Mutation (PDF)

Mutatio n  Failure in Mismatch Repair Generates a Mutation, a Change in the DNA Sequence A change to the sequence of bases in an organism’s DNA is called a mutation. Mutagens are substances or energy sources that can cause mutations. New alleles arise as a result of mutation...

Mutatio n  Failure in Mismatch Repair Generates a Mutation, a Change in the DNA Sequence A change to the sequence of bases in an organism’s DNA is called a mutation. Mutagens are substances or energy sources that can cause mutations. New alleles arise as a result of mutations. Most genetic mutations are neutral or harmful. A mutation may consist of change in a single base or a large-scale change involving chromosomal abnormalities. Normal Gene Function Depends on DNA Repair DNA repair requires a three-step process: 1. Recognition 2. Removal 3. Replacement Each step requires different sets of repair proteins using specialized enzymes to identify, remove, and replace the damaged DNA. Failure to Repair DNA Damage Can Lead to Serious Diseases, Including Cancer Child with xeroderma pigmentosum (XP); large skin cancer lesion on chin Changes in Genetic Material Mutation: a permanent change in the base sequence of DNA Mutations may be neutral, beneficial, or harmful Mutagens: agents that cause mutations Spontaneous mutations: occur in the absence of a mutagen  Types of Mutations Base substitution (point mutation) Change in one base in DNA Missense mutation (non-synonymous) – Base substitution results in change in an amino acid Nonsense mutation – Base substitution results in a nonsense (stop) codon Frameshift mutation – Insertion or deletion of one or more nucleotide pairs; Shifts the translational “reading frame” Figure 8.18a-b Types of Mutations and Their Effects on the Amino Acid Sequences of Proteins DNA (template strand) Transcription mRNA Translation Amino acid sequence Met Lys Phe Gly Stop Normal DNA molecule DNA (template strand) mRNA Amino acid sequence Met Lys Phe Ser Stop Missense mutation Stop Figure 8.18a-c Types of Mutations and Their Effects on the Amino Acid Sequences of Proteins DNA (template strand) Transcription mRNA Met Stop Translation Nonsense mutation Amino acid sequence Met Lys Phe Gly Stop Normal DNA molecule Figure 8.18a-d Types of Mutations and Their Effects on the Amino Acid Sequences of Proteins DNA (template strand) Transcription mRNA Translation Met Lys Leu Ala Amino acid sequence Met Lys Phe Gly Stop Normal DNA molecule Frameshift mutation Radiation Ionizing radiation (X-rays and gamma rays) causes the formation of ions that can oxidize nucleotides and break the deoxyribose-phosphate backbone UV radiation causes thymine dimers Chemical Mutagens Nitrous acid: causes adenine to bind with cytosine instead of thymine Nucleoside analog: incorporates into DNA in place of a normal base; causes mistakes in base pairing  Ultraviolet light Figure 8.21 The Creation and Repair of a Thymine Dimer Caused by Exposure to ultraviolet light Ultraviolet Light causes adjacent thymines to become cross-linked, forming a thymine dimer and disrupting Thymine dimer their normal base pairing. An endonuclease cuts the DNA, and an exonuclease removes the damaged DNA. New DNA DNA polymerase fills the gap by synthesizing new DNA, using the intact strand as a template. DNA ligase seals the remaining gap by joining the old and new DNA. Mutations Can Alter Protein Function Even single-base changes can alter protein function enough to produce a harmful phenotype such as a disease. Frameshift mutations alter the protein so extensively that they invariably destroy the normal function of the protein and produce a severe phenotype. A silent mutation causes no change in the structure of the protein, and therefore no change in the phenotype of The replacement of glutamic acid by valine the organism. changes the shape of hemoglobin. The accumulation of large amounts of the deformed Rarely, a mutation can be protein distorts the shape of red blood cells in beneficial and improve the people with sickle-cell efficiency or functionality of a anemia. protein. Regulation The Operon Model of Gene Expression Promoter: segment of DNA where RNA polymerase initiates transcription of structural genes Operator: segment of DNA that controls transcription of structural genes Operon: set of operator and promoter sites and the structural genes they control Figure 8.12 An Inducible Operon Operon Control region Structural genes I P O Z Y A DNA Regulatory Promoter Operator gene Structure of the operon. The operon consists of the promoter (P) and operator (O) sites and structural genes that code for the protein. The operon is regulated by the product of the regulatory gene (I) Pre-Transcriptional Control Repression inhibits gene expression and decreases enzyme synthesis Mediated by repressors, proteins that block transcription Default position of a repressible gene is on Induction turns on gene expression Initiated by an inducer Default position