Chromosomes and Chromatids

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hromosomes and chromatids occur as rod or thread-shaped structures in the nucleus of eukaryotic cells. Prokaryotes usually have only a single circular chromosome (in the webseries we will not focus on this group). The name chromosome (from the Greek chroma = color and soma = body) originates from the fact that chromosomes can be observed as stainable bodies in a light microscope during cell division. Chromosomes and chromatids contain chromatin, which mainly consists of extremely long stands of DNA material (Deoxyribonucleic acid) that functions as carrier of genes and regulatory elements. Besides, chromatin contains Histones (chromosome proteins) and other proteins involved in the packaging of the DNA strands during condensation at cell division (see figure E here below) and small quantities of RNA. aantallen chromosomen, chromatiden, zusterchromatiden, homogole chromosomen, ploïdie number of chromosomes, chromatids, sister chromatids, homolog chromosomes, ploidy Sister chromatids (with -tid at the end), are two identical (= exactly the same) parts (Chromatids) arising from replication of a chromosome. (In the figure here next the sister chromatids A1 and A2 are an exact copy of each other, as well as the set B1 and B2, whereas homologs A's and B's show small differences). These two parts retain the denomination chromatids as long as they are bound together through the centromere, which is for example the case during the entire S phase following duplication of DNA (replication). This connection is vible as a constriction during mitosis or meiosis. During anaphase in mitosis and anaphase II in meiosis the two sister chroamtids are pulled apart at the centromer. According to current definitions, each single chromatid is regarded as a own chromosome after separation of the chromatids during cell division. In most organisms chromosomes occur in pairs, the so-called

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