If animal DNA is a single strand, then what is a chromosome and why do different animals have different numbers of chromosomes?

Question

And how can animals descended from a common ancestor have different numbers of chromosomes?

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Answer 1

For most animals, DNA is not a single strand: each chromosome is a separate strand.

The chromosome count can change due to copying mistakes:

* a chromosome can get a spare copy. With time that copy can diverge into a different chromosome.
* a chromosome can be lost during copying
* a chromosome can be torn apart, and become two
* two chromosomes can be joined together

For example, human chromosome #2 (numbered by size – so, second largest) corresponds to two ape chromosomes, that were joined together.