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Updated: May 2 2017

Chromosomal Structure

  • Overview
    • Centromere
      • holds sister chromatids together
      • attachment of mitotic spindle fibers
        • allows chromosomes to be pulled to opposite poles during anaphase
      • variations in position
        • metacentric
          • centromere in the middle
        • submetacentric
          • centromere offset slightly towards one end
        • acrocentric
          • near complete displacement of centromere to one end
    • Nomenclature
      • long = q
      • short = p
        • remember: p = petite
      • translocation = t
      • deletion = del
  • Types of chromosomal alterations
    • Nondisjunction
      • homologous chromatids do not separate properly during meiosis
        • stage of nondisjunction affects gamete production outcome
          • nondisjunction in meiosis I results in 2 gametes with x 2 and 2 gametes x 0
          • nondisjunction in meiosis II results in 2 normal gametes, 1 gamete x 2 and 1 gamete x 0
        • zygote receiving 3 copies = trisomy
        • zygote receiving 1 copy = monosomy
      • risk greatly ↑ with ↑ in maternal age
        • more common in oogenesis than spermatogenesis
    • Translocation
      • exchange genetic info between nonhomologous chromosomes by breakage and repair
        • balanced
          • where exchanged fragment is still functional on another chromosome
        • unbalanced
          • where exchanged fragment cannot function properly
      • common in cancers
      • types
        • Robertsonian
          • balanced
          • always involve two acrocentric chromosomes
            • 13, 14, 15, 21, 22
          • results in loss of short arm and fusion of two long arms of different chromosomes
          • no clinical presentation because short arms of acrocentrics contain no vital info
            • is a translocation carrier
            • problems with gametogenesis and therefore reproduction
              • miscarriage, aneuploidy
              • depends on how chromosomes segregate during homologous pair separation
        • reciprocal
          • exchange of DNA between two non-homologous chromosomes
            • as long as no DNA is lost the phenotype is normal for that generation
              • is a translocation carrier
    • Inversion
      • type of rearrangement where part of chromosome is inverted in orientation
      • types
        • pericentric
          • inverted chromosomal segment includes centromere
          • remember: pericentric involves centromere
        • paracentric
          • inverted chromosomal segment does not include centromere
    • Ring chromosomes
      • causes
        • product of two breakage sites on the chromosome and the segment lost circularizes
        • ends of chromosomes join circularizing entire chromosome
      • usually lost during gametogenesis → monosomy
    • Isochromosome
      • replication of one arm of a chromosome with loss of the other
        • p-q → p-p'
      • lethal for autosomes
        • can be observed on sex chromosomes
    • Deletions
      • loss of chromosome segment
      • types
        • terminal
          • end of chromosome
        • interstitial
          • within the chromosome
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