How do changes in DNA affect gene products and phenotypes?
Topic 6.7 Mutations: explain the types of mutations and how they affect gene products, phenotype and the variation available to a population.
A focused answer to AP Biology Topic 6.7, covering point mutations (silent, missense, nonsense), frameshift mutations, chromosomal mutations, their effects on proteins and phenotype, and their role as the source of new variation, with a worked example.
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What this topic is asking
The College Board (Topic 6.7) wants you to explain the types of mutations (point mutations, frameshifts, chromosomal mutations) and how they affect the gene product, the phenotype, and the variation available to a population.
Point mutations
Frameshift and chromosomal mutations
Effects and inheritance
Whether a mutation is harmful, neutral or beneficial depends on the environment, not on the mutation alone: a change that is neutral now may become advantageous if conditions change. This is why mutation, despite being random and often harmful, is essential to evolution, it supplies the new variation that natural selection acts on, as the antibiotic-resistance example in Unit 7 shows. Mutations can be caused by errors in replication or by mutagens such as radiation and certain chemicals.
Try this
Q1. State the effect of a silent mutation on the protein. [1 point]
- Cue. None; the changed codon still codes for the same amino acid because the genetic code is redundant.
Q2. Explain why mutations are important for evolution. [2 points]
- Cue. Mutations create new alleles, the ultimate source of genetic variation; natural selection then acts on this variation, so without mutation there would be no new heritable traits to select.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2019 (style)4 marksSection II (long FRQ excerpt). (a) Distinguish between a point mutation and a frameshift mutation, and explain why a single-base insertion is usually more harmful than a single-base substitution. (b) Explain why mutations in body cells are not passed to offspring but mutations in gametes can be.Show worked answer →
A 4-point distinguish-and-explain FRQ on mutations.
(a) Distinguish and explain (2 points): (1 point) a point mutation changes a single base (substitution), affecting at most one codon, while a frameshift is an insertion or deletion that shifts the reading frame; (1 point) a single-base insertion is usually more harmful because every codon downstream is changed, so the whole protein from that point is altered, whereas a substitution changes at most one amino acid.
(b) Explain (2 points): (1 point) only mutations in gametes (or the cells that make them) are passed on, because offspring develop from the fertilized egg; (1 point) mutations in body (somatic) cells affect only that individual and are not inherited.
Markers reward distinguishing the mutation types, explaining the reading-frame effect, and the somatic versus germline distinction.
AP 2018 (style)1 marksSection I (multiple choice). A mutation that changes a codon to a stop codon, ending translation early, is called a: (A) silent mutation. (B) missense mutation. (C) nonsense mutation. (D) frameshift mutation.Show worked answer →
The correct answer is (C).
A nonsense mutation changes a codon into a stop codon, prematurely ending translation and producing a shortened, usually non-functional protein. A silent mutation (A) changes no amino acid; a missense mutation (B) changes one amino acid; a frameshift (D) shifts the whole reading frame.
Related dot points
- Topic 6.4 Translation: explain how the ribosome translates mRNA codons into a polypeptide, including the roles of tRNA and the genetic code.
A focused answer to AP Biology Topic 6.4, covering codons, the genetic code, the roles of mRNA, tRNA and ribosomes, the stages of translation, and using a codon table, with a worked translation problem.
- Topic 6.1 DNA and RNA Structure: describe the structure of DNA and RNA and explain how it suits their role in storing and transmitting genetic information.
A focused answer to AP Biology Topic 6.1, covering the double helix, antiparallel strands, complementary base pairing, the sugar-phosphate backbone, and the differences between DNA and RNA, with a worked base-pairing calculation.
- Topic 6.2 Replication: explain how DNA is replicated semiconservatively, including the roles of the key enzymes and the leading and lagging strands.
A focused answer to AP Biology Topic 6.2, covering semiconservative replication, helicase, DNA polymerase, the leading and lagging strands, Okazaki fragments and ligase, with a worked replication problem.
- Topic 7.2 Natural Selection: explain how directional, stabilizing and disruptive selection change the distribution of phenotypes in a population.
A focused answer to AP Biology Topic 7.2, covering directional, stabilizing and disruptive selection, sexual selection, and how each changes a phenotype distribution, with a worked interpretation of selection on a trait.
- Topic 5.6 Chromosomal Inheritance: explain the chromosomal basis of inheritance, including sex determination and the consequences of nondisjunction.
A focused answer to AP Biology Topic 5.6, covering the chromosome theory of inheritance, sex determination, linkage, nondisjunction and aneuploidy, with a worked example of nondisjunction.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)