Transcript Mutations
Chromatin Remodeling DNA is wrapped around histones to form nucleosomes Acetylation =acetyl groups are added to histones Chromosome packaging Exposes the primer so RNA polymerase can bind and transcription can begin Deacetylation =acetyl groups are removed from histones RNA Interference Occasionally, both DNA strands are transcribed Complementary strands bind to one another Gene sequence may allow formation of a “hairpin loop” RNA strand binds to itself Segments of dsRNA attract RNA-induced silencing complexes (RISCs) Can be used experimentally (clinically?) RNA Interference Mutations What is a Mutation? A change in the DNA sequence that is present in <1% of the population Mutations can happen at the DNA level or at the chromosome level Can affect any part of the genome (introns, exons, etc.) A polymorphism is also a change in a single nucleotide but occurs in >1% of the population Change in DNA Altered RNA Messed up protein Mutation refers to genotype while “mutant” refers to phenotype Types of Mutations Point mutations Splice site mutations Deletions and insertions Pseudogenes and transposons Expanding repeats Copy # variants Point Mutations Base substitution –may be good, bad, or neutral Transition Transversion Mutation causes change from one amino acid to another Nonsense Purine exchanged for pyrimidine (or vice versa) Missense Purine replaces purine (AG or GA) Pyrimidine replaces pyrimidine (CT or TC) Mutation causes change from an amino acid codon into a “STOP” codon Mutation in the promoter region Protein is normal but less protein is produced Base Substitution Normal gene A Base substitution U G A A G U U U G G C G C A A U G A A G U U U A G C G C mRNA Protein Met Lys Phe Gly Ala Met Lys Phe Ser Ala A Base Substitution –Sickle Cell Anemia Normal hemoglobin DNA C T T Mutant hemoglobin DNA Genotype C A T G U A mRNA mRNA G A A Sickle-cell hemoglobin Normal hemoglobin Glu Normal Red Blood Cell Phenotype Val Sickle-shaped Red Blood Cell Splice Site Mutations A type of point mutation that alters the way introns and exons are spliced Intron is translated or exon is skipped Deletions and Insertions Often cause a frameshift mutation Shift the entire reading frame (this is almost always REALLY BAD) May involve a single nucleotide or an entire piece of chromosome Normal gene A Base deletion U G A A G U U U G G C G C A A U Missing U G A A G U U G G C G C A mRNA Protein Met Lys Phe Gly Ala Met Lys Leu Tandem duplication An insertion mutation that repeats part of a gene’s sequence Ala His U Pseudogenes and Transposons Pseudogene DNA sequence that is very similar to that of a proteinencoding gene Sometimes transcribed but not translated May interfere with the normal gene, especially during crossing over Transposons “Jumping genes” May disrupt the site they jump from or the one they jump to Expanding Repeats Gene expands <40 copies of the repeated segment is transmitted normally but >40 are unstable Cause “anticipation” Phenotype gets worse and has younger onset with each generation Copy # Variants Sequences that are present in more than one place in the genome Two individuals may have similar DNA sequences but very different #’s of copies of those sequences Copies may be next to each other or on different chromosomes Effects of Mutations Loss of function Gene’s product is reduced or absent Tend to be recessive Gain of function The action of the gene product changes Tend to be dominant Germline vs. Somatic Mutations Germline mutation Change occurs during DNA replication before meiosis Resulting gamete and all cells that descend from it following fertilization have the mutation (all cells in the body) Somatic mutation Change occurs during DNA replication before mitosis All cells that descend from the changed cell are mutated (a subset of cells in the body) Spontaneous Mutations Usually an error in DNA replication Each gene has a ~1/100,000 chance of mutating We all likely have several mutations in our DNA but most DNA is non-coding Mutational Hotspots Regions in the DNA where mutations are more likely to occur Usually, repetitive sequences “Mississsippii” Induced Mutations A mutagen is an agent that causes mutation Ex: Ionizing radiation breaks the DNA sugarphosphate backbone Conditional Mutations Ex: The gene for glucose 6-phosphate dehydrogenase Used by red blood cells to extract energy from glucose Mutated in 100 million people The mutation can cause severe anemia but ONLY following exposure to: Fava beans Pollen in Baghdad Or certain anti-malaria drugs