A chromosome is an organized structure of DNA and associated proteins that carries genetic information within a cell. Its DNA contains genes and other sequences involved in the organization, maintenance, and use of the genetic material. Chromosomes serve as units of DNA replication and distribution during cell division. In eukaryotes, nuclear chromosomes typically contain long, linear DNA molecules; most bacterial chromosomes are circular. The name derives from Greek words meaning “color” and “body,” referring to their strong staining with certain dyes. (genome.gov)
Molecular organization
In the cell nucleus, DNA and its associated proteins form chromatin. DNA wraps around histone proteins to form nucleosomes, the basic repeating units of chromatin. Further folding and interactions with other proteins organize this material into compact structures. Packaging both accommodates long DNA molecules within the nucleus and influences how readily cellular machinery can access them. (genome.gov)
A chromosome is not necessarily an X-shaped object. During interphase, when a cell is not undergoing nuclear division, chromosomes are relatively extended and usually cannot be distinguished individually by ordinary light microscopy. They nevertheless remain chromosomes. Before and during division, their chromatin becomes more condensed, producing the recognizable structures commonly illustrated in textbooks. Chromatin describes the material composing nuclear chromosomes rather than an entirely separate form of genetic material. (ncbi.nlm.nih.gov)
Typical linear nuclear chromosomes possess multiple origins at which DNA replication begins, a centromere region that supports chromosome segregation, and telomeres at their ends. Telomeres help protect chromosome ends from degradation and inappropriate joining. The centromere provides the site for assembly of the kinetochore, a protein complex that connects a chromosome to the division spindle. Its position helps distinguish chromosome shapes and separates the short and long chromosome arms, conventionally designated p and q. (ncbi.nlm.nih.gov)
Replication and cell division
Before a typical cell divides, each chromosome’s DNA is copied. The resulting replicated chromosome consists of two sister chromatids, each containing a DNA molecule with essentially the same sequence. Sister chromatids remain associated until the appropriate stage of division. Thus, replication doubles the DNA content without immediately doubling the chromosome count: the attached sister chromatids are conventionally counted as one replicated chromosome. Once separated, each becomes an individual daughter chromosome. (ncbi.nlm.nih.gov)
During mitosis, replicated chromosomes condense, attach to spindle microtubules through their kinetochores, and align before their sister chromatids separate. The spindle distributes one sister from each chromosome to each daughter nucleus. Nuclear envelopes subsequently re-form, and the chromosomes become less condensed. This process generally preserves the chromosome complement through successive rounds of cell division. (ncbi.nlm.nih.gov)
Meiosis differs by using two divisions following one round of DNA replication to reduce the number of chromosome sets. Homologous chromosomes—corresponding chromosomes inherited from the two parents—separate in the first division, whereas sister chromatids separate in the second. In animals, this process contributes to the formation of eggs and sperm carrying a single chromosome set. Fertilization restores the paired complement. (genome.gov)
During meiosis, paired homologues can exchange corresponding DNA segments through crossing over. This produces new combinations of alleles, the alternative versions of genes, rather than simply transmitting each parental chromosome unchanged. Homologous chromosomes therefore should not be confused with sister chromatids: homologues have corresponding genetic regions but can carry different alleles, whereas sisters originate through replication of the same chromosome. (genome.gov)
Chromosome complements and biological diversity
Chromosome number and form vary among organisms. Humans typically have 46 nuclear chromosomes in most nucleated body cells, arranged as 23 pairs. These comprise 22 pairs of autosomes and one pair of sex chromosomes. Eggs and sperm normally contain 23 chromosomes. Common human sex-chromosome complements include XX and XY; the X chromosome carries many genes beyond those involved in sexual development. (medlineplus.gov)
Most bacteria possess one or two circular chromosomes rather than the multiple linear chromosomes characteristic of animal and plant nuclei. Bacterial DNA is also highly organized: DNA-binding proteins, folding, and supercoiling contribute to its compaction and distribution within the cell. Chromosome organization is therefore not restricted to organisms with nuclei. (genome.gov)
The nuclear chromosomes do not contain all the DNA of a human cell. The mitochondria contain a smaller, circular chromosome, which forms part of the human genome alongside the nuclear chromosome complement. Unlike nuclear chromosomes, human mitochondrial DNA is ordinarily inherited maternally. (genome.gov)
Chromosomal variation
Chromosomal changes can affect either number or structure. Numerical changes include additional or missing individual chromosomes. For example, trisomy 21 involves an additional copy of chromosome 21 and is associated with Down syndrome. Such changes differ in scale from alterations affecting only a small DNA sequence within a gene. (genome.gov)
Structural changes include deletions, duplications, inversions, and translocations. A deletion removes material, a duplication adds a copy, an inversion reverses a segment’s orientation, and a translocation transfers material between chromosomes. Their consequences depend on the affected regions, disruption of gene function, and whether genetic material is gained or lost. Some rearrangements have no apparent effect on health. Changes may be inherited or arise in reproductive cells, during development, or later in body cells; many cancers contain acquired chromosomal rearrangements. (medlineplus.gov)
Observation and analysis
Cytogenetics studies chromosome number, structure, and behavior. A karyotype denotes an individual’s chromosome complement and also an ordered laboratory image of chromosomes. Chromosomes are identified using size, staining-band patterns, and centromere positions. Karyotype analysis can reveal numerical differences and visible structural rearrangements, and is used in studies of development, tumors, and evolutionary relationships. It describes chromosomes at a different scale from analysis of individual DNA sequences. (genome.gov)