second chapter mutation

1. Mutagens are the agents responsible to cause mutation & are of three
types
1. Physical
2. Chemical
3. Biological
Physical
Radiation was first mutagen reported ( its effect on genes were first time
suggested to cause mutation). It was discovered in 1890.
Roentgen discovered X ray in 1896
Bacquerel discovered radioactivity in 1896
Marie & Pierre Curie discovered radioactive elements in 1896. These
discoveries lead to birth of atomic physics & our understanding of
electromagnetic radiations.
Atoms are made of -ve charged particles e- orbiting round nucleus.
Nucleus contains + charged neutrons & uncharged neutrons. E move from
one orbit to another when gets energy jumps to higher energy levels when
releases again moves to lower energy level. This energy is release in the
form of electromagnetic radiations
Visible light UV light, Xray, gamma ray are all electromagnetic radiations

2. Radiations break a chromosome by chemical reactions which requires
energy. Mutagenic effect of radiation depends upon the type & amount of
energy left in tissues
Electromagnetic radiations
energetic high energy radiations
Visible light
UV light X & Gamma
leaves energy in High energy radiations
form of heat & activations not only heat &
Leaves energy in leads to chemical change activate but leave
form of heat it is Energy in form of
Less energetic ionization in cell
Ionizing radiations penetrates deeper in cells than visible & UV light
Imparting much energy to absorbing tissue to ionize atoms/molecules.

3. High energy wave moving at high speed is stopped it releases energy. This
energy makes an atom to lose an e- & become charged or ion (process is
known as ionization). Free moving electrons causes other atoms to lose
electron & become +ly charged ions, two processes generate pairs of +ve
& -ve charged ions
A large number of ions may cluster together to form an ion track. These
ions undergo chemical reactions to neutralize their charges to reach to
stable configuration while doing so they produce breaks in chromosomes
thus inducing mutations.
Free ions may combine with O2 produce highly reactive chemicals which
may also react with DNA & cause mutation.
Ionizing radiations
X ray & Gamma rays are energetic enough &produce reactive ions
(charged atoms or molecules) when they react with biological molecules
are referred as Ionizing radiations
Term also includes corpuscular radiations
Stream of atomic & subatomic particles emitted by radioactive elements
(alpha & beta former He nuclei 2 protons & 2 neutrons ; later electrons.
UV light not ionize but they react with DNA & other biological molecules.

5. Biological effects of Radiations
Produce range of damage to cells /organisms mainly due to production of
free radicles of water (hydroxyl group). These free radicles posses
unpaired electrons& chemically very reactive interact with DNA, protein,
lipids in cell membranes
Xray cause DNA & protein damage –Organelle failure, block cell division
& cause cell death rapidly dividing cell types (blood cells forming areas of
bone marrow2, gastro- intestinal track lining are mostly affected by
ionizing radiations & severity depends on dose received.
A lot of information available on accidental exposure o nuclear plant 3
workers & victims of atomic bomb explosion such as those in Nagasaki &
Hiroshima. Sub lethal (100-200 rems) ; lethal (350-450 rems); supra lethal
(> 650 rems)
Ionizing radiations produce a range of effects on DNA through free
radicals & breaks in one or two strands (rearrangement or deletion of
chromiosome).

6. Free e- directly interacts with either e- or OH- can cause
DNA following types damage
e-
P+
H
O ss Break
H Deletion
cross linking
DS break
Hydroge atom consist of 1e- & 1 H+ e- may interact with water to produce
It is ionized OH free radicle which can damage the
DNA in same way as e-

7. • Zirkle (1930) reported that plants nucleus is more sensitive to ionizing
radiations than cytoplasm. Purines are less sensitive than pyrimidines
• UV Light
UV C UV B UV A
(180-290nm) (290-320nm) (320-visible)
Germicidal most Major lethal fraction deleterious effect
Energetic & lethal mutagenic fraction of creates O2 radicles pro
Absorbed by O3 layer sunlight duce few dimers

8. E Altenberg first to report that UV can cause mutation Longer WL can
not penetrate deeply effect superficial layers DNA has absorption max at
260nm. Results in formation of pyrimidine dimers linked by C=C bonds