Chromosomes

Chromosomes are linear (eukaryote) or circular (prokaryote) DNA molecules. In eukaryotes:

• DNA wraps around histone proteins to form nucleosomes (~146 bp per histone octamer).
• Nucleosomes coil into chromatin fibres (30 nm).
• During mitosis, chromatin condenses further into visible chromosomes.

A replicated chromosome consists of two identical sister chromatids joined at a centromere. Sister chromatids separate during anaphase of mitosis.

Ploidy.

• Haploid (n) — one copy of each chromosome (gametes: sperm, eggs).
• Diploid (2n) — two copies of each chromosome (somatic cells, after fertilisation).
• Polyploid — more than 2 copies (common in plants; e.g. wheat is hexaploid).

Humans: n = 23, 2n = 46.

Homologous pairs. In diploid cells, each chromosome has a homologous partner. Homologous chromosomes:

• Are the same SIZE and SHAPE.
• Have the same GENES at the same LOCI.
• May have DIFFERENT ALLELES at those loci (e.g. one parent contributed I^A, the other I^B at the ABO locus).
• One chromosome from the mother, one from the father.

Exception: sex chromosomes. In humans, females are XX (homologous), males are XY (X and Y not strictly homologous — only a small pseudoautosomal region).

Karyotype = the number and appearance of chromosomes in a cell.

A karyogram displays them photographed during metaphase, sorted by size and arranged in homologous pairs. Used to detect:

• Aneuploidy — abnormal chromosome number.
• Chromosomal rearrangements (translocations, deletions).
• Sex determination.

Human autosomes vs sex chromosomes.

• Autosomes — pairs 1–22; non-sex chromosomes.
• Sex chromosomes — pair 23:
  • XX = female.
  • XY = male.

The Y chromosome carries the SRY gene which triggers testis development.

Aneuploidies in humans:

Condition	Karyotype	Features
Down syndrome	Trisomy 21 (47, XX+21 or 47, XY+21)	Intellectual disability, distinctive features, cardiac defects
Turner syndrome	Monosomy X (45, X0)	Female, short stature, infertility
Klinefelter syndrome	47, XXY	Male, reduced fertility, taller

Cause: non-disjunction during meiosis — chromosomes fail to separate. Maternal age increases risk (eggs paused in meiosis I from birth).

Prenatal diagnosis uses amniocentesis (16 weeks; small miscarriage risk) or chorionic villus sampling (11 weeks) — followed by karyotyping or chromosomal microarray.