How do chromosomes relate to DNA? Why do they exist at all, if all genetic information is stored in a single strand of DNA within the cell? I read that chromosomes only form when cells are about to divide – if so, why is this necessary and what determines how many chromosomes are formed?
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A chromosome is what we call the condensed form of DNA. When the cell isn’t replicating, the DNA is kept loose and unwound, so that in can be accessed by the machinery that read it to make RNA and then proteins. The size of the loose DNA is huge, like HUGE so even in this state there is a lot of effort spent keeping the DNA condensed down in size, and only the most important bits are loose.
During cellular replication, however, the cells condense the DNA down as much as possible. This happens after the DNA has been copied, and makes splitting the DNA between the two daughter cells much easier.
A chromosome is what we call a specific piece of DNA. You have 23 chromosomes, each one is unique from the others based on what genes are encoded on it. You, me, and everyone in the world has the genes for a specific trait located in the same place, on the same chromosome.
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