Life sciences · Reference
What is a chromosome?
A chromosome is an organised, thread-like structure of DNA wound around proteins that carries an organism’s genes — the package in which genetic material is stored and passed on when cells divide.
Structure
A chromosome consists of one very long DNA molecule wrapped around histone proteins to form a material called chromatin. This packaging condenses DNA that would be metres long into a structure that fits within a microscopic cell nucleus. When a cell prepares to divide, the chromatin condenses further into the familiar X-shaped chromosomes visible under a microscope, each made of two identical copies (sister chromatids) joined at a region called the centromere, with protective caps called telomeres at the ends.
Chromosomes and inheritance
Genes are arranged in a fixed order along each chromosome, so a chromosome can be thought of as a long list of genes. Most human cells carry two copies of each chromosome — one inherited from each parent — making 23 pairs, or 46 in total.
Of these, 22 pairs are autosomes and one pair determines biological sex. Carrying chromosomes in matched pairs means most genes are present in two copies, which is the basis of allele inheritance.
Significance in research
Studying chromosomes — their number, structure, and behaviour — is the field of cytogenetics. Faithful copying and separation of chromosomes during cell division is essential, and errors in chromosome number or structure are an important subject of biological research. Reference genomes are organised by chromosome, so consistent chromosome naming and coordinates are central to making sequence data interoperable and reusable.
Key facts
At a glance
- Definition: an organised structure of DNA and protein carrying genes
- Made of: DNA wound around histone proteins (chromatin)
- Human chromosomes: 23 pairs (46 total)
- Autosomes vs sex chromosomes: 22 pairs plus 1 pair
- Key landmarks: centromere and telomeres
- Function: packages the genome for storage and division
Common questions
FAQ
What is a chromosome?+
A chromosome is a long molecule of DNA coiled around proteins into a compact structure that carries many genes. Chromosomes organise the genome so it can be stored, copied, and divided accurately when cells reproduce.
How many chromosomes do humans have?+
Most human cells contain 46 chromosomes, arranged as 23 pairs — one of each pair inherited from each parent. Twenty-two pairs are autosomes and one pair are the sex chromosomes.
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