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What is DNA ? • Experiments in bacteria and bacteriophages – DNA :hereditary material in living organisms Watson, Crick, and Franklin • Rosalind Franklin’s research produced x-ray diffraction images of DNA – Helped Watson and Crick build their DNA model – received the Nobel Prize Watson and Crick’s DNA model • two strands of nucleotides, coiled into a double helix • Each nucleotide has – A five-carbon sugar (deoxyribose) – A phosphate group – A nitrogen-containing base (adenine, thymine, guanine, or cytosine Base Pairing • Bases of two DNA strands pair in only one way – Adenine with thymine • (A-T) – Guanine with cytosine • (G-C) • DNA sequence (order of bases) varies among species and individuals DNA Replication and Repair • cell replicates its DNA before dividing • semiconservative – Enzymes unwind double helix – DNA polymerases • assemble complementary DNA strands – DNA ligase • seals gaps in new DNA strands • Two double-stranded DNA molecules result – One strand of each is new What are mutations? • DNA repair mechanisms fix DNA damaged by chemicals or radiation • Proofreading by DNA polymerases corrects most base-pairing errors • Uncorrected errors are mutations • The dog ate the cat • The oga tet hec at.. What is Cloning? • Clones – Genetically identical individuals – Produced by artificial twinning, nuclear transfers • To clone an adult animal – Cell’s DNA must be reprogrammed to function like an embryonic cell and direct development