Genetic Code and Mutation
Objectives
- Describe mechanisms by which mutations arise.
- Classify mutations based on the affected cells, the change to DNA, the effect on the coding sequence, the effect on the protein, and the effect on the cell or organism.
- Describe the difference between gain-of-function and loss-of-function mutation
- Distinguish between germline and somatic mutation
- Distinguish between indel and base substitution mutations
- Classify base substitution mutations as missense, nonsense, silent, or none of the above
- Recognize that not all indel mutations are frameshift mutations
- Recognize that not all mutations occur in the coding sequence of a gene, and that mutations in noncoding sequence may also affect phenotype.
- Recognize that not all mutations are bad.
- Predict the effect of a mutation on the function of a protein.
- Explain the connection between DNA damage and mutation.
- Describe the mechanisms by which DNA damage leads to mutation, including replication errors and damage to bases.
Source material
- Selected images and text remixed from Online Open Genetics (Nickle and Barrette-Ng), available through Biology LibreTexts[1]. Chapter 4: Mutation and Variation
- Selected images and text remixed from Open Genetics Lectures[2]. Chapter 11: Mutation Causes and 12: Mutation Consequences.
Introduction
Within any species, you will see normal phenotypic variation. For example, tomatoes may produce fruits of different size, shape, and color. Dogs can weigh five pounds, one hundred and fifty pounds, or any weight in between. Humans can have hair color ranging from blond to red to black. Those differences in phenotype are caused by differences in DNA sequence among individuals in the population.
Previous modules looked at how DNA acts as an information storage system and how that information is used to build RNA and proteins that perform many cellular functions. The structure and function of those RNA and protein molecules is specified by the sequence of the DNA, so any changes to the DNA sequence can affect the structure and function of those gene products. Changes to the function of gene products can impact the behavior of a cell or even a whole organism.
Changes to DNA sequence are called mutations. In general (non-science) usage, the word can have very negative connotations (when it is not associated with superheroes, of course!). But to a geneticist, the word mutation just means a change. Mutations may be harmful to an organism, beneficial to an organism, or neutral. Mutation also drives evolution: It is the source of variation in a population, and without variation, there is no evolution. (More on this in other modules!)
Some terminology regarding variations in DNA: A mutation is a change in DNA sequence from a reference organism. For model organisms in the lab, mutations are in relation to the most common, “wild-type” phenotype. But when talking about populations of organisms outside of the lab, including humans, it can be difficult to decide what is “normal”. For example, some people have blood type A, some have type B, some have type AB, and some have type O. None is more normal than another.
In these cases, the term polymorphism is used to describe the variation we see in a population. If an allele is seen in more than 1% of the population and is not associated with disease, we’d use the word polymorphism. If fewer than 1% of the population has a variation or if the variant is associated with disease, the word mutation would describe the sequence[3].
When we are describing these unusual variations in genetics, it can be commonplace for geneticists who study model organisms to describe an individual as mutant – as in “a mutant fruit fly with white eyes”. But this is not appropriate in human genetics, where the words “mutation” and “mutant” are limited to a description of gene or protein sequence, not people.
In this module and in others throughout this text, we see examples of mutations in human genes linked with genetic disorders. We can learn a lot about human genetics from studying rare differences in the population – and what we learn can help patients with a genetic disorder as well as further our understanding of human biology. But as you read, please keep in mind that the examples and images that we discuss are of real people who are deserving of respect. People who have consented to contribute their stories to education and research also deserve our gratitude. We should also remember that those differences do not indicate social worth, even when a genetic difference may cause health disparities.
- Nickle and Barrette-Ng. Open Online Genetics. in Open Online Genetics (2016). ↵
- Locke, J. ‘Open Genetics Lectures’ textbook for an Introduction to Molecular Genetics and Heredity (BIOL207). Borealis https://doi.org/10.7939/DVN/XMUPO6 (2017). ↵
- Karki, R., Pandya, D., Elston, R. C. & Ferlini, C. Defining “mutation” and “polymorphism” in the era of personal genomics. BMC Med. Genomics 8, 37 (2015). ↵
