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Mutations Some Se Non-Homologous Chromosomes Even Exchanged

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Mutations Some Se Non-Homologous Chromosomes Even Exchanged DO NOT EDIT--Changes must be made through “File info” CorrectionKey=A sectioN 6E Mutations 8.7 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 muta- tion can happen during replication and affect a single gene. A mutation can also happen during meiosis and affect a whole chromosome. Gene mutations There are different types of gene mutations. • A point mutation is when an incorrect Types of MutationS nucleotide is put into a DNA molecule A mutation is a change in an organism’s DnA. during replication. If the error is not fixed Normal Point mutation by DNA polymerase, the DNA is perma- DNA G A T C T C A G G C T A DNA G A T C T A A G G C T A mutated nently changed. For example, the figure to base the right shows a CTC codon that is changed A to a CTA codon. As a result, the wrong mRNA C U A G A G U C C G A U mRNA C U A G A U U C C G A U amino acid is added—aspartic acid instead of glutamic acid. protein protein • A frameshift mutation is the addition Leu Glu Ser Asp Leu Asp Ser Asp or removal of a nucleotide in the DNA Normal PointFrameshift mutation mutation (insertion) Frameshift mutation (deletion) sequence. This results in a change in the DNA G A T C T C A G G C T A DNA G A T C T A A G G C T A mutated reading frame. Recall the importance of the DNA G A T C T T C A G G C T A base DNA G A T C T C A G C T A A deleted reading frame from Section 8.5. Think back base addedA base mRNAto the sentenceC U A G “THEA G U CATC C ATEG A THEU RAT.” If mRNA C U A G A U U C C G A U G mRNA T mRNA the letter E is removed, or deleted, from the C U A G A A G U C C G A U C U A G A G U C G A U U proteinfirst “THE,” the reading frame is shifted. The protein protein protein result is “THCLeu ATAGlu TETSer HERAsp AT…” The Leu Asp Ser Asp Leu Glu Val Arg Leu Glu Ser Ile reading frame is also shifted if a nucleotide is Frameshift mutation (insertion) Frameshift mutation (deletion) added, or inserted. © Houghton Mifflin Harcourt Company Publishing DNA G A T C T T C A G G C T A DNA G A T C T C A G C T A A deleted Chromosomal mutations base added base Errors in meiosis can result in changes in large parts of a chromosome. G mRNA C A A A C C A T mRNA Recall that crossingU G over Gis Ua normalG processU in which chromosomesC U A G A G U C G A U U exchange pieces. Errors in crossing over or in other parts of meiosis can resultprotein in chromosomes with two copies of the same gene.protein Pieces of Leu Glu Val Arg Leu Glu Ser Ile non-homologous chromosomes might even be exchanged. What is the difference between a gene mutation and a chromosomal mutation? 146 Holt McDougal Biology DO NOT EDIT--Changes must be made through “File info” CorrectionKey=A 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. • Impact on the amino acid sequence A change in one codon can U G C 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 muta- tion that changes the last nucleotide of this codon would have no stop effect on the resulting amino acid. U G A • Impact on the resulting protein Some changes might not affect the resulting protein’s shape or function. Other changes might U G A prevent the protein from functioning. For example, a mutation This point mutation changed 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. © Houghton Mifflin Harcourt Company Publishing 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 DO NOT EDIT--Changes must be made through “File info” CorrectionKey=A • 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 Mark It Up Go back and highlight mutation frameshift mutation each sentence that point mutation mutagen has a vocabulary word in bold. 1. What is the difference between a mutation and a mutagen? 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. © Houghton Mifflin Harcourt Company Publishing 4. Give an example of one mutation that will affect an organism’s pheno- type and one mutation that will not affect phenotype. 148 Holt McDougal Biology.
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