8.7 Mutations KEY CONCEPT Mutations are changes in DNA that may or may not affect phenotype.  May occur in somatic cells (aren‘t passed to offspring)  May occur in gametes (eggs & sperm) and be passed to offspring  Mutations lead to genetic variation because they randomly change an organisms genetic code.

8.7 Mutations Types of Mutations

8.7 Mutations Some mutations affect a single gene, while others affect an entire chromosome. A gene mutation affects a single gene. Many kinds of mutations can occur, especially during replication. Types of Gene Mutations: A point mutation substitutes one nucleotide for another. Ex: Sickle Cell Anemia mutated base

8.7 Mutations Nonsense Mutation Type of point mutation Results in a premature stop codon and usually a nonfunctional protein

8.7 Mutations A frame-shift mutation inserts or deletes a nucleotide in the DNA sequence. Throws off the reading frame. THE CAT ATE THE RAT THC ATA TET HER AT What would an insertion look like? (draw in notes)

8.7 Mutations Gene Mutation Animation

8.7 Mutations Chromosomal mutations affect many genes and an entire chromosome. Chromosomal mutations may occur during crossing over.

8.7 Mutations Chromosome Mutations  Five types exist:  Deletion  Inversion  Translocation  Nondisjunction  Duplication

8.7 Mutations Deletion  Due to breakage  A piece of a chromosome is lost

8.7 Mutations Inversion  Chromosome segment breaks off  Segment flips around backwards  Segment reattaches

8.7 Mutations Translocation results from the exchange of DNA (piece of one chromosome) segments between non-homologous chromosomes. Sometimes chromosomes can fuse onto another chromosome.

8.7 Mutations Nondisjunction  Failure of chromosomes to separate during meiosis  Causes gamete to have too many or too few chromosomes (Ex: 3 chromosomes instead of 2 for any given set, or 1 instead of 2)

8.7 Mutations Nondisjunction  Can cause “Trisomy” (three copies of the same chromosome in an egg or sperm)  Trisomy 21 (Down syndrome)

8.7 Mutations Gene duplication results from unequal exchange of segments crossing over. Results in one chromosome having 2 copies of some genes and the other chromosomes having no copies of those genes.

8.7 Mutations Chromosome Mutation Animation

8.7 Mutations

Mutations may or may not affect phenotype. 1. Chromosomal mutations tend to have a big effect. 2. Some gene mutations change phenotype by: –causing a premature stop codon. –causing a change in protein shape or the active site. –causing a change in gene regulation. blockage no blockage

8.7 Mutations 3. Some gene mutations do not affect phenotype. A mutation may be silent because…. –It occurs in a non-coding region. –It may not affect protein folding or the active site.

8.7 Mutations Silent Mutations Some mutations have no effect and are called “silent” –Example: GUC changed to GUG Both code for the amino acid valine This would not affect the protein being made in any way

8.7 Mutations Mutations can be caused by several factors. 1. Replication errors can cause mutations. 2. Mutagens, such as UV rays and chemicals, can cause mutations. 3. Some cancer drugs use mutagenic properties to kill cancer cells. 4. Mutations happen regularly 5. Almost all mutations are neutral 6. Many mutations are repaired by enzymes. 7. Some types of skin cancers and leukemia result from somatic mutations 8. Some mutations may improve an organism’s survival (beneficial)

8.7 Mutations Several methods help map human chromosomes. A karyotype is a picture of all chromosomes in a cell. X Y

8.7 Mutations Karyotypes can show changes in chromosomes. –deletion of part of a chromosome or loss of a chromosome –large changes in chromosomes –extra chromosomes or duplication of part of a chromosome