What’s the difference between DNA, a genome, and a chromosome?
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Let’s start from the smallest building blocks. We can put a nucleobase (flavor A, T, C or G) together with a deoxyribose sugar to form a nucleoside. If we then attach a phosphate group to the ribose part of our nucleoside, it becomes a nucleotide capable of linking to others like it; deoxyribose alone won’t link to other deoxyriboses, so we need phosphates between them. Once you have several nucleotides connected to each other, you refer to the whole thing as **DNA** — deoxyribonucleic acid. It looks something like [this](https://humanbiology.pressbooks.tru.ca/wp-content/uploads/sites/6/2020/01/DNA.png). And since the nucleobases (the A/T/C/G parts) like to bind to each other, specifically A-T and C-G, two pieces of DNA will line up together like that. Complicated chemistry stuff means that lining-up happens at a slight angle, and that causes the typical double helix structure.
So that’s DNA. In living organisms, their total DNA isn’t just one long strand (double helix!); it’s divided into a number of separate strands. These are **chromosomes**, and in addition to just the strand of DNA, they also contain proteins named histones which the DNA is kind of wrapped around to help keep it organized. Humans have 23 unique chromosomes, and two of each (one from their mother, one from their father).
Then, the **genome** is pretty easy. It’s just the collective of genetic material (= DNA) in a person. It can also refer to the species as a whole, i.e. “the human genome is divided over 23 chromosome pairs”. That’ll usually be obvious from context.
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