A sestina, or sextain, is a poem comprised of six stanzas containing six lines each. These six-by-six stanzas are followed by a shorter (3-line) final stanza known as an envoi. This gives the sestina a total of 39 lines (6 * 6 + 3 = 39).
1 A nonsense mutation is a point mutation in a sequence of DNA that results in a premature stop codon, or a nonsense codon in the transcribed mRNA, and possibly a truncated, and often nonfunctional protein product.
Silent mutations are point mutations that do not change the amino acid sequence of the protein. These are most likely to have no effect. Redundancy of the Genetic Code reduces the chance that point mutations that result in a change in the third nucleotide of a codon will alter the specified amino acid.