DNA Damage PDF - B.Sc. III Notes

B.Sc. III Notes DNA Damage 1 Molecular Biology Unit- I “DNA Damage” Introduction: Any kind of permanent alteration in chemical nature of DNA sequence is called as DN...

B.Sc. III Notes DNA Damage 1 Molecular Biology Unit- I “DNA Damage” Introduction: Any kind of permanent alteration in chemical nature of DNA sequence is called as DNA damage and if it does not repair lead to Mutation. DNA damage is distinctly different from mutation, although both are types of error in DNA. DNA damage is an abnormal chemical structure in DNA, while a mutation is a change in the sequence of standard base pairs. DNA damage may be occurs due to Lesion (single nucleotide mismatch, thymine dimer, modification of existing nucleotide) and Breaks in strands of DNA. Modes of DNA damage: 1) Spontaneous DNA damage 2) Induced DNA damage 1) Spontaneous DNA damage: Damage in DNA is occurs in absence of any external known factors, in other word damage occurs naturally is called as spontaneous DNA damage. Following are the some natural reasons due to which damage occurs in DNA. a) Tautomeric Shift: Tautomer’s is one of the reasons for DNA damage. In this Amino form of bases are converted into Imino form i.e. NH2 group is converted in to double bond =NH) and Keto form of bases are converted into Enol form, means normal bases converted in to Tautomeric bases (Fig.1.1). Tautomeric forms are not so much different than normal bases that’s why they easily escape from proof reading activity. Amino Form Imino Form DNA Damage © Mr. R. J. Patil  B.Sc. III Notes DNA Damage 2 NORMAL BASE PARING BASE PAIRING AFTER TAUTOMERIC HIFT A=T (Amino form) (Imino) A*= C C=G (Amino form) (Imino) C*= A T=A (Keto form) (Enol) T*=G G=C (Keto form) (Enol) G*=T Due to tautomeric shift transition type of mutation occurs that means Purine (R) is converted into Purines i.e. Adenine is converted into Guanine and Pyrimidines(Y) are converted into Pyrimidine (Fig 1.2). A A A=T G=C T Replication C G A * C Replication T T A Fig. 1.2 Transition type of mutation. b) Spontaneous Deamination: In O2 rich environment generally, bases have tendency to deaminate e.g. deamination of Cytosine leads formation of Uracil (Fig 1.2). Due to which transition type mutations are occurs. Deamination Hypoxanthine Fig.1.3.Some examples of Deamination DNA Damage © Mr. R. J. Patil  B.Sc. III Notes DNA Damage 3 Normal Base Paring Pairing After Deamination A=T (Deamination A forms Hx.) Hypoxanthine (Hx.) = C C=G (Deamination C forms U.) Uracil (U) = C G=C (Deamination G forms X.) Xanthene = could not bind to any other bases. Xanthene could not bind to any other Nitrogen Bases lead to formation of apurinic site on DNA molecule. c) Spontaneous depurination: It is most common reason for DNA damage. It is occurs due to the breakage in Glycosydic bond presents in between Sugar and Nitrogen bases. Glycosydic bond is weaker bond so it easily breaks after small rise in Temperature (more than 45 0c.) due to which Apurinic sites (AP) are formed on DNA (Fig.1.4). Due to which transition type mutations are occurs. P S P P S P Glycosydic Bond A Temp. 450c. AP Site. T T A P P S P P S Fig.1.4 Spontaneous Depurination. d) Spontaneous Oxidative Damage: The reactive Oxygen present in all aerobic cell. It acts on H 2O to produce Hydrogen peroxide H2O2, Hydroxyl free radicle (-OH), they acts on Guanine and convert it to the 8 Oxo guanine. Due to Oxidative damage trans-version type of mutation occurs that means Purine (R) converted into the Pyrimidine i.e. Adenine is converted Thymine and Pyrimidines(Y) are converted into the Purine. DNA Damage © Mr. R. J. Patil  B.Sc. III Notes DNA Damage 4 8-Oxo-Guanine G G=C T=A G * Replication C T T G Replication A T C A Fig. 1.5. Transversion type of mutation. e) Slippage Error: Slippage errors occur due to the sliding of DNA polymerase. It is most observe during replication of certain repetitive sequence which is present in telomere region of chromosome. Generally DNA pol could not slip from DNA due to the PCNA clamp is associated with it, but by mistakely some time it slips. For e.g. Huntington gene of human. Generally Huntington gene contain 20-30 CAG repeats but due to Slippage error it increase up to 40-50 repeats due to which normal Huntington protein could not produce leads to Huntington Disease In humans. DNA Damage © Mr. R. J. Patil  B.Sc. III Notes DNA Damage 5 2) Induced DNA Damage: Damage in DNA occurs due to any known external factors (Mutagens) like Carcinogens, UV rays, Cigarate smoke (cigarate smoke contain Benzopyrine). The rate of mutation depends upon amounts of mutagens presents in environment. Following are the some agents which causes induced DNA damage. A) Physical agents. B) Chemical agents. C) Biological agents. A) Physical agents: Physical agents like radiations gives the breaks to the DNA strands or form the dimers of nitrogen bases during the DNA replication causes DNA damage. Most of the time this mutations are lethal. The radiations are of two types. Radiations Ionizing Radiation Non Ionizing Radiation Ionizing radiations are high energy Ionizing radiations are low energy strength radiation. strength radiation. It put double stranded or single It allow the covalent bond formation stranded breaks on DNA (breaks between two neighboring phosphodiester bond). Pyrimidine (Fig.1.6) For e.g. Clastogens like α,β,γ, rays For e.g. UV rays.(A, B,C,) emitted from radioisotopes and X-ray. Only UV- B acts as mutagen. UV- C also acts as mutagens but it is absorbed by ozone layer. UV-A is used for synthesis of Vitamin A. UV-B allows the covalent bond formation between two neighboring pyrimidine (Thymine = Thymine) due to which thymine could not binds to Adenine of other strand leads to formation of Apurinic sites (AP site) on DNA (Fig.1. 6 DNA Damage © Mr. R. J. Patil  B.Sc. III Notes DNA Damage 6 Thymine Dimer T T T T UV Light AP site A A Fig.1.6. Formation of thymine = Cyclobutane ring. thymine dimer or Cyclobutane ring (contain 4 carbon and 2 H ) T T B) Chemical agents: Certain chemicals like alkylating agents adds alkyl group (-CH3) to the bases e.g. o-6 methyl guanosine (This mythelation interfere with unwinding of the DNA molecule during replication. Most of the time this mutation is lethal. Alkylation C) Biological Agents: Biological agents like some viruses introduce their genes into host genome leads to alteration in host genome. E.g. retroviruses like HIV, HPLV, and Cauliflower mosaic virus etc. DNA Damage © Mr. R. J. Patil