To prevent confusion about the terms chromatin, chromosome and chromatide as they are employed during the cellcycle, a few features have been summarized here below:
| Definition: chromatin, chromosomes (n) and chromatids (c) |
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2n 2c means two homolog (diploid) but unreplicated chromosomes (two chromatides).
1n 1c one single chromosome (haploid) that is unreplicated.
2n 4c Two homolog chromosomes (diploid) consisting each of two sister chromatides (two yellow and two blue), thus 4c in total.
1n 2c one single chromosome in which DNA has been duplicated. |
| The number of chromosomes is indicated by a small letter n. |
| Chromatin is the product of which chromosomes are made. It consists primarily of DNA strands, histones (chromosome proteins around which DNA is folded during condensation (at cell division) and small amounts of RNA |
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| Chromosomes (from the Greek words chroma = color and soma = body; the number of chromosomes is indicated by the small letter n) are stainable structures that can be observed during cell division as rod-shaped bodies in a light microscope. Chromosomes consist of extremely long strands of DNA material and function as carriers of the genes and regulatory elements. The word chromosome designs two identical sister chromatids as long as they are bound together throught the centromer. According to current definitions, each single chromatid is regarded as a own chromosome after separation of the chromatids during cell division. |
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Chromatids (with tids at the end; the number of chromatids, that is the number of sets DNA molecules, is indicated by a small letter n) are the two parts (sister chromatids) of which a chromosome consists after replication. During mitosis or meiosis the region where the sister chromatids are connected (the centromer) is visible as a constriction. During anaphase in mitosis and anaphase II in meiosis the two sister chroamtids are pulled apart at the centromer. |
| Number of chromosomes (n) and chromatides (c) |
| Normal body cells of diploid organisms always contain pairs of homolog chromosomes. All these cells are in principe genetically identical to the zygote (the product resulting from fertilization) from which they descend. Each pair of homolog chromosome is a heritage of one set from mother side and another from father side. However, because of meiotic "reshuffling" (= crossing-overs), a different pool of genes is present in the inherited chromosomes than in the cells of the father and the mother!
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| Sexual reproductive cells have only one sample of each chromosome.
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| An unreplicated chromosome contains one double strand -DNA molecule.
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| A replicated chromosome contains two identical double strand -DNA- molecules, the chromatids, that are joined at their centromere.
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| Two single sets of pairs (daughter) chromosomes are formed by separation of the pair of chromatids during cell division (from anaphase on in mitosis and anaphase II in meiosis)
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| Haploid is the term for a cell or an organism with only one set of chromosomes (n). If this is valid for all cells, the entire organism is regarded as being haploid. A haploide cell remains in fact n after replication, but doubles from c to 2c. Each chromosome consists of two chromatides.
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| Diploid is the term for cells with a double number of chromosomes (2n), whereby one set of chromosomes is homolog to the other. (The sex chromosomes present in each human cell are an exception). A diploid cell remains 2n after replication, but doubles from 2c to 4c.
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| Humans usually have 22 pairs of homolog chromosomes (autosomes) and as sex chromosomes 2 x chromosomes (in female), or 1 x and 1 y chromosome (in males), thus in totall 46 chromosomes per body cell (see human karyogram; 2n = 46) and 92 chromatids per cell after replication.
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