DNA mutations.docx

Ways to cause DNA damage. Spontaneous Endogenous factors Exogenous factors Spontaneous DNA damage Cells are in aqueous environment therefore a lot of hydrolytic reactions can take place. Hydrolytic depurination- the bond between the sugar phosphate and the base is hydrolysed which the leaves with...

Ways to cause DNA damage. Spontaneous Endogenous factors Exogenous factors Spontaneous DNA damage Cells are in aqueous environment therefore a lot of hydrolytic reactions can take place. Hydrolytic depurination- the bond between the sugar phosphate and the base is hydrolysed which the leaves with the base on its own and sugar phosphate backbone. Hydrolytic deamination of bases- the bases that contain amino group are effected. Ammonia is removed which leaves with modified/different base. Endogenous factors Produced by the metabolism of the cell such as: Superoxide (O2-) Hydrogen peroxide (H2O2) Hydroxyl radicals (OH) Exogenous factors High energy radiation such as: UV X-rays Y-rays Organic compounds Inorganic compounds UV induced damage. The shorter the wavelengths, the higher energy it has therefore more damage it can cause. UV-C contains the most energy therefore causes the most DNA damage. UV-B contains less energy therefore mostly absorbed by the DNA. UV-A is relatively harmless to DNA. How does UV cause damage? Commonly, it cross links bases together This means DNA replication can’t take place as the bases can’t be recognised as they have different structures due to UV damage. Shorter wavelengths such as X-rays and y-radiation contains high energies therefore impacts the DNA more. Reactive oxygen species causes DNA base damage at multiple sites. Oxidises bases and causes different changes in DNA. Has similar consequences for DNA replication. X-rays and y-radiation produces single and double stand breaks. The backbone of DNA can also be effected. Alkylation agents also causes DNA damage. Alkylating agents transfer alkyl groups such as methyl, ethyl on electron rich atoms on DNA which are nitrogen and oxygen atoms. All the oxygen and nitrogen atoms are prone to changes by alkylating agents. Changes structure of DNA Chemicals can cross link DNA duplexes. Blocks transcription and DNA replication. Such as cisplatin. Can crosslink DNA strands together which stops cell division as the cell’s DNA cannot replicate further. Can also add protein on DNA to damage it. Consequences of DNA modifications Modification of bases can change base pairing. Can result in mutation. Double strand breaks can lead to gene rearrangement or loss of genes. Crosslinking of DNA strands inhibits transcription, translation and can kill a cell. Changes DNA sequence therefore changes reading frame. Types of mutations Point mutation- change of one base pair. Deletion- one or more base pair are lost. Insertion- one or more base pair are inserted. All the above causes frame shift Silent mutations= no consequences for protein sequence (degenerate- some amino acids can be encoded by several codons) Missense mutation= protein sequence changes due to change in amino acid due to different codon. Nonsense mutation= codon changes which produces a stop codon amino acid which can lead to premature stop in the translation. Mutations in the promotor/activator regions can also modify genes expression and regulation. DNA repair Important to conserve DNA sequence as the stability of DNA sequence is essential for life. If not repaired, it can lead to mutations and different diseases. Mechanisms to correct DNA damage Base and nucleotide Excision and elimination of the damaged nucleotide(s) via enzymes. DNA polymerase fills the gaps using the other strand as template. DNA ligase heals and seals the gap. Base excision repair DNA glycosylases scans DNA for alterations Altered bases are recognised. Modified bases are hydrolysed (sugar-base bond of damaged nucleotides) Endonuclease and phosphodiesterase remove sugar phosphate backbone as the whole nucleotide needs to be replaced. DNA polymerase fills in the gap and DNA ligase heals the nick. Nucleotide excision repair. Excision nuclease recognises the damage. Cuts further away from DNA damage on either side DNA helicase removes the entire portion of damaged strand. DNA polymerase fills gaps and DNA ligase heals the nick. Also reacts to distortions in the DNA structure caused by medications. Chemical reversal There are some enzymes available can directly reverse some of the damages and not all especially in prokaryotes. Doesn’t involve breakage of the phosphodiester backbone. Examples include: Photolyase reverses pyrimidine dimers Methyl guanine methyl transferase (MGMT) reverses methylation of guanine bases. Certain methylation of cytosine and adenine can also been reversed by cells