Reason : $mRNA$ is transferred from the nucleus into cytoplasm where ribosomes and amino acids are available for protein synthesis.
$i.$ Six codons do not code for any amino acid.
$ii.$ Codon is read in $mRNA$ in a contiguous fashion.
$iii.$ Three codons function as stop codons.
$iv.$ The initiation codon $AUG$ codes for methionine.