In G1, each chromosome is a single chromatid. During interphase (G1 + S + G2), chromosomes are fully or partially decondensed, in the form of chromatin, which consists of DNA wound around histone proteins (nucleosomes). Analogies aside, this is a phenomenal task, and when chromatid condensation fails, so too does proper chromosome segregation. Chromosomes condense and become visible by light microscopy as eukaryotic cells enter mitosis or meiosis. Although already a remarkable feat, during mitosis all that DNA would have to be condensed to line up along the midfield before being pulled toward the goals - and it must do all this without getting tangled or broken as the stadium splits in two. During interphase, all of that DNA would fill a typical football stadium from the pitch to the upper tier. If DNA could be scaled to something the size of an overhead power cable (with a one inch diameter), those imaginary DNA strands - pulled end to end - would stretch around the world from San Francisco to Tokyo to Paris and back. At the beginning of mitosis, the chromosomes condense, the nucleolus disappears, and the nuclear envelope breaks down, resulting in the release of most of the contents of the nucleus into the cytoplasm. Each replicated chromosome can now be seen to consist of two identical chromatids (or sister chromatids). For scale, consider the following: each human cell has two copies of its 3.3 billon base-pair genome. A unique feature of the nucleus is that it disassembles and re-forms each time most cells divide. The chromosomes condense into compact structures. Chromosomes condense and become visible by light microscopy as eukaryotic cells enter mitosis or meiosis. Given that the extended length of DNA is much, much greater than the diameter of the cell, the process of condensation enables the movement of chromosomes to nascent daughter cells without DNA entanglement or entrapment by the cleavage furrow. In order to faithfully and accurately segregate chromosomes, eukaryotes must first condense each chromatid pair into a compact structure that can be separated and transported to each daughter cell.
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