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.
Nonsense Mutations and Stop-Codon Mutations. Three codons in the genetic code tell the cell to stop adding amino acids to a protein because the end of the gene has been reached. In a nonsense mutation, a codon that stands for an amino acid mutates to one of these three stop codons.