the role of clays in shielding dna against x-ray radiation: possible implications on the origin of...
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The Role of Clays in Shielding DNA against X-Ray Radiation: Possible Implications on the origin of life
Angela Ciaravella
F. Scappini (2), M. Franchi (3), E. Gallori (3), C. Cecchi-Pestellini (4), M. Barbera (1),
R. Candia (1), G. Micela (1)
(1) INAF – Osservatorio Astronomico di Palermo , Italy
(2) ISMN –CNR , Bologna, Italy
(3) Dipartimento di Biologia Animale e Genetica, Univ. di Firenze, Italy
(4) Dipartimento di Fisica, Univ di Lecce, Italy
Firenze 28/02 – 1/03/05
First evidence of life on the Earth 3.8 × 109 yr
Earth Atmosphere :
a big unknow, very little screening from Sun radiation
Conditions on the Early Earth
Early X-ray Sun:
more active, hard and bright 3 × 1026 (0.1 keV)
< Lx > = 1027 erg/sec
5 × 1027 (0.2 keV)
Yohkoh-SXT 3-40 Å band
1-10 keV band:
the photon flux today is ~10-3 the flux at the age of Pleiades
Sun
@ m
inSu
n @
max
How and under which conditions life was created?
DNA Irradiation
Free DNA type B
Clay Adsorbed DNA type A
34 Å per turn B
27-28 Å per turnA
Why DNA? DNA is already life
An easy and well tested method to estimate damage
We irradiate free and clay absorbed DNA from Bacillus Subtilis
CLAYS: Montmorillonite Na0.2Ca0.1Al2Si4O10(OH)2(H2O)10
Kaolinite Al2Si2O5(OH)4
Vacuum tight container
X-ray, UV, Vis 2 -104 eV (1- 7000 A).
17 m stainless-steel vacum beam lineclass 1000 clean_room (Barbera et al 1999).
The XACT Facility:
Al 1.49 0.15 Ti 4.51 0.17 Cu 8.04 0.20
Flux (erg sec-1 cm -2 )
Line(keV )
DNA X-Ray Irradiation
Irradiation Dose : 102 - 5.8 × 104 ergExposure Time : 2m - 16h
10-5 minimum
3 × 10-3 maximum
10-1 large flares
The Sun today (1.5 –12.4 keV):
DNA Samples: Free 10μg DNA + 1400l of H2O Clay Adsorbed 10μg DNA + 2mg Clay + 1400l of H2O
DNA Damage Evaluation
The integrity of DNA molecules is estimated by transformation experiments :
efficiency of irradiated DNA to transform competent cell in a bacterial culture
for
Number of transformed bacterial cellsFtf =
Total number of bacterial cells
Comparing Ftf with Ftf0 the damage is evaluated
Typical errors on Ftf are 20 %
The transformation frequence is:
Transformation frequence for non irradiated DNA
Results and Conclusions I
*** Free DNA is severely damaged by X-rays
and the damage depends on the energy dose rather than the hardness of radiation
⃟ ⃟ ⃟ ⅏⅏ ⅏
Clay Adsorbed DNA is resistent to the X-ray irradiation
Can clays shield DNA against X-rays?Diluted clay is much more transparent than H2O to X-rays
The more compact configuration (A) of clay adsorbed DNA binding to the substrate by electrostatic and/or hydrogen bonds
can play protective role
Results and Conclusions II
Are X-rays directly responsible for DNA damage?
Num X photons (Cu 5 × 104 erg) 4 × 1012
Num H2O molecules 5 × 1022
Num DNA molecules 4 × 1011
Secondary UV
UV irradiation of free and clay adsorbed DNA give similar results (Scappini et al 2004)
2.5 mm
1 mm
0.2 mm
0.01 mm
Are clays essential in protecting the basic building blocks of life as well ?
DNA adsorbed on clays do survive X-rays and UV photodegradation