Updated: 5/2/2017

Chromosomal Structure

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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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