4. Microscopic and submicroscopic structure and function of chromosomes. Chromosomal morphology. Normal human karyotype. Chromosomal heteromorphism.
Covers the structure and function of chromosomes, from their condensed chromatin form and sister chromatids through centromeres, telomeres and secondary constrictions, to packaging levels from nucleosomes up to the metaphase chromosome. It then sets out chromatin types, centromere-based morphology classes, and G, Q, R and C banding techniques, before describing the normal 46-chromosome human karyotype and chromosomal heteromorphisms with examples of heterochromatic variants that carry no clinical significance.
Topic 4 of 65 in the MU Sofia syllabus for Genetics (Medicine, Year 3). The free AceMed note covers it in 11 sections, about 542 words (4 min of reading), with 15 flashcards.
In this note
Topic 4 – Chromosomes: Structure, Morphology, Karyotype, and Heteromorphism
I. Introduction
II. Microscopic Structure (Light Microscopy Level)
III. Submicroscopic Structure (Molecular/Ultrastructure)
Chromatin Types
IV. Chromosomal Morphology
Banding Patterns (Staining Techniques)
V. Normal Human Karyotype
VI. Chromosomal Heteromorphism
VII. Functional Aspects of Chromosomes
VIII. 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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