Chromatid

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Chromatid

A chromatid is one among the two identical copies of DNA making up a replicated chromosome

Chromosome

A chromosome is an organized structure of DNA and protein that is found in Cell . A chromosome is a single piece of DNA that contains many genes, regulatory sequence and other genetic sequence....
, which are joined at their centromere

Centromere

A centromere is a region of DNA typically found near the middle of a chromosome where two sister chromatids come in contact. It is involved in cell division as the point of mitotic spindle....
s, for the process of cell division

Cell division

Cell division is a process by which a cell , called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle....
(mitosis

Mitosis

Mitosis is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus, into two identical sets in two daughter nuclei....
or meiosis

Meiosis

In biology or life science, meiosis is a process of reductional division in which the number of chromosomes per cell is halved. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores....
). The term is used so long as the centromeres remain in contact. When they separate (during anaphase

Anaphase

Anaphase, is from the ancient Greek ??? and f?s?? , is the stage of mitosis when chromosomes separate in a eukaryote cell . Each chromatid moves to opposite poles of the cell, the opposite ends of the mitotic spindle, near the microtubule organizing centers....
of mitosis and anaphase 2 of meiosis), the strands are called daughter-chromosomes.

In other words, a chromatid is "one-half of a replicated chromosome".

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Quantity

In humans, for example, there are normally 23 pairs of homologous chromosomes in each cell (N=23). However, the quantity of chromatids will be a multiple of 23. It can be either 4N, 2N or 1N. N does not refer to haploid or diploid; it refers to the number of chromatids in the cell as a multiple of the haploid number of chromosomes for the organism. For example, because a human haploid germ cell has 23 chromosomes, then "N" refers to a multiple of 23. (e.g. 2N=46 chromatids). The last is only seen in haploid gametes, with only one of each homologous chromosome pair. Such are created in gametogenesis

Gametogenesis

Gametogenesis is a process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes....
.

4N

In a cell with 4N chromatids, there are 23 chromosome pairs (46 chromosomes), and each chromosome has 2 chromatids. Thus, there are 92 chromatids in each cell (4N). It occurs after the S phase of interphase. (See cell cycle

Cell cycle

The cell cycle, or cell-division cycle, is the series of events that take place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission....
).

2N

Immediately after a mitosis

Mitosis

Mitosis is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus, into two identical sets in two daughter nuclei....
, where a cell has divided in two, but not yet duplicated its DNA in S phase, there are still 23 chromosome pairs (46 chromosomes). However, each chromosome only has one chromatid. Thus there are 46 chromatids (2xN)

Alternatively, a haploid cell with two chromatids per chromosome also has 46 chromatids. However, this doesn't occur naturally in humans.

1N

Immediately after meiosis

Meiosis

Gamete

A gamete is a Cell that fuses with another gamete during fertilization in organisms that sexual reproduction. In species which produce two morphologically distinct types of gametes, and in which each individual produces only one type, a female is any individual which produces the larger type of gamete?called an ovum ?and a male produces th...
, only has half the amount of chromosomes (23 chromosomes). Furthermore, each chromosome only has one chromatid. Thus, there are 23 chromatids (1xN)

Etymology

The term chromatid was proposed by Clarence Erwin McClung

Clarence Erwin McClung

Clarence Erwin McClung was an United States Biologist who discovered the role of chromosomes in sex determination.References ...
(1900) for each of the four threads making up a chromosome

Chromosome

Meiosis

Mitosis

Mitosis is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus, into two identical sets in two daughter nuclei....
.

The term derives from the Greek chroma (colour); for the derivation of -id, see cytokinesis

Cytokinesis

Cytokinesis is the process where the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation to the next....
.