New answers tagged human-genome
Sequences that don't appear in a genome are called "nullomers". That article claims that there are no 10bp sequences that don't appear in the human genome, and 80 11bp sequences that don't, citing this paper.
The chromosomes in your picture do have sister chromatids, they are just very close together. A chromosome needs to pass through replication before it can compact into the typical metaphase chromosome shape depicted above. When I prepare metaphase chromosomes, I usually see a mixture of chromosome sets where the chromatids are close together (like above) and ...
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