Chromosome

A eukaryotic chromosome is a highly organised nuclear structure composed of DNA and associated proteins, forming the fundamental unit of genetic organisation within eukaryotic cells. These chromosomes contain part or all of an organism’s genome and display a dynamic architecture that influences gene expression, cell division, heredity and overall cellular function.

Structural Organisation of Eukaryotic Chromosomes

Eukaryotic chromosomes consist of long, linear DNA molecules tightly associated with histone proteins, creating a compact and stable framework known as chromatin. Histones, with the assistance of chaperone proteins, bind to DNA to form nucleosomes, the primary structural units that enable condensation and maintain genomic integrity. A DNA segment wraps around a histone octamer to form a nucleosome, producing a characteristic ‘beads-on-a-string’ appearance corresponding to the 10 nm chromatin fibre.
Further coiling leads to the formation of the 30 nm fibre, a higher-order structure found predominantly in euchromatin during interphase. These fibres fold into loops and ultimately form the highly condensed metaphase chromosome visible during cell division. Each eukaryotic chromosome contains a centromere, which defines the kinetochore attachment site and can generate characteristic shapes depending on its position, such as metacentric, submetacentric or acrocentric forms.
In addition to nuclear chromosomes, most eukaryotes possess a maternally inherited mitochondrial genome that exists as a circular DNA molecule. Some organisms may also contain additional small cytoplasmic chromosomes.

Chromatin States and the Cell Cycle

During interphase, chromatin exists in two distinct states. Euchromatin represents the transcriptionally active regions where DNA is relatively decondensed, permitting gene expression. Heterochromatin, in contrast, is tightly packed and transcriptionally inactive. Constitutive heterochromatin, typically found near centromeres, contains repetitive DNA sequences and remains permanently silent. Facultative heterochromatin may vary in its activity depending on developmental or environmental conditions.
As cells enter mitosis or meiosis, chromatin undergoes progressive condensation. The transcriptional machinery becomes inactivated, and the chromosomes assume a compact and transportable configuration suitable for accurate segregation. Chromosomes reach their maximum condensation during anaphase in many animal cells.

Replication and Chromosome Duplication

Before a cell divides, each chromosome is replicated during the S phase of the cell cycle. The two identical DNA copies, termed sister chromatids, remain attached at the centromere. Depending on the centromere’s position, the metaphase chromosome may appear X-shaped or present a two-armed morphology. These chromatids separate during mitosis or meiosis, ensuring the accurate distribution of genetic material to daughter cells.
Errors in chromosome duplication or segregation may lead to chromosomal instability, which can result in translocations, aneuploidy or mitotic catastrophe. If damage is extensive, the cell typically activates apoptotic pathways. However, mutations that suppress these pathways may allow defective cells to survive, contributing to oncogenesis.

Three-Dimensional Chromosomal Architecture

Eukaryotic chromosomes occupy defined territories within the nucleus rather than existing in a random distribution. This spatial arrangement forms a complex three-dimensional network that influences transcriptional regulation, DNA repair and genome stability. Chromosome territories ensure that interactions between enhancers, promoters and regulatory proteins occur with high precision, contributing to the fidelity of gene expression.

Etymology and Early Terminology

The term ‘chromosome’ derives from the Ancient Greek words for ‘colour’ and ‘body’, referring to the strong staining properties of chromosomal material when treated with specific dyes. Heinrich Wilhelm Waldeyer introduced the term, building upon Walther Flemming’s earlier description of chromatin. Early cytological terminology has since evolved, though historical terms such as chromosom or early references to chromatin underscore the initial emphasis on staining characteristics rather than molecular composition.

Historical Discovery and Development of Chromosome Theory

The structures now recognised as chromosomes were first identified by Otto Bütschli in the late nineteenth century. Theodor Boveri made seminal contributions by demonstrating chromosome individuality and continuity, establishing that each chromosome carries a unique genetic load. These ideas, supported by Wilhelm Roux’s earlier hypotheses, laid the groundwork for modern chromosome biology.
The rediscovery of Mendel’s work at the beginning of the twentieth century allowed Boveri to connect Mendelian inheritance to chromosomal behaviour. His findings influenced American cytologists such as Edmund Beecher Wilson, Nettie Stevens, Walter Sutton and Theophilus Painter, many of whom collaborated closely with him.
Wilson later synthesised the work of Boveri and Sutton into the Boveri–Sutton chromosome theory of inheritance, which posited that chromosomes are the carriers of hereditary information. Although initially contested by figures including William Bateson, Wilhelm Johannsen, Richard Goldschmidt and T. H. Morgan, the theory gained definitive support through chromosomal mapping conducted in Morgan’s laboratory.
An important milestone in human cytogenetics was the determination of the correct human chromosome number. In 1923 Painter incorrectly reported 48 chromosomes, a figure accepted for decades. In 1956 Joe Hin Tjio’s research established the accurate number as 46, forming the basis of modern human karyotyping.

Comparison with Prokaryotic Chromosomes

Prokaryotes differ significantly from eukaryotes in chromosome structure and organisation. Bacteria and archaea usually possess a single circular chromosome, though some species may have linear chromosomes or multiple replicons. Genome sizes vary widely, from extremely reduced endosymbiotic genomes of only about 130,000 base pairs to more than 14 million base pairs in certain soil-dwelling bacteria.
Prokaryotic genes are commonly arranged in operons and rarely contain introns. DNA replication typically initiates from a single origin in bacteria, whereas archaea may possess multiple origins similar to eukaryotes.
Because prokaryotes lack a nucleus, their chromosomes reside within the nucleoid, a defined but flexible region of the cytoplasm. This structure is maintained by histone-like proteins in bacteria and by proteins resembling eukaryotic histones in archaea. Prokaryotes often carry additional DNA molecules such as plasmids, which facilitate horizontal gene transfer and contribute to adaptation, antibiotic resistance and metabolic versatility.
Supercoiling is a common structural feature of both prokaryotic chromosomes and plasmids. To permit transcription and replication, DNA must be temporarily relaxed by topoisomerase enzymes, ensuring controlled access to the genetic code.

Significance of Chromosomes in Genetic Processes

Chromosomes perform essential roles in heredity, gene expression and genome stability. Their structural organisation ensures that genetic information is accurately stored, replicated and transmitted across generations. Meiotic recombination reshuffles genetic material, promoting diversity and enabling evolutionary adaptation. Cellular mechanisms that maintain chromosomal integrity protect against mutations and chromosomal abnormalities, though errors in these processes can have profound consequences, including developmental disorders and cancer.