Re: Some junk ain't junk


"Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx> wrote in message news:...

"Keith Hudson" <keithhudson@xxxxxxxxxxx> wrote in message news:dsqcu6$869$1@xxxxxxxxxxxxxxxxxxxxxx
I agree with this. The carrying of the vast amount of "junk" is a huge
energy burden on the organism every time a cell duplicates. It must
have some very good purpose.

It is not clear to me that the energy burden is significant, at least
in comparison to the energy burden of duplicating all of the other
cellular machinery. My guess is that the energy burden of building
lipids and proteins FAR outweighs the energy for duplicating DNA,
even in organisms loaded with junk. Anyone have some numbers?

Ask and you shall receive. Amazon just delivered "The Vital Force:
A Study of Bioenergetics" by Franklin Harold. And, I find the answer
to my question (for heterotrophic prokaryotes, anyways) in table 5.2.

It turns out that replicating DNA is not a big item in the cell's
energy budget. Perhaps 5%. I was wrong about lipid synthesis, though.
That is less than 1%. The big ticket item is synthesis of proteins
at more than 50%. Other major costs are synthesis and turnover of
RNA, making cell-wall materials (polysacharides), and active transport
of materials across membranes.

Comparable figures for eukaryotes were not provided. However,
considering that eukaryotes, while they have much larger genomes,
do not have a greater fraction of cell mass devoted to DNA than
do prokaryotes, I doubt that the main conclusions are much different.
The presence of 'junk' just is not a huge energy burden.

Also, if an autotrophic organism had been evaluated, I suspect that
the cost of making lipids would have been comparable to or greater
than the cost of making DNA. By weight, there is at least three
times as much of it.


.

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