2. Physical basis of heredity: chromosomes as the principal cellular structures of heredity, microscopic and submicroscopic organisation, behaviour during cell division · Genetics · MU Sofia

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.

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.