Medical Genetics · MU Plovdiv

5. Chromosomal diagnostics – methods (direct and indirect). Banding techniques – diagnostic possibilities. Chromosomal polymorphism. Heterochromatin markers. Sex chromatin – essence, diagnostic value.

Inside: 11 sections · 10 labelled pictures · 1 table · 29 flashcards · 7 min

Topic 5 of 33 in the MU Plovdiv syllabus for Medical Genetics (Medicine, Year 4). The free AceMed note covers it in 11 sections, about 1,015 words (7 min of reading), with 29 flashcards.

In this note

  1. Introduction
  2. I. Chromosomes – introduction
  3. II. Microscopic structure (light microscopy level)
  4. III. Submicroscopic structure (molecular / ultrastructure)
  5. IV. Chromosomal morphology
  6. V. Banding patterns (staining techniques)
  7. VI. Normal human karyotype
  8. VII. Chromosomal heteromorphism (polymorphism)
  9. VIII. Functional aspects of chromosomes
  10. IX. Key points
  11. X. Cytogenetic methods: FISH (fluorescence in situ hybridization)

Introduction

This topic covers the structure and morphology of chromosomes, banding techniques, the normal human karyotype, chromosomal heteromorphism (polymorphism, heterochromatin markers), sex chromatin (Barr body) and the cytogenetic methods used in chromosomal diagnostics, including molecular cytogenetics (FISH) and its comparison with other methods.

I. Chromosomes – introduction

  • Definition: chromosomes are the organized, highly condensed form of chromatin, visible during cell division (especially metaphase).
  • Function: store and transmit genetic information; ensure equal distribution of DNA during mitosis and meiosis; regulate gene activity through chromatin state.
  • Composition: DNA + histone proteins + non-histone proteins.
  • Each chromosome = 1 long DNA molecule (~50–250 million bp) coiled with proteins.

II. Microscopic structure (light microscopy level)

Observed under light microscopy (metaphase spreads, Giemsa-stained):

  • Chromatid: each chromosome consists of 2 sister chromatids (identical DNA copies after S phase).
  • Centromere (primary constriction): appears as a narrow “waist”; essential for spindle attachment (via kinetochore); defines p arm (short arm) and q arm (long arm).
  • Telomeres: repetitive DNA sequences (TTAGGG repeats in humans) at chromosome ends. Function: protect against degradation and end-to-end fusion. Maintained by the telomerase enzyme.
  • Secondary constrictions: e.g., nucleolar organizer regions (NORs) → sites of rRNA gene clusters (on acrocentric chromosomes 13, 14, 15, 21, 22).
  • Satellite bodies: small chromatin segments attached to acrocentric short arms by stalks; contain repeated rDNA.

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