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Prokaryote Structure

Prokaryotes are tiny cells that always have a cell wall and don't have a nucleus.

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Arsenic and New Life?
Teacher Contributed

Arsenic and New Life?


That Ain't Your Mother's DNA

All life we know of is based around DNA; that's why people were stunned when NASA announced a "new" form of life that wasn't based on DNA...at least not the DNA with which we are familiar. All known life forms use nucleic acids to encode and distribute genetic information, and they use phosphate (PO4-3), an oxidized form of phosphorus (P: element 15), as the molecular scaffolding to build these molecules. This "new" form of life used arsenate (AsO4-3), an oxidized form of arsenic (As: element 33), as opposed to phosphate to form the scaffolding of its nucleic acids (DNA and RNA). At least that's the way it was initially announced.

You can get a little background here:

As hinted at in the above clip, there was more to come. Here is the NASA press conference where the discovery was announced. Pay particular attention to the "curmudgeon" chemist.

The scientific process is a marvelous thing, designed to be self-correcting. This means that scientists must submit their work to peer review, a process where their work is sent out to other scientists who are recognized authorities in the pertinent field. These "reviewers" comment on the paper, suggest revisions, and question conclusions. This process of peer review often leads to much improved manuscripts, which are then published in a journal that is available for anyone to read. In cases of extraordinary "discoveries," other scientists often submit responses to the original article to the journal that published the article, specifically stating their arguments as to why they question the findings of the paper and often suggesting further work to clarify these findings. Part of this whole process is that the publishing scientists are expected to submit all their work and their samples to any peer that requests them so that they can replicate the experiments to confirm the findings. In this case, there was a swarm of responses questioning both the methods used in the experiments, as well as the interpretation of the results (in the resources cited below, you can find the original paper, some of these responses, and the paper's main author's response to the responses). None of the "responders" questioned that this was an extraordinary bacterium that could grow at such high arsenic concentrations. They questioned that arsenic was in fact incorporated into DNA and other biomolecules. The authors of the original paper welcomed people to repeat their experiment and confirm their findings. Unfortunately, things didn't work out the way they had wished, and their findings of arsenic being incorporated into DNA were not confirmed in this case. You can go to these links to read the papers of the scientists who repeated the experiments.

The scientific process works, especially if you listen closely to the curmudgeons.

Extension Investigation

If you need help scratching a mental itch, use the resources below:

  1. Do you feel these experiments prove that arsenic cannot be used as a replacement for DNA? Explain your reasoning.
  2. What do you think of the scientific process? Explain your reasons for your opinion.
  3. Can you think of any ways the scientific process can be improved?
  4. What are some of the values of interdisciplinary teams when conducting research into complex questions?
  5. List three ways in which organisms use phosphate.
  6. Locate phosphorus and arsenic on the periodic table. How are the locations of these two elements related?

Resources Cited

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