4/2/2002 · Quiz cancellation: Thursday (4/4) quiz · Video on 4/11 - "Who are you?" (MV-98208) · Reminder: Med School Loyola alums panel discussion, Thursday, 12:30, Octavia I & II · Early registration begins next week - sign up for advising now! Last remarks on Cancer and Genetics… Chromosomal mutations: the other mutation type Chromosomal structural rearrangements = Type of eukaryotic chromosome mutation requiring breakage and reunion of chromosome parts. · may result in abnormal gene number or position · may result in the inactivation or reactivation of genes · provide means of creating blocks of genes adaptive to specific environments · may study this character (chromosome arrangement and karyotype) to study evolution and speciation cytogenetics = study of chromosome structure and behavior (normal and abnormal) in relation to their genetic properties Features of chromosomes · Chromosome size · Centromere position - Primary constriction, kinetochore, telocentric, acrocentric, metacentric, acentric, diffuse centric, molecular structure (base pair sequence) known, p-arm and q-arm · Telomeres - chromosome ends - molecular structure known, telomerase · Nucleolar organizer region position - nucleolus, rRNA regions, secondary constriction, number per genome varies · Heterochromatin and other banding patterns - polytene chromosomes of fruit fly with chromomere patterns, heterochromatin = densely staining regions that are more condensed than euchromatin, C-bands = constitutive heterochromatin, G-bands = giemsa bands (usually not found in plants), Q-bands = quinicrine stained, R-bands = reverse giemsa · Homologous chromosome pairing in polytene and meiosis (prophase I) - chromosomes contort to pair if necessary · Broken ends reactive and tend to join with other broken ends - normal chromosome ends (telomeres) are not reactive Types of rearrangements 1. Deletions 2. Duplications 3. Inversions 4. Translocations Deletions origin Interstitial - two simultaneous breaks and fusion of broken ends. Terminal - breaks on chromosome ends; probably lethal since telomeres would be lost, so that "apparent" terminal breaks probably include two breaks with one near telomere. Deletion detection · -mid-prophase I deletion loop or polytene loop when heterozygous · not found in homozygous condition (lethal); in heterozygous sometimes referred to as "partial monosomy" (e.g., Cri du chat) · such mutations cannot revert · pseudodominance effect - unmasking of recessive alleles lead to detection of deletion · Spontaneous deletions in somatic cells can lead to tumor formation. · Deletion mapping using pseudodominance in Drosophila or in situ hybridization and deletion cell lines from patients (Fig. 17-4 and 17-7).