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BIOL 205
Ian D Chin- Sang

BIOL205:week 9, lecture 25 Chromosome numerical variations • polyploidy: extra complete sets of chromosomes, e.g., 3n (triploidy) • this is found in some spontaneously aborted human fetuses • aneuploidy:”wrong number” of chromosomes, ex: ◦ 2n + 1 = trisomy, the presence of an extra copy of one specific chromosome ◦ 2n - 1 = monosomy, the absence of one copy of a specific chromosome → very rare in humans ◦ origins of aneuploidy most frequently lie in meiotic nondisjunction Common human aneuploidies • there are no viable human monosomies except that of the X chromosome: ◦ X0, Turner syndrome • other sex-chromosome aneuploidies associated with viability are XXY (Klinefelter syndrome) and XYY • most human autosomal trisomies are embryonic lethals ◦ but Ts 13 (Patau), 18 (Edwards) and 21(Down) survive in many cases • the most viable human trisomy is Ts21, Down syndrome • incidence of nondisjunction-related Ts21 increases with increasing maternal age ◦ good test to check for this: PCR and don't need a big sample • abnormalities and lethality associated with monosomy and trisomy suggest tight dosage control, with no dosage compensation i.e only 1 transcriptionally active X –so XX females make same amount of product as XY males ◦ dosage compensation: if have 3 copies of gene doesn't necessarily mean get more than if had 2 copies of the gene ◦ XX females make same amount of product as XY males (X down regulating) infertile Problem:metaphase duringmeiosis - breakage ofspindle - failure ofcentromeres to split → Most disjuctionproblems happeninthe meiosis I, but those in meiosis II more commonlyknown Chromosomal structural aberrations • The most frequent structural aberrations include: deletions, duplications, inversions, and translocations. • These abnormalities of structure can give rise to abnormalities in meiotic segregation of chromosomes as well as further structural abnormalities after crossing over. • Translocations can be reciprocal or non-reciprocal ◦ DNAcan be moved quite a long distance through a translocation Things to know about changes in chromosome structure • Each chromosome is a single dsDNAmolecule • First event in rearrangement (normally) – two or more double-strand breaks • Double strand breaks are potentially lethal unless repaired • Repair systems try to join broken ends together • The only chromosome rearrangements that survive are those that produce a • DNAmolecule with a centromere and 2 telomeres • Acentric chromosomes (lacking centromere): lost ◦ if not at the middle during metaphase, chromosome is lost • Dicentric: 2 centromeres – pulled in opposite directions – usually break • Duplications or deletions affect gene balance ◦ the larger the duplication or deletion the larger the problem Unbalanced rearrangements • deletions – can be within a gene (intragenic) or multigenic (many genes) takenout C and D • duplications – can be within same chromosome or different chromosome ◦ may express protein at a deletrious level • Balanced Rearrangements: Change gene order but do not remove or duplicate DNA ◦ results from 1) inversions 2) reciprocol translocations – 2 non-homologous chromosomes break and rejoin. Chromosome abnormalities • error in the cell division • groups: 1) Change in the amount of chromosome (piece or whole) Loss (deletion) Gain (duplication) e.g. Trisomy 21 (Down syndrome) 2) Change in the structure of chromosome Translocation (change of location) Inversion (change of orientation Summary Origins of chromosomal rearrangements → most of the time come from non-allelic, non-homologous recombination → human genome ~ 45% repetitive DNA: gives lots of oppertunities to find different sequences that are similar Chromosome banding Technique • the chemical treatment of chromosomes to reveal characteristic patterns of horizontal light and dark bands, p-arm e.g. G-banding. • a suitable dye or enzyme is applied after cells have been arrested during cell division by chemicals. • analysis of distribution of bands on individual chromosomes allows the identification of structural chromosomal abnormalities. ◦ created a numerical map of the bands q-arm Through banding pattern Can visualize deletion in Chromosome 5 in Cri du chat syndrome → changed shortened end to become telomere • Down’s syndrome can also result from a Robertsonian translocation • Carried by one of the parents germline – almost a complete chromosome 21 translocated. q-armof21 joinrs onto q- armof14 Unaffected childrenmaybe normal Onlytwo mutations involved directlyin or Translocationcarriers DownSyndrome (?) theycode for a cell adhesionmolecule, proteinkinase The chimeric fusion proteins • the chimeric gene, thro
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