Inside the nucleus, DNA does not float freely. It wraps around histone proteins and folds into compact thread-like structures called chromosomes. Each chromosome contains a single, long DNA molecule plus the proteins that package and regulate it.

Human somatic cells normally contain 46 chromosomes—23 pairs. During cell division the chromosomes condense further and become visible under a light microscope. Centromeres hold sister chromatids together until they separate. Telomeres protect the ends from degradation.

Genes are arranged linearly along chromosomes. The location of a gene is its locus. Chromosome number and structure are species-specific; disruptions cause a range of genetic conditions. Karyotyping arranges stained chromosomes by size and banding pattern for clinical analysis.

• Thread-like structure of DNA and proteins
• Carries genes in linear order
• Humans have 46 in somatic cells
• Condenses during cell division