Unstable hemoglobin due to chain elongation

If one of the globin chains is too long, it will
destabilize the tetrameric hemoglobin
molecule. Hemoglobin Cranston (HbCr) (1)
arises from the insertion of two nucleotide
bases (adenine and guanine) into positions 1
and 2 of codon 145 (tyrosine) of the ! chain,
which leads to a shift of the reading frame. This
changes the normal stop codon UAA into AGU,
the RNA codon for threonine (Thr). As a result,
the normally nontranslated sequences that follow
the stop codon are now translated, and a
polypeptide is formed that is 11 amino acids too
long, extending to position 157.With hemoglobin
Constant Spring (2), the " chain is
lengthened by mutation of the stop codon UAA
to CAA, which codes for glutamine (Gln). The
sequences that normally follow the stop codon
now become translated, and a peptide that is 31
amino acids too long is formed. A number of
other chain-elongation mutations due to similar
mechanisms, such as with hemoglobin
Ikaria