Radiation Resistance

Facts about the cockroach is interesting. Source: The Hindu

It can:

  • run three miles in one hour — the fastest insect alive
  • hold its breath for 40 minutes
  • live a week without a head, only dying of thirst because it has no mouth to drink water
  • squeeze into cracks that are 1.6 millimetres thick — the equivalent of you trying to fit into a football
  • survive temperatures as low as 0 degrees centigrade but when it gets really cold, it likes snuggling with humans or any other warm body.
  • recognise members of its own family just by their smell.

Its heart is a simple tube that can pump blood both backwards and forward and even stop at will without harming the insect. The cockroach is the greatest escape artist of all time with an uncanny ability to sense danger, with its hair

It was thought until the 1950’s that cockroaches will be the only living being which will survive in the event of a nuclear calamity. This notion changed with the discovery of a radioresistant bacterium. Dr. Karl S. Kruszelnicki from Australia, writes

But there really wasn’t a lot research into the field of Being able to survive radiation until the late 1940s and 1950s. Around this time, three factors had emerged. First, there was the existence of the victims of the two Atom Bombs dropped on Japan, second, there was the start of the Cold War and the nuclear standoff between the Superpowers, and finally, there was the search for peaceful uses for nuclear power. As a result, we discovered that we humans are much more susceptible to radiation than insects, and will die after a dose of some 400 – 1,000 rads. For example, some people as far as 21 kilometres from Ground Zero at Hiroshima received doses of 1,200 rads – and suffered slow and agonising deaths. But insects turned out to be much more radiation resistant. Wood-boring insects and their eggs were able to survive doses of 48,000 to 68,000 rads with no apparent ill effect. In 1959, Drs. Wharton and Wharton found that it took 64,000 rads to kill the fruit fly, and a colossal 180,000 rads to be sure of killing the parasitoid wasp, HabrobraconSource

While for cockroach, it only takes 10 times more dose to kill it. The rad is a unit of absorbed radiation dose in terms of the energy actually deposited in the tissue. The table below shows typical dose examples. 1 krad is 1000 rad and 1 Mrad is 1000000 rad.

25 rad -lowest dose to cause clinically observable blood changes
0.2 krad -local dose for onset of erythema in humans
0.4 krad -whole body LD50 for acute radiation syndrome in humans
1 krad -typical radiation tolerance of ordinary microchips
4 to 8 krad -typical radiotherapy dose, locally applied
10 krad -fatal whole-body dose in Wood River Junction criticality accident
1 Mrad -typical tolerance of radiation-hardened microchips

Daly et al., write in the Journal of Bacteriology

Deinococcus (formerly Micrococcus) radiodurans Rl was discovered by Anderson and coworkers in X-ray-sterilized canned meat that was found to have undergone spoilage. Culture yielded a red-pigmented nonsporulating gram-positive bacterium that was extremely resistant to ionizing radiation and many other agents that damage DNA. The deinobacteria are the most ionizing and UV-radiation-resistant organisms known, with full survival reported at exposures of 0.5 to 3 megarads (mega is a million), which is 10 times higher than killing the parasitoid wasp, Habrobracon.

Since its discovery, studies on D. radiodurans have shown
that its extreme resistance is attained via extraordinarily efficient DNA repair (Moseley, B. E. B. 1983. Photobiology and radiobiology of Micrococcus (Deinococcus)  radiodurans. Photochem. Photobiol. Rev. 7:223-274.). For example, ionizing radiation produces double-strand breaks (dsb) in the DNA of D. radiodurans with the same efficiency as in the DNA of other organisms; however, wild-type D. radiodurans can mend >100 dsb per chromosome without lethality or mutagenesis, whereas most other organisms can repair no more that 2 or 3.

Daly et al., were able to isolate a mutant strain which is sensitive to radiation and in this paper they report that a key protein is missing in the mutant which prevents the DNA repair. Since then there has been several papers on Deinococcus and can be searched on Pubmed.

My first article for the journal club during my PhD at DTU was an article on Deinococcus, titled “The RecA proteins of Deinococcus radiodurans and Escherichia coli promote DNA strandexchange via inverse pathways“. It was related to a project I was doing on gene shuffling of hexose transporters in Saccharomyces cerevisiaeUffe Mortensen, who was my project advisor, from the lab where I worked at DTU, Denmark is working on the rad52 protein in yeast to understand the molecular mechanism behind DNA double strand break repair. Other people who are working on DNA repair and whose paper we had discussed in our journal club are the following:

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