LECTURE 6 Gene Mutation (Chapter 16.1-16.2)

Words to know… Deleterious: (Adj) Causing harm or damage Truncated: (Adj) Cut short Restriction: The state of being limited in purpose or capacity

INTRODUCTION Mutation = heritable change to DNA Wild type allele → mutant allele Creates an unusual allele If it occurs with a gene, it is usually deleterious On the positive side, mutations are the foundation for evolutionary change On the negative side, mutations are much more likely to be harmful than beneficial to the individual and often are the cause of diseases

Mutagen (e.g. UV light) Gene

16.1 CONSEQUENCES OF MUTATIONS Mutations can be divided into three main types 1. Chromosome mutations Changes in chromosome structure 2. Genome mutations Changes in chromosome number 3. Gene mutations Relatively small change in DNA structure that affects a single gene Type 3 will be discussed in this chapter

Genome Mutation

Chromosomal Mutation

Gene Mutation Small change in DNA sequence not visible on a karyotype

Point Mutations A Point mutation is a change of a single base pair 5’ AACGCTAGATC 3’ 3’ TTGCGATCTAG 5’ 5’ AACGCGAGATC 3’ 3’ TTGCGCTCTAG 5’ A transition is a change of a pyrimidine (C, T) to another pyrimidine or a purine (A, G) to another purine A transversion is a change of a pyrimidine to a purine or vice versa Transitions are more common than transversions Is the above change a transition or a transversion?

Effects of Point Mutations Point mutations in the coding sequence of a structural gene can have various effects on the polypeptide Silent mutations are those base substitutions that do not alter the amino acid sequence of the polypeptide Due to the degeneracy of the genetic code

Effects of Point Mutations Missense mutations change the amino acid coded by the codon May be deleterious, beneficial, or neutral Deleterious example: Sickle-cell anemia Glu →Val

Beneficial Example: The sickle cell mutation is also beneficial! Confers resistance to malaria in heterozygotes

Cystic fibrosis (severe) Cystic fibrosis (mild) Neutral Example: Over 900 mutations have been documented in CFTR gene; many have no phenotypic effect Cystic fibrosis (severe) Cystic fibrosis (mild)

Nonsense mutations change an amino acid coding codon to a stop codon Leads to a truncated polypeptide that is usually non-functional

Deletion of four base pairs Addition of four base pairs Additions or deletions affect short sequences of DNA 5’ AACGCTAGATC 3’ 3’ TTGCGATCTAG 5’ 5’ AACGCTC 3’ 3’ TTGCGAG 5’ 5’ AACAGTCGCTAGATC 3’ 3’ TTGTCAGCGATCTAG 5’ Deletion of four base pairs Addition of four base pairs

Effects of Deletions and Insertions Deletions and insertions can be divided into Those that cause reading frame shifts Number of base-pairs deleted or inserted is not divisible by 3 (e.g. 1, 2, 4, 5…) Usually result in a truncated non-functional polypeptide with a deleterious phenotype But can also be neutral of even beneficial Those that don’t cause reading frame shifts Number of base-pairs deleted or inserted is divisible by 3 (e.g. 3, 6, 9, 12…) Usually less harmful than reading frame shift mutations

Beneficial Example: Mutation in CD4 receptor on lymphocytes 32 bp deletion results in a reading frame shift; receptor non-functional; prevents HIV virus entry into the cell

Gene Mutations outside of coding sequences can affect phenotype Mutations in the core promoter can change levels of gene expression Up mutations increase expression. Down mutations decrease expression Other important non-coding mutations are in Table 16.2

Example: Replication error FIRST OPPORTUNITY FOR DNA REPAIR SECOND OPPORTUNITY FOR DNA REPAIR MUTATION IS NOW “FIXED”

Other ways to Categorize Mutations In a natural population, the wild-type is the relatively prevalent genotype. Genes with multiple alleles may have two or more wild-types (variations). A forward mutation changes the wild-type genotype into some new variation A reverse mutation changes a mutant allele back to the wild-type It is also termed a reversion

As we’ve seen, they are often characterized by their phenotypic eddect Mutations can also be described based on their effects on the wild-type phenotype As we’ve seen, they are often characterized by their phenotypic eddect Deleterious mutations decrease the chances of survival The most extreme are lethal mutations Beneficial mutations enhance the survival or reproductive success of an organism The environment can affect whether a given mutation is deleterious or beneficial Some mutations are conditional They affect the phenotype only under a defined set of conditions An example is a temperature-sensitive mutation

Mutations can also be divided into the type of cells affected Germ-line cells Cells that give rise to gametes such as eggs and sperm Somatic cells All other cells Germ-line mutations are those that occur directly in a sperm or egg cell, or in one of their precursor cells Refer to Figure 16.4a Somatic mutations are those that occur directly in a body cell, or in one of its precursor cells Refer to Figure 16.4b AND 16.5

Copyright © The McGraw-Hill Companies, Inc Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Germ-line mutation Gametes Somatic mutation The size of the patch will depend on the timing of the mutation Embryo The earlier the mutation, the larger the patch An individual who has somatic regions that are genotypically different from each other is called a genetic mosaic Patch of affected area Mutation is found throughout the entire body. Mature individual Therefore, the mutation can be passed on to future generations Therefore, the mutation cannot be passed on to future generations Half of the gametes carry the mutation. None of the gametes carry the mutation. Figure 16.4 (a) Germ-line mutation (b) Somatic cell mutation

16.2 OCCURRENCE AND CAUSES OF MUTATION Mutations can occur spontaneously or be induced Spontaneous mutations Result from abnormalities in cellular/biological processes Errors in DNA replication, for example Underlying cause originates within the cell Induced mutations Caused by environmental agents Agents that are known to alter DNA structure are termed mutagens These can be chemical or physical agents Refer to Table 16.4

Spontaneous Mutations Are Random Events Are mutations spontaneous occurrences or causally related to environmental conditions? This is a question that biologists have asked themselves for a long time Jean Baptiste Lamarck: Physiological adaptation theory Proposed that physiological events (e.g. use and disuse) determine whether traits are passed along to offspring Charles Darwin: Random mutation theory Proposed that genetic variation occurs by chance Natural selection results in better-adapted organisms

Random Mutations Can Give an Organism a Survival Advantage Joshua and Ester Lederberg(1950s) devised an ingenious way to test these alterative theories experimentally Studied the resistance of E. coli to infection by bacteriophage T1 tonr (T one resistance) Hypothesis: E. coli cells that survive T1 infection were already resistant to the phage prior to exposure Due to random mutations "Replica plating"

The Lederbergs' experiment: Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Master plate containing many colonies that were grown in the absence of T1 phage A velvet cloth (wrapped over a cylinder) is pressed gently onto the master plate and then lifted. A little bit of each bacterial colony adheres to the velvet cloth, thereby creating a replica of the arrangement of colonies on the master plate. A few tonr colonies were observed at the same location on both plates!!! This indicates that mutations conferring tonr occurred randomly on the primary (nonselective plate) The presence of T1 in the secondary plates simply selected for previously occurring tonr mutants This supports the random mutation theory Velvet cloth The replica is then gently pressed onto 2 secondary plates that contain T1 phage. Petri plate with T1 phage Petri plate with T1 phage Incubate overnight to allow bacterial growth. Figure 16.7 Replica plating