A chromosome is a package of DNA that contains part or all of the genetic material of an organism. In eukaryotic cells, chromosomes are long linear DNA molecules bound to histones and other proteins and are organized within a membrane‑bound nucleus. Prokaryotes are typically haploid and possess a single circular chromosome located in a region of the cytoplasm called the nucleoid. Viruses may have one or more DNA or RNA molecules that are sometimes referred to as chromosomes.
Explanation
The DNA in a chromosome carries genes that encode proteins, enzymes and stable RNAs required for cellular function. Eukaryotic chromosomes are composed of chromatin—DNA wrapped around histone proteins—and may have millions of base pairs. Each chromosome has structural features such as centromeres, which are attachment points for spindle fibers during mitosis and meiosis, and telomeres at the ends that protect against degradation. During the cell cycle, chromosomes duplicate and condense so that sister chromatids can segregate evenly during cell division. Most eukaryotes are diploid, carrying two copies of each chromosome; variations in chromosome number can lead to genetic disorders. Bacterial and archaeal chromosomes, by contrast, are usually a single circular double‑stranded DNA molecule without true histones; these chromosomes must be supercoiled and compacted to fit within the small cell and are anchored to the plasma membrane at the origin of replication. Because prokaryotes are haploid, mutations expressed in their single chromosome can have immediate phenotypic effects. Some bacteria also carry plasmids—small, extrachromosomal DNA molecules—that can provide antibiotic resistance or other traits. Understanding chromosome organization has illuminated mechanisms of gene regulation, replication, recombination and heredity.
Examples and Significance
The intestinal bacterium Escherichia coli has a single circular chromosome of about 4.6 million base pairs, while Bacillus subtilis has a slightly larger chromosome. Saccharomyces cerevisiae, a unicellular yeast used in baking, has sixteen linear chromosomes. Humans have forty‑six chromosomes arranged in twenty‑three pairs; karyotyping reveals abnormalities such as trisomy 21 (Down syndrome) or structural rearrangements. In microbiology, antibiotic drugs like fluoroquinolones inhibit bacterial topoisomerases that maintain chromosome supercoiling. Viruses such as influenza have segmented RNA genomes referred to as chromosomes, and reassortment of segments can create new strains. Chromosome analysis is fundamental to genetics, taxonomy, evolutionary biology and clinical diagnostics.
Chromosomes are the carriers of hereditary information in living organisms. Their organization, number and structure influence how genes are expressed and inherited and form the basis for understanding cell division, evolution and disease.