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Tuesday, August 21, 2012

DNA Damage & Repair

Here's a great video explaining DNA damage and repair mechanisms.  It's from Prof. Jim Boyne of the University of Bradford.  Unfortunately, what that means is that in addition to technical jargon, you've got heavily accented technical jargon.  If you're not up on basic molecular biology, you might want to pass.  If you're wishy-washy you might want to read this first.

He does a good job (though not intentionally) of addressing a common health physics denialism claim that goes something like:

1.  We have lots of DNA damage happening all the time.

2.   Some event happens causing low dose radiation exposure.

3.  There is no increased risk from the radiation because of 1.

Mathematically, it's like claiming 100,000 + 1 = 0, whereas the correct answer is 100,001.
Prof. Boyne also highlights that the damage and repair mechanisms associated with radiation are different from those associated with intrinsic causes.

I recommend you enjoy it with a cup of tea and some biscuits:


  1. I would be interested to hear your opinion of this recent research, which appears to contradict LNT by directly observing repair processes in living cells:

    Here's the money quote:

    "Says co-author Mina Bissell of Lawrence Berkeley National Lab: 'Our data show that at lower doses of ionizing radiation, DNA repair mechanisms work much better than at higher doses. This non-linear DNA damage response casts doubt on the general assumption that any amount of ionizing radiation is harmful and additive.'”

    The full paper is free at the link below:

    Neumaier et. al., “Evidence for formation of DNA repair centers
    and dose-response nonlinearity in human cells”, Proc. Natl. Acad. Sci. 108, not yet printed

    Sincerely yours,

    An LNT "denialist" while being a firm believer in linearity above 200 mSv or so

    1. You seem to have a cartoonish view of LNT. See Figure 10-1 at the bottom of the page here:

      We don't just linearly extrapolate from high doses to low doses. We know that at low doses biological repair mechanisms have greater fidelity. We incorporate a term called the Dose & Dose Rate Effectiveness Factor (DDREF) to account for this.

      The LBL study illustrates the DDREF "in action".

      There are basically only 2 points in the study, so obviously a line (linear) can be drawn between them, but it is of downward slope:

      15 RIF/Gy x 2 Gy = 30 RIF

      64 RIF/Gy x 0.1 Gy - 6.4 RIF

      The authors' mathematical model didn't agree with the evidence. They need to change their model.

    2. You accuse me of having a "cartoonish" view, yet you link me to a page with a graphic showing an approximation (straight line at low dose/dose rate) to a "hypothetical" curve, based on a "presumed" curvature?

      At least you have a sense of humor.

      I'm an engineer. I prefer data to presumptions.

    3. The intent of the link was to explain the DDREF.

      Epidemiologists stratify the cancer data using dose groupings. Currently we see are seeing a higher than expected (based on linear model) cancer rate in the 0-100 mGy range with the A-bomb survivors. (And lower than expected in the 300-700 mGy range).

      This doesn't mean we only see down to 100 mGy as that strata consists of lots of people with doses below 100 mGy. The more one stratifies, the lower the sample number within each strata which makes it harder to get statistically significant results within each strata, especially as the dose approaches backgroud. It's a trade-off, but it's the best DATA and means to analyze that we have.

      We use mice DATA (large numbers, well defined doses) to help fill in the human epidemiology because they seem to respond to radiation similarly to humans.

      When we statistically calculate the Maximum Likelihood Estimator for a threshold using human DATA, the the result is 0.

      We also incorporate human and animal DATA from molecular biology similar to the study you provided. However those studies have less evidentiary weight (see my page "LNT Model & Radiation Studies", upper right of this page, especially the Pyramid Of Evidence).

  2. Ooopps, should say, "64 RIF/Gy x 0.1 Gy EQUALS 6.4 RIF"