of an inducible gene is off The Operon Model of Gene Expression In an inducible operon, structural genes are not transcribed unless an inducer is present LAC operon In the absence of lactose, the repressor binds to the operator, preventing transcription In the presence of lactose, metabolite of lactose– allolactose (inducer)–binds to the repressor; the repressor cannot bind to the operator and transcription occurs Figure 8.12 An Inducible Operon RNA polymerase I P Z Y A Transcription Repressor Active mRNA repressor protein Translation Repressor active, operon off. The repressor protein binds with the operator, preventing transcription from the operon. Figure 8.12 An Inducible Operon I P O Z Y A Transcription Operon mRNA Translation Allolactose (inducer) Inactive Transacetylase repressor Permease protein β-Galactosidase Repressor inactive, operon on. When the inducer allolactose binds to the repressor protein, the inactivated repressor can no longer block transcription. The structural genes are transcribed, ultimately resulting in the production of the enzymes needed for lactose catabolism. Positive Regulation Catabolite repression inhibits cells from using carbon sources other than glucose Cyclic AMP (cAMP) builds up in a cell when glucose is not available cAMP binds to the catabolic activator protein (CAP) that in turn binds the lac promoter, initiating transcription and allowing the cell to use lactose  Bacteria Figure 8.14 The Growth growing on Rate of E. Coli on glucose as the Glucose and Lactose sole carbon Glucose source grow faster than on Lactose lactose. Bacteria All glucose growing in a consumed medium containing glucose Glucose Lactose used and lactose first used Lag consume time the glucose and then, after a short lag time, the lactose. During the lag  Promoter Figure 8.15 Positive lacl lacZ Regulation of the Lac DNA Operon CAP-binding site RNA Operator polymerase can bind and transcribe cAMP Active CAP Inactive lac repressor Inactive CAP Lactose present, glucose scarce (cAMP level high). If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces large amounts of mRNA for lactose digestion. Promoter lacl lacZ DNA Operator RNA CAP-binding site polymerase can't bind Inactive lac Inactive repressor CAP Lactose present, glucose present (cAMP level low). When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription. Summary of lac operon responses CAP Repressor Level of Glucose Lactose activation binds transcription No + - - + transcription Low-level + + - - transcription No - - + + transcription Strong - + + - transcription OpenStax College, Biolog The Operon Model of Gene Expression In repressible operons, structural genes are transcribed until they are turned off Excess tryptophan is a corepressor that binds and activates the repressor to bind to the operator, stopping tryptophan synthesis Figure 8.13 A Repressible Operon Operon Control region Structural genes I P O E D C B A DNA Regulatory Promoter Operator gene Structure of the operon. The operon consists of the promoter (P) and operator (O) sites and structural genes that code for the protein. The operon is regulated by the product of the regulatory gene (I) Figure 8.13 A Repressible Operon RNA polymerase I P O E D C B A Transcription Repressor mRNA Operon mRNA Translation Polypeptides Inactive comprising the repressor enzymes for protein tryptophan synthesis Repressor inactive, operon on. The repressor is inactive, and transcription and translation proceed, leading to the synthesis of tryptophan. Figure 8.13 A Repressible Operon P I E D C B A Active repressor protein Tryptophan (corepressor) Repressor active, operon off. When the corepressor tryptophan binds to the repressor protein, the activated repressor binds with the operator, preventing transcription from the operon. Post-Transcriptional Control The riboswitch is a part of an mRNA molecule that binds to a substrate and changes the mRNA structure. translation is initiated or stopped microRNAs (miRNAs) base pair with mRNA to make it double-stranded Double-stranded RNA is enzymatically destroyed, preventing production of a protein Epigenetic Control Methylating nucleotides turn genes off Methylated (off) genes can be passed to offspring cells  DNA Figure 8.16 MicroRNAs Control a Wide Range of Transcription of miRNA occurs. Activities in Cells miRNA binds to target miRNA mRNA that has at least six complementary bases. mRNA mRNA is degraded.  Gene Expression Can Be Regulated at Several Levels Tight packing of DNA prevents access to its gene regulatory DNA, making that segment of DNA transcriptionally inactive. Regulation of transcription enables the cell to conserve resources when it does not need a particular gene product. By limiting the life span of many types of mRNA, a cell prevents the wasteful synthesis of proteins. Regulation of translation keeps mRNA ready to direct rapid protein synthesis when needed. Proteins can be directly regulated by modification following translation.