Definition and number
Chromosomes are thread‑like nuclear structures that carry genes.
Human somatic cells contain 46 chromosomes arranged as 23 pairs – 22 autosomal pairs and one pair of sex chromosomes (XX in females, XY in males).
Karyotyping
Chromosomes are stained, spread on a slide and examined under a microscope.
The arranged set (karyotype) reveals the size, shape and number of each chromosome and helps detect numerical or structural defects.
Barr body
In female cells one of the two X chromosomes becomes transcriptionally inactive and condenses to form a Barr body.
Presence of a Barr body indicates a cell is genetically female (XX).
Chromosomal abnormalities
Abnormalities arise from changes in chromosome number (aneuploidy) or structure (deletions, duplications, translocations, inversions).
Examples important for exams:
Down syndrome – trisomy 21 (extra chromosome 21)
Turner syndrome – monosomy X (45,X)
Klinefelter syndrome – extra X in males (47,XXY)
Role in cell division – mitosis (somatic)
Prophase → Metaphase → Anaphase → Telophase → Cytokinesis
During prophase chromosomes condense; at metaphase they align at the equatorial plate; anaphase separates sister chromatids; telophase reforms nuclei and cytokinesis divides the cytoplasm, giving two daughter cells with the same 46 chromosomes.
Role in cell division – meiosis (germ)
Meiosis I: Prophase I → Metaphase I → Anaphase I → Telophase I → Cytokinesis (produces two cells, each with 23 chromosomes)
Meiosis II: Prophase II → Metaphase II → Anaphase II → Telophase II → Cytokinesis (produces four haploid gametes).
Simple text diagram of a chromosome
|====| short arm (p) | | |====| centromere | | |====| long arm (q)
The diagram shows the two arms (p and q) joined by the centromere, the basic unit that ensures equal segregation during cell division.