Genetics · MU Sofia

3. The human karyotype: chromosomal heteromorphism and its practical significance; mitochondrial chromosomes.

Inside: 19 sections · 11 labelled pictures · 16 flashcards · 9 min

Topic 3 of 79 in the MU Sofia syllabus for Genetics (Medicine, Year 3). The free AceMed note covers it in 18 sections, about 1,240 words (9 min of reading), with 16 flashcards.

In this note

  1. Topic 4 – Chromosomes: Structure, Morphology, Karyotype, and Heteromorphism
  2. I. Introduction
  3. II. Microscopic Structure (Light Microscopy Level)
  4. III. Submicroscopic Structure (Molecular/Ultrastructure)
  5. Chromatin Types
  6. IV. Chromosomal Morphology
  7. Banding Patterns (Staining Techniques)
  8. V. Normal Human Karyotype
  9. VI. Chromosomal Heteromorphism
  10. VII. Functional Aspects of Chromosomes
  11. VIII. Key Points
  12. Topic 3 – Organization of the Mitochondrial Genome
  13. II. General Features of the Human Mitochondrial Genome
  14. III. Gene Content
  15. IV. Replication and Transcription
  16. V. Inheritance and Mutation
  17. VI. Clinical Relevance
  18. VII. Key Points

I. 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)

When 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 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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