7 Mechanisms of Mutation and DNA Repair

Mutations Spontaneous mutation : occurs in absence of mutagenic agent Rate of mutation: probability of change in DNA sequence during a single generation Induced mutation: caused by exposure to mutagenic agent=mutagen

Mutations: Phenotypic Effects Mutations can be classified by their phenotypic effects Germ-line mutations: affect gametes (inherited) Somatic mutations: may affect any type of body cell except gametes (not inherited)

Mutations Conditional mutations: produce phenotypic changes under specific (restrictive) conditions but not others (permissive conditions) Temperature-sensitive mutations: conditional mutation whose expression depends on temperature

Mutations: Genotypic Effects Mutations can be classified by their effect on gene function Loss-of-function (null): totally nonfunctional gene product Hypomorphic: reduces level of expression

Mutations: Genotypic Effects Hypermorphic: above normal level of expression Gain-of-function: many are dominant and may cause expression at an abnormal time or in an abnormal place

Molecular Basis of Mutation Mutations result from changes in the base sequence of DNA: Base substitutions -one pair of of DNA nucleotides is replaced by another pair : -Transition mutations- a purine is substituted for a purine or a pyrimidine is substituted for a pyrimidine

Molecular Basis of Mutation -Transversion mutations- a purine replaces a pyrimidine or vice versa Base substitutions are point mutations which alter one DNA base pair without adding or deleting any base pairs Point mutations may affect gene expression in several ways

Point Mutations Types of point mutations: Silent substitutions are base substitutions which do not alter the amino acid composition of the protein encoded by a gene: -silent mutations may affect the noncoding portion of a gene or may occur in the coding portion but may not alter codon usage

Point Mutations Missense mutations change a single amino acid as a result of a change in codon specification: -missense mutations can have serious consequences on the biological properties of a protein - sickle cell anemia results from a single amino acid substitution in hemoglobin which alters its structure

Point Mutations Point mutations can also alter signals used to regulate gene expression: Promoter mutations may block transcription Splice site mutations may block splicing or create new splice signals Nonsense mutations change a codon to a stop codon which results in a premature termination of translation

Insertions and Deletions Insertions add one or more nucleotide pairs to DNA sequence Deletions remove one or more nucleotide pairs from DNA sequence Insertions or deletions involving a multiple of 3 DNA base pairs = in-frame since they do not alter the reading frame of the genetic code

Insertions and Deletions Insertions or deletions which involve a non-multiple of 3 DNA base pairs = frameshift mutations since they alter the codon translation reading frame Large deletions may remove genes-no gene product is made Insertions can result from gene amplification which can result in the overproduction of gene products

Insertions and Deletions Deletion mutations in the dystrophin gene cause muscular dystrophy Gene amplifications are often observed in human malignancies Insertion and deletion mutations may result from unequal crossing-over during recombination or replication slippage during replication of simple tandem repeat sequences

Transposable Elements Transposable elements are found in prokaryotes and eukaryotes Transposable elements are called selfish DNA because these elements maintain themselves through replication and transposition

Transposable Elements Transposition= movement of genetic elements from one chromosome location to another transposase=enzyme which catalyzes movement of genetic element

Transposable Elements Steps in transposition: -transposase binds to terminal inverted repeat sequence -enzymatic cleavage results in transfer to different chromosomal site -insertion site is random and involves duplication of 2-12 base pairs Transposable elements cause mutations by inactivating genes at sites of insertion

Transposable Elements Reverse transcriptase: enzyme using RNA transcript as a template for a DNA daughter strand LTR retrotransposons: long terminal repeats Non-LTR retrotransposons: no terminal repeats –LINE and SINE: most abundant transposable elements in mammalian genomes

Spontaneous Mutations Lederberg’s replica plating: bacterial colonies are transferred to velvet pad and from pad to new plate to test for the frequency of phage resistant colonies in a population

Mutation Hot Spots Mutation hot spots have a higher mutation rate than most DNA: Cytosine deamination to uracil is often detected at hot spots Sites of cytosine methylation result in deamination which converts 5- methylcytosine to thymine Both mutations result in GC to AT transitions

Mutation Hot Spots Cytosine deamination can be repaired by DNA uracil glycosylase which recognizes the incorrect GU base pair and removes uracil AP endonuclease then removes the ribose sugar Single-strand gap is repaired by DNA polymerases and nick is sealed by ligase

Induced Mutations Base analogs such as 5-bromouracil may be incorporated into DNA during replication instead of thymine and pairs with guanine resulting in AT to GC transition Nucleotide analogs can inhibit DNA replication

Chemical Mutagenesis Nitrous acid converts amino groups to keto groups altering the base pairing properties of the bases to produce transition mutations Alkylating agents add alkyl groups to bases resulting in transition mutations or depurination = loss of guanine

Radiation Mutagenesis Ultraviolet radiation (UV) causes adjacent thymines to become covalently linked = pyrimidine dimers Ionizing radiation causes formation of free radicals, highly reactive ions which can damage DNA producing serious mutagenic effects

DNA Repair Mechanisms Mismatch Repair consists of the excision of a segment of DNA that contains a base mismatch followed by repair synthesis Photoreactivation repairs UV- induced pyrimidine dimers by breaking the covalent linkage between the thymine bases

Excision Repair Excision repair is a multistep process in which a segment of damaged DNA is removed and replaced by Resynthesis using the undamaged strand as a template

DNA Repair Mechanisms Postreplication repair involves replication of damaged DNA strand which results in a gap at the damaged DNA site A segment of the template DNA from the other strand is inserted to repair the gap by recombination The gap in the template is repaired