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Types of Mutations A mutation is a permanent change in the DNA sequence of a gene. Mutations in a gene's DNA sequence can alter the amino acid sequence of the protein encoded by the gene. Like words in a sentence, the DNA sequence of each gene determines the amino acid sequence for the protein it encodes. The DNA sequence is interpreted in groups of three nucleotide bases, called codons. Each codon specifies a single amino acid in a protein. Mutate a sentence! We can think about the DNA sequence of a gene as a sentence made up entirely of three-letter words. In the sequence, each three-letter word is a codon, specifying a single amino acid in a protein. Have a look at this sentence: Thesunwashotbuttheoldmandidnotgethishat. If you were to split this sentence into individual three-letter words, you would probably read it like this: The sun was hot but the old man did not get his hat. This sentence represents a gene. Each letter corresponds to a nucleotide base, and each word represents a codon. What if you shifted the three-letter "reading frame?" You would end up with T hes unw ash otb utt heo ldm and idn otg eth ish at. Or Th esu nwa sho tbu tth eol dma ndi dno tge thi sha t. As you can see, only one of these three "reading frames" translates into an understandable sentence. In the same way, only one three-letter reading frame within a gene codes for the correct protein. Now, going back to the original sentence: Thesunwashotbuttheoldmandidnotgethishat. See how you can mutate the reading frame of this sentence by inserting or deleting letters within the sentence. It's easy to make mutations that create "nonsense" sentences. Can you make mutations that maintain or change the meaning of the sentence without creating such nonsense? I. Point Mutations : Changes in a single base pair. A. "Silent Mutations" : do not affect the protein. • GGC mutates to GGU. Both code for glycine. B. Insertion of a stop codon : protein is cut off before it is finished. • UAC (tyrosine) mutates to UAG (stop). C. Base substitution codes for a different amino acid : can be either beneficial (rare), no effect or harmful. I. Frameshift Mutation : addition or deletion of a base causes the "reading frame" to be off. THE DOG BIT THE CAR. THE DOB ITT HEC AR II. Chromosomal Mutation : mutation at the level of the entire chromosome; results from incorrect joining, breaking off of pieces; nondisjunction Questions: #1 - #8 refer to the DNA master strand listed below: AATGCCAGTGGTTCGCAC 1. If a `G' were added to the original master strand of DNA after the third nucleotide (T), what would the resulting mutated M-RNA look like? 2. Write the protein fragment (amino acid chain) that results from the DNA molecule described in #1. 3. Explain how the protein fragment (amino acid chain) in #2 has changed as a result of the mutation described in #1. 4. Name the kind of mutation described in #1. (Be specific!) 5. If the `G' in the fourth nucleotide position were to be cut out of the original DNA strand, what would the resulting M-RNA look like? 6. Write the protein fragment (amino acid chain) that would result from the DNA molecule described in #5. 7. Explain how the protein fragment (amino acid chain) in #6 has changed as a result of the mutation described in #5. 8. Name the kind of mutation described in #5. (Be specific!) Put the correct letter for which example shows the following mutations: _____Inversion _____Polyploidy _____Translocation _____Trisomy _____Deletion _____Monosomy _____Nondisjunction 9. What type of mutation is this? 10. What type of mutation is this? 11. Look at the first row of amino acid sequence. Then compare this row to the row below it. What kind of mutation is this? Met ATG TAC Thr ACC TGG Asp GAC CTG Glu GAG CTC Met ATG TAC Thr ACC TGG Asp GAC CTG Glu GAG CTC --------- Stop TAC ATC --------- Met ATG TAC Lys AAA TTT --------- MeT ATG TAC Lys AAA TTT 12. Look at the first row of amno acid sequence. Then compare this row to the row below it. What kind of mutation is this? Met ATG TAC Met ATG TAC Thr ACC TGG Thr ACC TGG Asp GAC CTG Asp GAC CTG Glu GAG CTC ARG CGA GCT GLU GAG CTC --------- ------- --------- Met ATG TAC Met ATG TAC Lys AAA TTT Lys AAA TTT
