Human karyotype with bands and sub-bands

A chromatid (Greek ''khrōmat-'' 'color' + ''-id'') is one half of a duplicated

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

. Before replication, one chromosome is composed of one

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

molecule. In replication, the DNA molecule is copied, and the two molecules are known as chromatids. During the later stages of cell division these chromatids separate longitudinally to become individual chromosomes. Chromatid pairs are normally genetically identical, and said to be

homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mos ...

. However, if mutations occur, they will present slight differences, in which case they are

heterozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mos ...

. The pairing of chromatids should not be confused with the

ploidy Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Here ''sets of chromosomes'' refers to the number of maternal and paternal chromosome copies, ...

of an organism, which is the number of homologous versions of a chromosome.

Sister chromatids

Chromatids may be sister or non-sister chromatids. A sister chromatid is either one of the two chromatids of the same

joined together by a common

centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fiber ...

. A pair of sister chromatids is called a dyad. Once sister chromatids have separated (during the

anaphase Anaphase () is the stage of mitosis after the process of metaphase, when replicated chromosomes are split and the newly-copied chromosomes (daughter chromatids) are moved to opposite poles of the cell. Chromosomes also reach their overall maxim ...

of mitosis or the anaphase II of

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

during

sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...

), they are again called chromosomes, each having the same genetic mass as one of the individual chromatids that made up its parent. The DNA sequence of two sister chromatids is completely identical (apart from very rare DNA copying errors). Sister chromatid exchange (SCE) is the exchange of genetic information between two sister chromatids. SCEs can occur during

mitosis Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...

. SCEs appear to primarily reflect DNA recombinational

repair The technical meaning of maintenance involves functional checks, servicing, repairing or replacing of necessary devices, equipment, machinery, building infrastructure and supporting utilities in industrial, business, and residential installat ...

processes responding to DNA damage (see article Sister chromatid exchange). ''Non-sister chromatids'', on the other hand, refers to either of the two chromatids of paired

homologous chromosome Homologous chromosomes or homologs are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during meiosis. Homologs have the same genes in the same locus (genetics), loci, where they provide points along e ...

s, that is, the pairing of a paternal chromosome and a maternal chromosome. In

chromosomal crossover Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' sister chromatids, non-sister chromatids that results in recombinant chromosomes. It is one of the fina ...

s, non-sister (homologous) chromatids form chiasmata to exchange genetic material during the prophase I of

(See Homologous chromosome pair).

References

{{Chromo Chromosomes Molecular genetics Cell biology Mitosis Telomeres

Sister chromatids

See also

References