The basic unit of chromosomes is the DNA double-stranded helix. It is composed of nucleotides and a sugar-phosphate backbone.
DNA helices are wound around histones, which are proteins that act as spools. Histones allow for compact storage of genetic information (which can be large in eukaryotes) and also play a role in gene regulation.
Nucleosomes are bundles of eight or more histones. They compact together to reduce the size of DNA.
Chromatin is fundamentally made up of nucleosomes but is largely uncondensed, with nucleosome bundles spaced out and connected by wound-up DNA.
The chromosome is condensed chromatin and is present when the cell is about to divide. In prophase, chromatin condenses to form a chromosome. The chromosome decondenses back to chromatin after cell division. In the chromatin state DNA can be replicated and transcribed.
The telomere is a repeated nucleotide sequence at the end of a chromosome (or chromatid). Telomeres prevent exonucleases from deteriorating the ends of chromosome, and prevent fusion with nearby chromosomes.
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