Transcript Mutations
Mutations What is a mutation? • Mutation – A change in the DNA that affects inherited genetic information • They may be gene mutations which result from single gene changes, or chromosome mutations. • Mutations may be positive, negative or neutral. • Mutations drive evolution. Mutagens A substance that causes mutations. • chemicals • ionizing radiation • UV rays • X-rays GENE MUTATIONS • • • • • point mutations frame shift mutation deletion addition substitution point mutations A mutation that results from a change in one DNA base pair. Three examples of point mutations: • deletion (subtraction) of a base pair • insertion (addition) of a base pair • substitution of a base pair Point Mutations can be: • nonsense mutations: code for the stop codon • missense mutations: code for a different amino acid • silent mutations (neutral) : code for the same or a different amino acid but without any functional change in the protein Deletion (subtraction) • If one nucleotide is removed, all the codons from that point are changed. (This results in different amino acids in the protein, which may not be effective for its intended purpose.) • This is known as a (reading frame shift) a frame shift. A deletion with no frame shift: If a three-base codon is deleted, then there is no frame shift, but there will still be a problem with the protein formed. This is what happens to cause the disorder cystic fibrosis. Cystic Fibrosis, or CF, is a common fatal genetic disease. CF is most common among people whose ancestors came from Northern Europe. The disease is caused by a recessive allele on Chromosome #7. Children with CF have serious digestive problems. In addition, they produce a thick, heavy mucus that clogs their lungs and breathing passageways. The disease takes a heavy toll. Only half of the children born with Cystic fibrosis survive into their 20s. a nebulizer which dispenses medicine in a fine mist to help thin mucus in the lungs. HOW DOES DELETION CAUSE CF? • A protein in the membrane of cells allows chlorine ions (Cl-) to pass across biological membranes. • Most cases of CF involve a deletion of a codon from the gene for this protein. As a result, the protein does not fold properly. Therefore it cannot be transported to the cell membrane and is destroyed. Because of this cells do not transport chlorine ions properly and tissue throughout the body malfunction Insertion (addition) • The addition of a nucleotide changes the codons, and therefore the amino acids in the protein. • A frame-shift occurs here as well. SUBSTITUTION • It effects one nucleotide. No frame shift occurs. One or more bases are changed. • A famous example is the substitution that causes sickle-cell anemia. Sickle cell anemia is a defect in the hemoglobin of blood cells that causes then to assume a sickle shape. This causes clotting and organ damage. A person with sickle-cell anemia may die as a young adult. The substitution that occurs in sickle-cell anemia causes the amino acid valine to be substituted for the amino acid gluatamic acid in the hemoglobin protein. Valine causes a portion of the Red blood cell to become hydrophobic and twist out of shape to avoid the water in the blood plasma. Chromosome mutations • Deletion: loss of part of a chromosome • Duplication: a repeated segment of a Inversion: a portion of the chromosome is reversed in its orientation Translocation: one part of a chromosome breaks off and attaches to another nonhomologous chromosome. Usually, two translocations happen at the same time as the nonhomologous chromosomes exchange portions. Some Animal Mutants Two-faced Cat One-Eye Shark Oldest living 2-faced cat = 12 yrs (9/30/11) Referred to as a “Janus” cat holoprosencephaly – forebrain portion not developed caused the malformations below: cyclops kitten - died shortly after birth