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

When a cell divides, it needs to duplicate its DNA so that the new cells each have a complete copy. Part of doing this is to divide the DNA strand into chunks. These chunks are “chromosomes.” I don’t know the exact mechanism for how the DNA “decides” to divide itself up into a different number of chromosomes, but different organisms have different numbers of chromosomes because it divides its DNA up into different numbers of chunks when replicating.

During this replication process, there can be errors. Sections of DNA can be rearranged, copied, or deleted. In this manner, an organism can end up with new cells that have different number of chromosomes. If this mutation happens in the creation of gamete cells, then those mutations can be passed on to potential offspring.