I'm by no means an expert in the field, merely a curious visitor, but I've been thinking about this and Google isn't of much help. Do we know of any lifeforms that don't have the conventional double-helix DNA as we know it? Have any serious alternatives been theorized?
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2$\begingroup$ I don't quite know if viruses and prions can be treated as "alive"... $\endgroup$– user132Commented Dec 15, 2011 at 14:28
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$\begingroup$ What is considered alive? This is a good question. I think it is safe to define something as alive when it is (1) capable of sustaining and replicating itself and (2) able to interact with its environment. $\endgroup$– Bart JacobsCommented Dec 15, 2011 at 15:00
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1$\begingroup$ Irregardless if viruses and prions are "alive", those that we know cannot survive in a word without RNA/DNA. $\endgroup$– Nick TCommented Dec 15, 2011 at 15:17
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$\begingroup$ I agree with @Poshpaws it is great question $\endgroup$– DarqerCommented Dec 19, 2011 at 12:53
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5 Answers
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To follow up what mbq said, there have been a number of "origin of life" studies which suggest that RNA was a precursor to DNA, the so-called "RNA world" (1). Since RNA can carry out both roles which DNA and proteins perform today. Further speculations suggest things like a Peptide-Nucleic Acids "PNA" may have preceded RNA and so on.
Catalytic molecules and genetic molecules are generally required to have different features. For example, catalytic molecules should be able to fold and have many building blocks (for catalytic action), whereas genetic molecules should not fold (for template synthesis) and have few building blocks (for high copy fidelity). This puts a lot of demands on one molecule. Also, catalytic biopolymers can (potentially) catalyse their own destruction.
RNA seems to be able to balance these demands, but then the difficulty is in making RNA prebiotically - so far his has not been achieved. This has lead to interest in "metabolism first" models where early life has no genetic biopolymer and somehow gives rise to genetic inheritance. However, so far this seems to have been little explored and largely unsuccessful (2).
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I just saw this popular article in New Scientist which also discusses TNA (Threose nucleic acid) and gives some background reading for PNA, GNA (Glycol nucleic acid) and ANA (amyloid nucleic acid).
(1) Gilbert, W., 1986, Nature, 319, 618 "Origin of life: The RNA world"
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$\begingroup$ There actually was a study where they were able to create (using theoretical early Earth conditions) two of the four RNAs. Though I always have a tough time finding the video interview for it I did find this. wired.com/2009/05/ribonucleotides $\endgroup$– FrankyGCommented Apr 5, 2016 at 16:33
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There has been a recent report on Science, which had much return in the general press, in which a bacteria was identified that could live in an environment where arsenic was subsituted to phosphorus (one of the components of DNA, forming the backbone of the double helyx).
This is the original paper:
A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus
and the commentary appeared on Nature
Arsenic-eating microbe may redefine chemistry of life
There is, however, much critique on the methodology used in the paper, and on whether arsenic would really be incorporated in DNA instead of phosphorus.
Science published several of these critiques in an Editor's Note And here you will find the Response of the Authors
Other than that... well if you consider virus as life-forms, there's plenty that do not have double stranded DNA, but have instead single strand DNA or single strand RNA or double strand RNA.
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1$\begingroup$ I am also sceptical - but fascinating if correct ! $\endgroup$– PoshpawsCommented Dec 15, 2011 at 16:07
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4$\begingroup$ I don't want to get too deep into the controversy over this study, but from reading the paper myself and talking to others, the weight of current opinion is certainly against the arsenic result. $\endgroup$– yamadCommented Dec 16, 2011 at 0:25
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3$\begingroup$ @KatieBanks, this is not my field so those interested should read the 8 critiques. One main criticism, though, is that their As+/P- medium has sufficient trace phosphorus to support the growth they attribute to arsenic incorporation. The authors argue that their As-/P- control, which shows no growth at all, indicates that it isn't just trace P supporting growth. This is a good point, but this still doesn't really require an interpretation that As is incorporated into the DNA. Their other data has also been criticized for insufficient purity in their samples/preps. $\endgroup$– yamadCommented Jan 9, 2012 at 15:24
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2$\begingroup$ this was debunked: news.nationalgeographic.com/news/2012/07/… $\endgroup$ Commented Aug 29, 2014 at 14:16
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1$\begingroup$ Also: ncbi.nlm.nih.gov/pubmed/22798070 $\endgroup$– nicoCommented Aug 29, 2014 at 14:42
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It depends whether you call prions a life form, but prions do not make (direct) use of DNA to propagate themselves. They force other proteins into a misfolded protein state.
Again, the question remains whether prions should be considered "alive".
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4$\begingroup$ The question of them being alive is moot; prions are ultimately dependent on DNA for their propagation as the host must generate additional substrate for them to turn over. $\endgroup$– Nick TCommented Dec 15, 2011 at 15:19
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$\begingroup$ That is the reason I put "direct" between parentheses. Strictly speaking, prions do not make use of DNA to propagate. However, as you indicate, there is definitely room for debate. $\endgroup$ Commented Dec 15, 2011 at 15:38
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$\begingroup$ Prions can't however change any protein into a misfolded state... they are a misfolded state of the PrP protein, which is normally synthetised by the organism, and for which a gene exists. $\endgroup$– nicoCommented Dec 15, 2011 at 15:43
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$\begingroup$ Luckily they can't. However, keeping the question above in mind, I don't think that this is of any relevance. $\endgroup$ Commented Dec 15, 2011 at 20:46
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There are serious speculations that the origins of life were using RNAs both as enzymes and genetic information carrier.
Later this informative RNAs evolved into a more stable and less reactive DNAs, enzymatic role was delegated to proteins and RNA only remained into most crucial parts of expression chain (mRNA and rybosome) and some regulation mechanisms.
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This recent Cell paper mentions a ribozyme (RNA enzyme) that ligates two oligonucleotides into itself. Given a sufficient source of input oligonucleotides and the correct conditions, it can catalyse its own replication and undergo Darwinian evolution, and can be thought of as a rudimentary form of RNA-based life.
The authors hypothesise that ligase-based RNA replicators could have been the first RNA replicators, which were later replaced by the now-standard polymerisation:
A somewhat different approach relies on RNA enzymes with RNA-templated RNA ligase activity to join oligonucleotide substrates to form complementary RNA products. It has been proposed that the first replicating, evolving systems on Earth operated by this mechanism and only later came to depend upon residue-by-residue polymerization
It should be noted that RNA ribozyme polymerases already exist, but many of them require proteins in addition to the RNA structure.
