Cytogenetics and DNA Overview

Cytogenetics
  • Definition: Branch of genetics that studies the organization and arrangement of genes and chromosomes using microscopy techniques.
Karyotyping
  • Process:
    • Sample of cells taken from sources like blood smears (WBCs), skin, cheek cells, amniocentesis, chorionic villi.
    • Cells are fixed (development arrested) and stained to visualize chromosomes.
    • A picture is taken during metaphase and chromosomes are arranged into pairs (1-22 plus sex chromosomes).
  • Purpose:
    • Detect alterations in chromosome number (used for sex determination).
    • Identify gross structural abnormalities through banding patterns.
Variations in Chromosome Number
  1. Aneuploidy: Less than the standard number of chromosomes (46).
    • Example: Turner Syndrome (45, X).
  2. Polyploidy: More than the standard number of chromosomes.
    • Example: Klinefelter Syndrome (47, XXY).
  3. Trisomy: Presence of an extra chromosome.
    • Example: Trisomy 21 (Down syndrome).
Structural Abnormalities in Chromosomes
  • Identified via banding patterns:
    1. Inversion: A segment of a chromosome is inverted.
    • Example: ABCDE appears as ADCBE. (All genes are still present).
    1. Deletion: A segment of a chromosome is missing.
    • Example: ABCDE appears as ABE. (Missing genetic info can affect phenotype).
    1. Translocation: Segments from one chromosome are swapped with segments from another.
    • Example: ABCDE appears as ABF G C D HIJ. (All genes present, but deletions might occur at cell division).
Study Questions: DNA
  1. Griffith, Avery, MacLeod, and McCarty's Contribution: Demonstrated that DNA is the genetic material through experiments that showed transformation in bacteria.
  2. Components of a Nucleotide:
    • Three components: Sugar, Phosphate group, Nitrogenous base.
    • Four nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).
    • Nucleotides form a double-helix structure, resembling a twisted ladder with sugars and phosphates forming the sides and nitrogenous bases forming the rungs.
  3. Watson and Crick's Contribution: Utilized data from Chargaff's base pairing rules and X-ray diffraction images. They arranged molecular shapes for the double helix structure, winning the Nobel Prize.
  4. Chargaff’s Rules: Proved that A pairs with T and C pairs with G by comparing base ratios from different organisms.
  5. DNA Structure Drawing:
    • Label: Sugars, Phosphates, Nitrogenous Bases, Phosphodiester bonds, Hydrogen bonds.
  6. Duplicated Chromosome Drawing:
    • Label: Telomere, Centromere, Sister Chromatid, Duplicated Chromosome.
    • Define homologous pair; total pairs in the human nucleus: 23.
  7. Karyotype Definition: A visual representation of an individual's chromosomes.
    • Karyotyping Process: Performed using metaphase chromosomes from cell samples; searching for abnormalities or variations in number.
  8. Chromosome Number Variations: Can observe aneuploidy and polyploidy through karyotyping.
  9. Chromosomal Abnormalities: Identified via specific banding patterns (inversions, deletions, translocations).
  10. Danger of Chromosomal Deletions: Often more critical than translocations since they can lead to loss of essential genetic information affecting gene function, whereas translocations may retain genetic material.