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Major_t0Ad t1_jbnnlhz wrote

There is selection pressure to strip away genes you don't need because the investment for offspring is smaller. Environmental bacteria tend to lose genes quite quickly, e.g. when they enter simple growth medium in the lab because they don't have to "fight" for survival in harsh conditions.
So when you find organisms they kind of already have the 'minimum set' for their respective environment. The smallest DNA for a free-living organism is around 1.3 Mbp with 1354 encoding genes (Marine bacteria Pelagibacter ubique)

https://doi.org/10.1126/science.1114057 Publication figure 1 shows different species, also mycoplasmum, in a scatter plot genome size over number of genes.

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PatrickKieliszek t1_jbnvk7i wrote

How does this selection pressure tie in with the large amount of non-coding DNA in complex organisms?

Reading Wikipedia on this didn't leave me with the impression that there is consensus on the why of DNA that doesn't code for proteins or change transcription sites.

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Dr_Vesuvius t1_jbochke wrote

Well first we must distinguish between “non-coding” and the pop-science concept of “junk” DNA. While the most interesting thing about DNA is its ability to code for genes, that is not the only thing it does.

Most of our DNA has some kind of function. That could be coding for RNA that isn’t supposed to be transcribed. It could be structural, like telomeres and centromeres. It could be about regulating transcription or replication.

All the same, human DNA is much more prone to accumulating dead genes than bacterial DNA due to our generation time. We can carry around a bunch of pseudogenes or ancient viruses that managed to get themselves added to our genome. Selection pressure is much less and much slower when it takes 20-40 years to reproduce as opposed to 20-40 minutes.

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Mithridates12 t1_jbp6rmn wrote

Off topic, but I really appreciate when people like you who seem to know what they’re talking about share some insights that laymen like myself can understand. Just makes me want to read up on things

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Major_t0Ad t1_jbwx7av wrote

Adding to this awesome answer: ecologically speaking, the investment for offspring is much less dependent on sheer DNA mass for complex organisms than it is for bacteria. Bacteria optimize things unheard of for complex organisms, they have crazy selection pressure.

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