Variation in chromosome structure and number

• Cytogenetics


  Study of the structure and normal and pathological functioning of chromosomes
• Cytogeneticists


  • Use 3 main features to identify and classify chromosomes: location of the centromere, size, banding patterns
• Centromeric locations


  • Metacentric
  • Submetacentric
  • Acrocentric
  • Telocentric
• Metacentric


  Centromere present in the middle of chromosome and divides chromosomes into 2 equal arms
• Submetacentric


  Centromere located near the middle towards the top
• Acrocentric


  Centromere located near the end of the chromosome
• Telocentric


  Centromere located on one end such that the p arms are not, or barely visible
• 1. banding


  Allows each chromosome to be identified by its characteristic banding pattern, can distinguish chromosomal abnormalities or structural rearrangements
• Chromosomal abnormalities


  • Deletions
  • Duplications
  • Inversions
  • Translocations
  • Aneuploidies
• Deletions


  Loss of chromosomal segment when a chromosome breaks and a fragment is lost, can be terminal or interstitial
• Duplication


  Repetition of a chromosomal segment compared to the normal parent chromosomes caused by abnormal events during recombination
• Inversions


  Change in the direction of part of the genetic material along a single chromosome
• Translocations


  • Simple translocation
  • Reciprocal translocation
• Aneuploidies


  Loss or gains of entire chromosomes
• Cytogenetics in the diagnostic setting

  • Constitutional abnormalities
  • Acquired abnormalities
• Constitutional abnormalities


  Can be passed down to the next generation, e.g. Turner syndrome, Klinefelter Syndrome, Down syndrome
• Acquired abnormalities


  Cancer diagnosis and research
• Leukaemia


  Cancer of the white blood cells, develops when a blood cell undergoes transformation into malignant cell capable of uncontrolled growth
• Leukaemia is a clonal disease
• Chromosomal abnormalities in leukaemia

  • Numerical abnormalities
  • Structural abnormalities
• Differences in specific genes are denoted by the term allelic variation
• Transposable elements are examples of repetitive sequences
• Homologous genes are two or more genes that are derived from the same ancestral gene
• Comparative genomic hybridization can be used to detect deletions and duplications in chromosome structure
• DNA segments of 1000 bp or more in length which exist in differing numbers of copies among individuals within the same species are described by the term copy number variation (CNV)
• The process which involves the formation of a double-stranded DNA molecule from single-stranded DNA molecules of two different samples is termed hybridisation
• Inversions are classified based on the position of the centromere
• A piece of a chromosome that has lost its centromere is called an acentric fragment
• Telomeres are repeated sequences of DNA that are found at the end of chromosomes
• An inversion is a chromosomal structural change in which a segment of DNA is rotated 180 degrees
• An organism with a chromosome number that is an exact multiple of a chromosome set is referred to as euploid
• 3 sets of chromosomes = triploid, 4 sets of chromosomes = tetraploid