DNA Synthesis and repair basic science oncology

Basic science oncology

1. DNA Synthesis and Repair Desak Pramesti , MD

2. THE STRUCTURE OF DNA DNA is composed of four nucleotides, each containing: adenine, cytosine, thymine, or guanine. The amounts of A =T, G = C, and purines = pyrimidines [Chargaff’s Rule]. DNA is a double-stranded helix with antiparallel strands [Watson and Crick, 1953]. Nucleotides in each strand are linked by 5’-3’ phosphodiester bonds Bases on opposite strands are linked by hydrogen bonding: A with T, and G with C.

3. DNA Replication THE BASIC PRINCIPLE: BASE PAIRING TO A TEMPLATE STRAND A. Semiconservative with respect to parental strand Each 2-stranded daughter molecule is only half new • One original strand was used as a template to make the new strand B. Bidirectional with multiple origins of replication C. Primed by short stretches of RNA D. Semidiscontinuous with respect to the synthesis of new DNA

4. PROTEINS INVOLVED IN DNA SYNTHESIS A. DNA polymerases B. DNA Helicases C. DNA Primases D. Single-stranded DNA binding proteins E. DNA ligase F. Topoisomerases G. Telomerase

5. THE MECHANISM OF DNA REPLICATION DNA replication is catalyzed by DNA polymerase which needs an RNA primer • RNA primase synthesizes primer on DNAstrand • DNA polymerase adds nucleotides to the 3 ’ end of the growing strand

6. PROTEIN SYNTHESIS The information content of DNA is in the form of specific sequences of nucleotides along the DNA strands • The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins • The process by which DNA directs protein synthesis, gene expression includes two stages, called transcription and translation TRANSCRIPTION AND TRANSLATION Cells are governed by a cellular chain of command • DNA  RNA  protein T ranscription Is the synthesis of RNA under the direction of DNA • Produces messenger RNA (mRNA) Translation is the actual synthesis of a polypeptide, which occurs under the direction of mRNA • Occurs on ribosomes

7. MRNA PROCESSING

8. We can divide translation into three stages Initiation Elongation Termination The AUG start codon is recognized by methionyl-tRNA or Met Once the start codon has been identified, the ribosome incorporates amino acids into a polypeptide chain RNA is decoded by tRNA (transfer RNA) molecules, which each transport specific amino acids to the growing chain Translation ends when a stop codon (UAA, UAG, UGA) is reached

9. Directionality Ribosom unit akan mulai bekerja dekat methyl cap

10. DNA REPAIR SYSTEMS is necessary  cells are continuously exposed to environmental mutagens but also mutations would otherwise occur spontaneously during DNA replication. Mismatch repair Mismatch repair deals with correcting the mismatches of normal bases that fail to maintain normal Watson-Crick base pairing (A to T, C to G) recognition of a mismatch is accomplished by several different proteins including those encoded by MSH2, MLH1, MSH6, PMS1 , and PMS2 genes  an inherited form of colon cancer (hereditary nonpolyposis colon cancer [HNPCC]) at a young age.

11. B. Base excision repair to correct the spontaneous depurination and spontaneous deamination (removal of amine groups) that happen to bases present in DNA. pontaneous deamination of cytosine causes it to be converted to uracil, normally found in RNA and not in DNA. C. Nucleotide excision repair necessary to remove ultraviolet light–induced DNA damage as well as DNA damage from environmental chemicals. UV lights can nevertheless form pyrimidine-pyrimidine dimers from adjacent pyrimidine bases (cytosine and guanine) in DNA. Thus, sunlight is mutagenic, causing both sunburn and skin cancer Xeroderma pigmentosum is a genetic disorder of DNA repair in which patients carry mutations in the nucleotide repair enzymes. D. Double-stranded DNA repair 1. Homologous recombination This type of repair takes advantage of sequence information available from the unaffected homologous chromosome for proper repair of breaks. BRCA 1 and 2 proteins normally play a role in the homologous recombination process. Mutations in these genes increase breast cancer risk 2. Nonhomologous end joining his process permits the joining of ends even if there is no sequence similarity between them. It is error prone since it can also introduce mutations during repair.

12. Thank you