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Comment by pfisherman | Hacker News Reader

> The most significant part of Crick’s idea — and the part that Watson ignored in his oversimplification — was that there are three flows of information that cannot occur, due both to lack of experimental evidence and any plausible biochemical mechanisms. These were protein → protein, protein → RNA and, above all, protein → DNA.

These are categorically untrue. Post translational modification of proteins (by proteins) is a thing. Post transcriptional modification of RNAs (by proteins) is a thing. Transposition of DNA sequences (by proteins) is a thing.

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> Post translational modification of proteins (by proteins) is a thing.

Yes prions are real but the article points out that prions modify the way other, already-existing proteins fold. Prions do not create new proteins nor do they change the amino acid sequences of other proteins, which means prions don't violate the dogma.

The article addresses your other points as well.

Folding is clearly a form of information transfer though (and proteins are deeply involved in folding other proteins, even outside of the prion case). As stated in the screenshot it is fine "information here means the sequence of amino acid residues..." but that's a much narrower definition than is commonly communicated.

Proteins also do change the amino acid sequences of other proteins - they cleave them! The results of the cleavage are then important for cell biology, and although it seems plausible that the the results could have traditional "protein" style functions, e.g. as an enzyme, I not sure this is ever actually the case. But the end result is still that the rule has to be understood very narrowly.

I take these points to be correct, but they seem tangential to what is (at least according to the article) the main issue at stake: can proteins modify the information passed on to descendants? If this were the case, there would be a mechanism whereby Lamarckian evolution could, at least in theory, occur.

In that regard, I feel this is particularly relevant:

"During an organism’s life, environmental conditions cause certain genes to get switched on or off. This often occurs through a process known as methylation, in which the cell adds a methyl group to a cytosine base in a DNA sequence. As a result, the cell no longer transcribes the gene.

"These effects occur most frequently in somatic cells — the cells that make up the body of the organism. If epigenetic marks occur in sex cells [however], they are wiped clean prior to egg and sperm formation. Then, once the sperm and eggs have fully formed, methylation patterns are re-established in each type of cell, meaning that the acquired genetic regulation is reset to baseline in the offspring."

Wait until you read about proteins that methylate dna and how this can persist across cell divisions and even generations.

https://pmc.ncbi.nlm.nih.gov/articles/PMC8683130/

Biology is messy and imperfect, and generally not suited to dogmatic and other normative claims - but that is also covered in the article, which mentions how Crick was on the lookout for exceptions.

I would guess that, in the ordinary cases of cell division, the replication of methylation is, on balance, desirable, and that it probably requires an additional mechanism beyond minimal DNA base-sequence replication. In that case, its near-absence specifically within gamete production seems likely to be significant. The cases where methylation persists through generations should not be ignored, and neither should the existence of a mechanism that apparently exists to prevent (or at least constrain) it (or a mechanism to

The original article does discuss methylation.

These exceptions covered toward the end of the article. The core claim by Crick boils down to “no reverse translation”.

I think this is a much clearer expression of the claim than the commonly given one.

In the diagram/text description it is specifically referring to replication, in the sense of transmitting genetic information to daughter cells. In that sense there is no protein-protein transfer; no means of copying one protein or rebuilding nucleic acids from the amino acid sequence.

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