599 amino acids long.