20. Submicroscopic and microscopic structure of chromosomes. Human karyotype. Methods of karyotyping and chromosome analysis.

Question 1

Submicroscopic and microscopic structure of chromosomes

Answer 1

General:

-Structure: Dense, compact; visible during metaphase.
-Shape: Rod-shaped, straight or bent.
-Staining: Basophilic (stains with hematoxylin).
-Size: Length 3-8 μm; thickness 0.5-2 μm.

Chromosome Parts:
-Sister Chromatids: Each chromosome has two, each a single DNA molecule.

-Arms: Short and long sections from centromere to telomere.

-Centromere: Attachment point for sister chromatids; primary constriction.

-Kinetochore: Links centromere to spindle microtubules during division.

-Telomeres: Ends of chromosomes, contain DNA repeats; maintained by telomerase.

-Telomerase: Inactive in adult somatic cells, leading to telomere shortening and aging; activation in somatic cells can cause tumors.

-Secondary Constriction: (Nucleolar organizer) Contains genes for rRNA, separates satellite region.

Question 2

Human karyotype

Answer 2

Definition:

Karyotype: Number, size, and shape of metaphase chromosomes in a eukaryotic cell.
Species Specific: Persistent and unique to each species.

Human Karyotype:

Total Chromosomes: 46 (22 pairs of autosomes + 1 pair of sex chromosomes, XX or XY).

Chromosomal Diseases:

-Down Syndrome: Trisomy 21, karyotype 47 chromosomes.
-Klinefelter Syndrome: Boys with karyotype 47XXY.
-Turner Syndrome: Girls with karyotype 45X0.

Question 3

Methods of karyotyping and chromosome analysis

Answer 3

Preparation of Microscopic Preparations:

1. Cells Used: Dividing cells in metaphase.
   -Direct Method: High mitotic activity cells (cornea, thymus, bone marrow).
   -Indirect Method: Cultured cells (peripheral blood lymphocytes, skin/lung fibroblasts, fetal cells).

2.Culturing: Use medium with phytohemagglutinin to stimulate division.

3. Metaphase Arrest: Add colchicine to stop division.
4. Cell Swelling: Suspend cells in hypotonic solution (potassium chloride or sodium citrate).

5.Fixation and Staining: Observe unique dark and light bands for chromosome identification.

6.Microscopy: View, photograph metaphase plates, and create karyogram.

Staining Methods:

-Total Staining: Romanowski-Giemsa method stains entire chromatin.
-Differential Staining (Banding): Shows light and dark bands (hetero- and euchromatin).
-G-Banding: Dark bands (heterochromatin), light bands (euchromatin). Essential for identifying homologous pairs and diagnosing mutations.