Chromosomes

Chromosomes are large, discrete structures that package and organize genetic material in the form of DNA and proteins. They carry the genes that govern development, physiology, and inheritance. The number and structure of chromosomes vary among species.

In eukaryotes, chromosomes are linear DNA molecules housed in the nucleus. DNA winds around histone proteins to form nucleosomes and higher-order chromatin. Each chromosome has a centromere, which constrains the shape, and telomeres at the ends that protect chromosome ends. The region where DNA replication begins is the origin of replication, and the chromosome is partitioned into arms labeled p (short) and q (long).

In humans, somatic cells are diploid with 46 chromosomes: 22 autosomes and 1 pair of sex chromosomes (XX in typical females and XY in typical males).

During cell division, chromosomes are replicated; sister chromatids remain attached at the centromere until anaphase. Mitosis distributes identical copies to two daughter cells, while meiosis produces haploid gametes with genetic recombination.

Prokaryotes usually have a single circular chromosome located in the nucleoid, and many also carry smaller circular DNA molecules called plasmids. Eukaryotic chromosomes are linear and include regulatory elements and origins of replication.

Chromosome abnormalities, such as aneuploidy (extra or missing chromosomes) or structural changes (deletions, duplications, inversions, translocations), can cause developmental and health disorders. Karyotyping and molecular tests help diagnose these conditions. The organization of chromosomes within the nucleus influences gene expression through chromatin structure, with regions of active euchromatin and repressive heterochromatin shaping access to the genetic code.