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section 8.7 6E Mutations Key Concept Mutations are changes in DNA that may or may not affect phenotype. Some mutations affect a single gene, while others affect an entire chromosome. In biology a mutation means a change in an organism’s DNA. A mutation can happen during replication and affect a single gene. A mutation can also happen during meiosis and affect a whole chromosome. Gene Mutations DNA A mutation is a change in an organism’s DNA. Normal Point m DNA G A T C T C A G G C T A DNA mRNA C U A G A G U C C G A U mRNA protein Leu Glu mutated G A T C T A A G G C T A G A T C T T C A G G C T A base mRNA mRNA C U A G A U U C C G A U C U A G A A G U C C G A U protein protein Leu Leu Asp Glu Ser Val added A base T Asp Arg Frameshift mutation (deletion) G A T C T C A G C T A A added base What is the difference between a gene mutation and a chromosomal mutation? Holt McDougal Biology Asp DNA DNA Errors in meiosis can result in changes in large parts of a chromosome. mRNA mRNA T C U A G A A G U C C G A U C U A G A G Recall that crossing over is a normal process in which chromosomes exchange pieces. Errors in crossing over or in other parts of meiosis can protein protein result in chromosomes with two copies of the same gene. PiecesLeu of Glu Val Arg Leu Glu non-homologous chromosomes might even be exchanged. 146 Ser Point mutation Frameshift mutation (insertion) DNA G A T C T T C A G G C T A Chromosomal Mutations Types of Mutations U C G A U U Ser Ile deleted base G © Houghton Mifflin Harcourt Publishing Company There are different types of gene mutations. • A point mutation is when an incorrect nucleotide is put into a DNA molecule during replication. If the error is not fixed by DNA polymerase, the DNA is perma nently changed. For example, the figure to the right shows a CTC codon that is changed to a CTA codon. As a result, the wrong amino acid is added—aspartic acid instead of glutamic acid. • A frameshift mutation is the addition or removal of a nucleotide in the DNA Normal sequence. This results in a change in the A T C Recall DNA G importance G C T A T C Athe readingGframe. of the reading frame from Section 8.5. Think back to the sentence RAT.” If mRNA C U A G “THE C C ATE A G UCAT G A THE U the letter E is removed, or deleted, from the first “THE,” the reading frame is shifted. The protein Leu Glu TET SerHERAsp result is “THC ATA AT…” The reading frame is also shifted if a nucleotide is Frameshift mutation (insertion) added, or inserted. protein Frames DNA mRNA protein © Houghton Mifflin Harcourt Publishing Company Mutations may or may not affect phenotype. Whether a mutation affects an organism depends on many different things. • Type of mutation A point mutation only affects one codon. A frameshift mutation usually has a bigger effect because it changes the whole reading frame and can affect many codons. U G C • Impact on the amino acid sequence A change in one codon can still have a big effect. For example, if a codon for an amino acid is changed into a stop codon, transcription would end at the wrong U G C cysteine place. A point mutation may also have no effect. Recall that more than one codon can code for the same amino acid. For example, CGU, CGC, CGA, and CGG all code for arginine. A point mutation that changes the last nucleotide of this codon would have no U G A stop effect on the resulting amino acid. • Impact on the resulting protein Some changes might not affect U G A the resulting protein’s shape or function. Other changes might This point mutation changed prevent the protein from functioning. For example, a mutation could change the active site of an enzyme and prevent the enzyme a codon for cysteine into a stop codon. from binding to its substrate. • Type of cell Recall that mutations that occur in germ cells can be passed on to offspring. Mutations in body cells cannot be passed on to offspring. Give one example of a mutation that would not affect an organism’s phenotype. Mutations can be caused by several factors. Mutations happen. But cells have tools to repair them. For example, DNA polymerase has a “proofreading” function to fix errors. However, mutations can happen faster than the body’s repair system can work. Some mutations are the result of errors that happen normally in the cell. Other mutations are caused by things in the environment. • Replication errors DNA polymerase proofreads replication, but a small number of errors are not fixed. Over time, there are more and more errors. Eventually these mutations affect how the cell works. There is evidence that a build-up of mutations is a major cause of aging. Interactive Reader 147 • Mutagens Some conditions and substances in the environment can cause DNA mutations—such as UV light and some chemicals. Things in the environment that can change DNA are called mutagens. If mutations cause changes that affect the control over cell division, cancer may result. What are two examples of mutagens? 8.7 Vocabulary Check mutation point mutation frameshift mutation mutagen Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. 2. What is the difference between a point mutation and a frameshift mutation? 8.7 The Big Picture 3. Is a mutation that happens during meiosis likely to affect a single gene or an entire chromosome? Explain. 4. Give an example of one mutation that will affect an organism’s phenotype and one mutation that will not affect phenotype. 148 Holt McDougal Biology © Houghton Mifflin Harcourt Publishing Company 1. What is the difference between a mutation and a mutagen?