Chromatin Remodeling DNA is wrapped around histones to form nucleosomes DNA is wrapped around histones to form nucleosomes Chromosome packaging Chromosome packaging Chromosome packaging Chromosome packaging Acetylation =acetyl groups are added to histones Acetylation =acetyl groups are added to histones Exposes the primer so RNA polymerase can bind and transcription can begin Exposes the primer so RNA polymerase can bind and transcription can begin Deacetylation =acetyl groups are removed from histones Deacetylation =acetyl groups are removed from histones

RNA Interference Occasionally, both DNA strands are transcribed Occasionally, both DNA strands are transcribed Complementary strands bind to one another Complementary strands bind to one another Gene sequence may allow formation of a “hairpin loop” Gene sequence may allow formation of a “hairpin loop” RNA strand binds to itself RNA strand binds to itself Segments of dsRNA attract RNA-induced silencing complexes (RISCs) Segments of dsRNA attract RNA-induced silencing complexes (RISCs) Can be used experimentally (clinically?) 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  Purine replaces purine (A  G or G  A)  Pyrimidine replaces pyrimidine (C  T or T  C) Transversion  Purine exchanged for pyrimidine (or vice versa)  Missense Mutation causes change from one amino acid to another  Nonsense 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 mRNA Protein A Lys Phe GlyAla AAA CC GGGGUUUUUG Met Base substitution MetLysPhe Ser Ala UAG CA AAUUUGG A GC

Base Substitution –Sickle Cell Anemia Normal hemoglobin DNA Normal hemoglobin Glu mRNA C G A A TT Mutant hemoglobin DNA Sickle-cell hemoglobin Val mRNA C G A T AU Normal Red Blood CellSickle-shaped Red Blood Cell Genotype Phenotype

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  Tandem duplication An insertion mutation that repeats part of a gene’s sequence Normal gene mRNA Protein A Lys Phe GlyAla AAA CC GGGGUUUUUG Met Base deletion Missing A Lys LeuAla His UGCAAGU U UGG C G U A Met

Pseudogenes and Transposons  Pseudogene DNA sequence that is very similar to that of a protein- encoding 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 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 sugar- phosphate 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