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According to 23andme's Twitter

image of text

Image reads,

Approximately 8% of DNA is originally from viruses, which infected your ancestors and became integrated into their cells.

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So, 43% of cells in our bodies is "human", and the "human" cells have 8% viral/other DNA. No wonder it's so hard for some people to be "human". – user2338816 Mar 17 at 8:16
For anyone who is interested, here is the sourced link from the bottom of the image:… – Insane Mar 17 at 11:32
Note too that the reverse can also be true: – LarsH Mar 20 at 2:33
up vote 124 down vote accepted

Normal viruses infect cells in order to take advantage of cellular mechanisms to reproduce themselves. A class of viruses known as "retroviruses" have a slightly peculiar mechanism to achieve this - they synthesise a special enzyme called "reverse transcriptase", which translates their own RNA into DNA which is incorporated into the host cell's genome. The host cell then replicates the virus' DNA alongside the rest of its own.

Should a retrovirus happen to infect a germ line cell - i.e., a sperm or an ovum - the retroviral DNA will be copied into the offspring's DNA. And their offspring. That's called an "endogenous retrovirus". Some cause or contribute to cancers - and sometimes (for example) that DNA even gets co-opted for a useful purpose.

Given thousands or millions of generations, these endogenous infections accumulate. It doesn't even have to be independent infections - when a cell is infected it produces new copies of the RNA virus which can insert itself again somewhere else in the germline genome - even across the generations. They eventually tend to get deactivated by one mutation or another, but phylogenetic studies have shown that the some 98,000 retroviral elements and fragments in the human genome come from relatively few infections.

Estimates vary - many of the retroviral sequences have been modified by mutation over the aeons, making it hard to find them all - but recent estimates do come to about 8% of the human genome having a retroviral origin. There's a fair chunk of virus in all of us.

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This good answer explains how virus DNA is believed to enter our genome. But how do we know what sequences come from (retro)viruses; or to put it another way, how do we know that any sequence comes from a retrovirus? – LarsH Mar 16 at 21:56
Read the linked paper. – JDługosz Mar 16 at 23:47
Not just these.. There are LINE (make up 17% of the human genome) and SINE retrotransposons. – WYSIWYG Mar 17 at 7:04
@JDługosz: which one? – LarsH Mar 20 at 2:17
"Relatively few infections". – JDługosz Mar 20 at 2:18

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