chromosome
News •
-
What is a chromosome?
A chromosome is a compact, threadlike structure within a cell that carries hereditary information in the form of genes. Found in the nucleus of eukaryotic cells, chromosomes consist of DNA tightly wound around proteins.
Each species has a specific number of chromosomes. In humans, somatic cells contain 23 pairs, totaling 46. During cell division (mitosis), chromosomes duplicate, forming two identical chromatids joined by a centromere. These chromatids separate, ensuring each daughter cell receives a complete set of chromosomes. Chromosome abnormalities can lead to various disorders.
4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
Where are chromosomes found in a cell?
In eukaryotic cells, chromosomes are found within the nucleus, a specialized structure enclosed by a double membrane. The nucleus serves as the information center of the cell, housing the genetic code that determines the amino acid sequence of proteins.
Each chromosome consists of DNA tightly bound to proteins, forming a compact structure called chromatin. During cell division, chromatin condenses into a dense structure called a chromatid, which ensures accurate distribution of chromosomes to daughter cells.
Prokaryotic cells, like bacteria, lack a nucleus, and their DNA is not separated from other cellular components.
5 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What do chromosomes do in living organisms?
Chromosomes are microscopic structures within cells that carry hereditary information in the form of genes. They ensure accurate division and distribution of genetic material to daughter cells during cell division.
During cell division, chromosomes are replicated into two identical chromatids, which separate and are distributed to new cells. Chromosomes also determine the inheritance of characteristics, with autosomes controlling most traits and sex chromosomes determining sex-linked ones. Aberrations in chromosome structure or number can lead to genetic defects and diseases.
4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How are chromosomes related to genes and DNA?
Chromosomes, genes, and DNA are related components of heredity. Chromosomes are microscopic structures within cells that carry hereditary information in the form of genes. Genes, the units of heredity, are physically located on chromosomes.
Chromosomes are made of protein and DNA, with DNA being the chemical component chiefly responsible for the staining properties of chromosomes. DNA contains the genetic instructions for the development and function of all living organisms.
3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How do chromosomes behave during cell division?
During cell division, chromosomes undergo a series of precisely coordinated movements to ensure accurate distribution of genetic material to daughter cells. Prior to cell division, chromosomes replicate and condense into compact structures.
During prophase, the initial stage of mitosis, chromosomes condense and become visible. In metaphase, they align along the midline of the cell. Anaphase involves the separation of sister chromatids, which are identical copies of a chromosome formed during DNA replication. These chromatids then move toward opposite poles of the cell, ensuring each daughter cell receives a complete set of chromosomes. Finally, in telophase, the chromosomes decondense, and new nuclear membranes form around each set of chromosomes, completing the process of cell division.
9 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
Why do humans have 46 chromosomes, and what happens if the number is different?
Humans have 46 chromosomes because that is the typical chromosome number for human somatic cells. These chromosomes are arranged in 23 pairs, with one member of each pair inherited from each parent. Having the correct number of chromosomes is essential for normal development and function.
When there is an abnormal number of chromosomes (aneuploidy), it can result in various genetic disorders. For example, Down syndrome is caused by an extra copy of chromosome 21 (trisomy 21). Turner syndrome results from females having only one X chromosome, while Klinefelter syndrome occurs in males with one extra X chromosome (XXY). These conditions can lead to a range of health issues, including intellectual disability, physical abnormalities, and sterility.
4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
chromosome, the microscopic threadlike part of the cell that carries hereditary information in the form of genes. A defining feature of any chromosome is its compactness. For instance, the 46 chromosomes found in human cells have a combined length of 200 nm (1 nm = 10− 9 metre); if the chromosomes were to be unraveled, the genetic material they contain would measure roughly 2 metres (about 6.5 feet) in length. The compactness of chromosomes plays an important role in helping to organize genetic material during cell division and enabling it to fit inside structures such as the nucleus of a cell, the average diameter of which is about 5 to 10 μm (1 μm = 0.00l mm, or 0.000039 inch), or the polygonal head of a virus particle, which may be in the range of just 20 to 30 nm in diameter.
The structure and location of chromosomes are among the chief differences between viruses, prokaryotes, and eukaryotes. The nonliving viruses have chromosomes consisting of either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid); this material is very tightly packed into the viral head. Among organisms with prokaryotic cells (i.e., bacteria and blue-green algae), chromosomes consist entirely of DNA. The single chromosome of a prokaryotic cell is not enclosed within a nuclear membrane. Among eukaryotes, the chromosomes are contained in a membrane-bound cell nucleus. The chromosomes of a eukaryotic cell consist primarily of DNA attached to a protein core. They also contain RNA. The remainder of this article pertains to eukaryotic chromosomes.
Every eukaryotic species has a characteristic number of chromosomes (chromosome number). In species that reproduce asexually, the chromosome number is the same in all the cells of the organism. Among sexually reproducing organisms, the number of chromosomes in the body (somatic) cells is diploid (2n; a pair of each chromosome), twice the haploid (1n) number found in the sex cells, or gametes. The haploid number is produced during meiosis. During fertilization, two gametes combine to produce a zygote, a single cell with a diploid set of chromosomes. See also polyploidy.
" height="20" alt="HEW Logo">
Somatic cells reproduce by dividing, a process called mitosis. Between cell divisions the chromosomes exist in an uncoiled state, producing a diffuse mass of genetic material known as chromatin. The uncoiling of chromosomes enables DNA synthesis to begin. During this phase, DNA duplicates itself in preparation for cell division.
Following replication, the DNA condenses into chromosomes. At this point, each chromosome actually consists of a set of duplicate chromatids that are held together by the centromere. The centromere is the point of attachment of the kinetochore, a protein structure that is connected to the spindle fibres (part of a structure that pulls the chromatids to opposite ends of the cell). During the middle stage in cell division, the centromere duplicates, and the chromatid pair separates; each chromatid becomes a separate chromosome at this point. The cell divides, and both of the daughter cells have a complete (diploid) set of chromosomes. The chromosomes uncoil in the new cells, again forming the diffuse network of chromatin.
Among many organisms that have separate sexes, there are two basic types of chromosomes: sex chromosomes and autosomes. Autosomes control the inheritance of all the characteristics except the sex-linked ones, which are controlled by the sex chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes. All act in the same way during cell division. For information on sex-linked characteristics, see linkage group.
Chromosome breakage is the physical breakage of subunits of a chromosome. It is usually followed by reunion (frequently at a foreign site, resulting in a chromosome unlike the original). Breakage and reunion of homologous chromosomes during meiosis are the basis for the classical model of crossing over, which results in unexpected types of offspring of a mating.
The trusted destination for professionals, college students, and lifelong learners.
SUBSCRIBE
