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Lecture 8

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Queen's University
BIOL 205

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, e.g., – 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 – Origins of aneuploidy most frequently lie in meiotic nondisjunction Common human aneuploidies  There are no viable human monosomies except that of the X chromosome: XO, 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.  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. XO Turner Syndrome XXY Klinefelters 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 Things to know about changes in chromosome structure • Each chromosome is a single dsDNA molecule • 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 DNA molecule with a centromere and 2 telomeres Acentric chromosomes (lacking centromere)—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) A B C D E A B E Loss of A b c d E heterozygosity A b c d E Deleted fragments are lost • Duplications– can be within same chromosome or different chromosome A B C D E A B C C D E Balanced Rearrangements • Change gene order but do not remove or duplicate DNA Results from 1) inversions A B C D E A C B D E 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 Non-allelic Non-homologous Origins of chromosomal rearrangements recombination Human genome ~ 45% repetitive DNA Will focus on deletions and translocations! Chromosome banding Technique • The chemical treatment of Sub-band #3 chromosomes to reveal characteristic Band #4 Region #3 pa erns of horizontal light and dark bands, 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
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