Submitted by emelrad12 t3_zhujsn in askscience
There are some answers discussing how it was detected, or what they do, but none touch the topic of how viruses become part of our DNA. Specifically by what mechanism can anything external get integrated in anyone else dna?
The life cycle of a virus goes as follows. A virus attaches to their specific host, then inject their genetic material into the host. The host then gets "fooled" into reading this material and making more and more viruses. Once there's so many viruses that the host can no longer perform the function to keep itself alive, it essentially explodes and releases the next generation of viruses into the world.
Viruses can copy their DNA into their host for this purposes, but sometimes the virus is over powered or the cell survives infection and the genetic material the virus input is kept. If this happens in a gamete (sperm or egg cell) that goes on to be fertilized, that viral input is copied from the gamete to every daughter cell and voila, you have inserted viral genes into a multicellular organism.
> next generation of viruses
When you say next generation, it is only referring to the fact that a virus fooled the host body into making more viruses, right ?
Or is it more about the next generation being different from the virus that attached itself to a host ?
I am trying to understand if this is similar to a variant.
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In theory each generation would be an exact copy of its "parent" but mistakes can and do happen in the copying process so there can be slight differences in the next generation. Thats how the virus evolves. If these random errors in the copy end up making the new virus more successful there will me more of the variant
Viruses are essentially little balls filled with genetic information and a couple of proteins that act like machines to help the viral genetic information get copied inside the cell. This involves taking the viral genome, converting it into a usable form, sticking it inside host cell ribosomes (protein making machines), and forcing them to make viral proteins. Most of the time, this happens outside the nucleus and often never involves DNA. However, for a certain type of viruses called retroviruses, converting the viral genome into a usable form involves a protein called reverse transcriptase. This takes viral genetic material and converts it into DNA just like what we have in our own cells. These retroviruses then trick host cells into taking that DNA inside the nucleus where it is slipped inside a chromosome.
> mistakes can and do happen in the copying process
What are some causes for these mistakes ? Is it a crucial 'ingredient' the host body cannot provide at that moment ?
Mutations are just natural mistakes that happen when replicating DNA. There’s so many bases to read that a mistake every now and then is inevitable. It happens a lot more than you’d think, but you only notice when a section of DNA that is important gets screwed up. There are huge stretches of DNA that don’t code for anything in humans, including the viral DNA that was introduced long ago.
Copying DNA is not a perfect process. Mutations naturally occur every now and then.
There's no particular cause, mutations are just a fact of life.
HIV is a great example of this, it falls into the category of retroviruses which are viruses that integrate into the hosts genome. Once the virus has entered the cell, it comes in the form of RNA so it uses a reverse transcriptase to synthesize a complementary strand of DNA from the RNA. If that DNA is not detected in the cytosol by the cell (we have special detectors that look for intercellular viral DNA, like AIM2 inflammasomes or cGAS STING) then it will make its way to the nucleus where it will get incorporated into our genome by our own machinery
https://en.m.wikipedia.org/wiki/Retrovirus
The family is characterised by its RNA genome, the conversion of single stranded RNA (their own genome) into double stranded DNA is accomplished by the reverse transcriptase which can then be integrated into the host genome by their own integrase (at random/ semi random positions) both proteins are part of the mature viron.
Other virus families based on double stranded DNA, can end up with their genome integrated randomly at double stranded breaks of the genome.
The function of the DNA you referred to, that used to be called ‘junk DNA’, is still subject to research. It might not code for known proteins our bodies make under conditions that are so far researched but to say it doesn’t code for anything, I would say is wrong since we don’t know. Also, humans have inherited viral DNA that is a functional part of our genome, often functioning only if triggered by certain factors.
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I think u/Katamirand was referring to the "mistakes" in DNA replication during cell division. You should know that there are double-stranded DNA viruses that can infect and survive in the nucleus of the host cells while there are other viruses that have single-stranded RNA. In short, the polymerase in most RNA viruses cannot repair replication errors so these errors would be causes for mutations. So it's not so much of 'what the host cannot provide' that causes replication errors/mutations, but just think of it as a natural occurrence that not only can be caused by a multitude of factors, but also simply just by chance.
Here's a 200-page book, copy it by hand.
You may make a mistake.
That's DNA.
A virus's DNA is about 300 000 base pairs - which is about equivalent to a 200-page book. Granted, DNA has 4 characters while a book is written in closer to 64 - so maybe it's only a 65 page book; but you still have to copy it. Mistakes are going to happen.
Life generally takes two solutions to this: make a lot of copies as fast as you can and hope most of the books are still good; or take your time to make sure there are as few mistakes as possible.
Viruses are an extremely diverse group of microorganisms. They have diverse lifestyles. Some viruses use single- or double-stranded RNA genomes, some use single- or double-stranded DNA genomes. Some can reverse transcribe RNA into DNA.
Some viruses are strictly “lyric” meaning that their lifecycle relies on the immediate replication of virus and death of the cell. Some viruses (famously bacteriophage) can participate in a “lysogenic” lifestyle. This involves the viral genome becoming maintained by the host as part of its genome. In bacteria, this can happen as a plasmid or through integration of the viral genome into the chromosome.
In humans, lysogenic viruses exist. Chickenpox is an example. Initial infection and illness happens and subsides, then decades later the virus re-emerges and causes the disease shingles. This is caused by latent viral genomes stored in host cells.
Ancient viral DNA that has been domesticated in the human genome is kinda like this. You don’t have to be afraid of these latent viruses re-emerging because they have likely lost some important regulatory mechanisms that allow them to pop back out and cause illness. Now that piece of DNA just exists as a part of the human genome.
Hope that helps!
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There are specific viruses that insert their DNA directly into the host genome where it "goes along for the ride" with it's DNA being replicated along with the hosts until times get tough whereupon the virus takes over the cells machinery to make copies of itself.
https://en.wikipedia.org/wiki/Retrovirus
However sometimes the inserted virus loses the ability to make full virus particles, and now it's stuck.
This is almost correct, the cell will only explode with viroids if the virus is in the lytic stage. Otherwise it will remain dormant.
what happens to the host after that? taking useless genetic information. won't the host DNA become longer overtime?
> where it is slipped inside a chromosome.
To expand upon this point, the retroviruses also have an enzyme called integrase that inserts their DNA into the host genome. Integrase inhibitors are a class of anti-HIV drugs.
> If this happens in a gamete (sperm or egg cell) that goes on to be fertilized
Not only gamete, but any germline cell (that eventually develops into a gamete)
Yes, the host genome will grow with integrations like this. Genomes are often inflated due to integration of viral sequences and other selfish genetic elements. This helps explain the extreme variation in genome size among eukaryotes.
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A virus is really just a piece of DNA or RNA surrounded by a carrying container. Retroviruses integrate their genomes into host genomes, other virus genomes can coincidentally be integrated at times. Look at figure 1 in article below for description of retrovirus integration mechanism.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7120651/
Our genomes are maybe 10% or more viral dna that no longer has open reading frames/is not expressed.
[deleted] t1_izok1vw wrote
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