Heres my example...DNA for B Haem.
Normal DNA for B Haem
=
-CTT-
=
normal mRNA sequence
-GAA-
=
Amino acid sequence in B Haem polypeptide- Glutamine
=
normal red blood cells. Flow freely through capillaries. Carry O2 to cells.
HOWEVER IF A MUTATION OCCURS......
Instead of CTT, it becomes CAT.
mRNA therefore transcribes the wrong codon....instead of GAA it becomes GUA.
The amino acid that corresponds with this codon is different....Valine instead of glutamine. The result is sickle shaped red blood cells, which block capillaries so sufficient O2 cannot be provided for cells= sickle cell anaemia.
Reading this over, it sounds confusing. Its much easier if you draw it as a flow chart. Hope it helps