7 Mechanisms of Mutation and DNA Repair. Mutations Spontaneous mutation : occurs in absence of...
-
date post
22-Dec-2015 -
Category
Documents
-
view
220 -
download
1
Transcript of 7 Mechanisms of Mutation and DNA Repair. Mutations Spontaneous mutation : occurs in absence of...
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