Chromosome
A chromosome is a tightly coiled structure of DNA and protein that carries many genes and is passed to new cells during division.
Definition
A chromosome is a bundle of DNA wound tightly around proteins called histones so it fits inside the nucleus, carrying hundreds or thousands of genes along its length. Humans have $46$ chromosomes arranged as $23$ pairs, $22$ pairs of autosomes plus one pair of sex chromosomes (XX in females, XY in males), while during division each chromosome appears as two identical sister chromatids joined at a centromere; prokaryotes carry a single circular chromosome instead. At the molecular level a eukaryotic chromosome is a linear DNA duplex packaged into nucleosomes and folded into chromatin fibers and loop domains, with a centromere for spindle attachment and telomeres of repetitive DNA protecting its ends, and whether that chromatin is open (euchromatin) or densely packed (heterochromatin) controls which genes are accessible. Mitosis compacts chromosomes roughly $10{,}000$-fold, and abnormalities in their number (aneuploidy, such as trisomy 21) or structure (deletions, duplications, inversions, translocations) can cause disease.
Example
Chromosome number does not track complexity: humans have $46$, dogs have $78$, a fruit fly has only $8$, and a fern can exceed $1{,}000$, far more than a human. The two chromosomes in a human pair carry the same genes at the same positions, one from each parent, and chromosome 1 alone carries about $250$ million base pairs and more than $2{,}000$ genes. Telomeres shorten by roughly $50$ to $100$ base pairs with each division in most somatic cells, one limit on replicative lifespan, while the Philadelphia chromosome, a translocation between chromosomes 9 and 22, creates the BCR-ABL fusion gene that drives chronic myeloid leukemia.
Key Insight
Sorting $46$ packages during cell division is far more reliable than sorting $20{,}000$ loose genes, which is exactly the problem chromosomes solve. Packaging is itself a form of regulation: a gene buried in dense heterochromatin stays silent even with a perfect sequence, which is why a liver cell and a neuron can use an identical genome to build entirely different structures.