Viruses contain genetic material (RNA/DNA) encoding instructions to make more of themselves. They infect cells to feed them those instructions and handle the actual production. Many viruses can simply hang out inside the cell and hand their RNA, or DNA with an extra step, to the protein-building machinery of the cell directly. Retroviruses take a different approach and actually insert their genetic material into that of the host cell, then allow it to be produced from there.
That insertion is permanent and will stick with the host cell until it dies sooner or later (usually sooner, once the immune system catches on). But what if this cell happens to be an egg/sperm cell that goes on to grow into a entire new individual? Now that bit of retrovirus DNA is an integral part of the individual’s genome. It will be passed on to all their future descendants as well. At that point you can describe the retroviral DNA as endogenous; originating from within the organism.
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Viruses contain genetic material (RNA/DNA) encoding instructions to make more of themselves. They infect cells to feed them those instructions and handle the actual production. Many viruses can simply hang out inside the cell and hand their RNA, or DNA with an extra step, to the protein-building machinery of the cell directly. Retroviruses take a different approach and actually insert their genetic material into that of the host cell, then allow it to be produced from there.
That insertion is permanent and will stick with the host cell until it dies sooner or later (usually sooner, once the immune system catches on). But what if this cell happens to be an egg/sperm cell that goes on to grow into a entire new individual? Now that bit of retrovirus DNA is an integral part of the individual’s genome. It will be passed on to all their future descendants as well. At that point you can describe the retroviral DNA as endogenous; originating from within the organism.