REVIEW DNA Structure

Deoxyribonucleic Acid DNA Deoxyribose sugar Double helix A -2-T, C-3-G Strands are complementary Purines: A and G Pyrimidines: T and C

Ribonucleic Acid RNA Ribose Sugar Single-stranded A-2-U, C-3-G Purines: A and G Pyrimidines: U and C

DNA Replication Many proteins work together in DNA rep and repair The two strands are complementary, each will act as a template for the new strand in rep (a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C. (b) The first step in replication is separation of the two DNA strands. (c) Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand. (d) The nucleotides are connected to form the sugar-phosphate backbones of the new strands. Each “ daughter ” DNA molecule consists of one parental strand and one new strand. A C T A G A C T A G A C T A G A C T A G T G A T C T G A T C A C T A G A C T A G T G A T C T G A T C T G A T C T G A T C Figure 16.9 a–d

DNA Replication is semiconservative Each of the two new daughter molecules will have one old strand (parent strand) and one newly made strand Figure a–c Conservative model. The two parental strands reassociate after acting as templates for new strands, thus restoring the parental double helix. Semiconservative model. The two strands of the parental molecule separate, and each functions as a template for synthesis of a new, comple- mentary strand. Dispersive model. Each strand of both daughter mol- ecules contains a mixture of old and newly synthesized DNA. Parent cell First replication Second replication

Origins of Replication Sites where the two strands are separated  Unwound by Helicase Euks: may have hundreds or even thousands Stabilized by SSBPs until replication is complete

Elongation Begins at the replication fork Is catalyzed by DNA polymerases, which add nucleotides to the 3’ of a growing strand Figure New strandTemplate strand 5 end 3 end Sugar A T Base C G G C A C T P P P OH P P 5 end 3 end 5 end A T C G G C A C T 3 end Pyrophosphate 2 P OH Phosphate

Leading vs. Lagging DNA poly only add nucleotides to the 3’ ends Leading strand: the strand on which DNA polymerase can synthesize a comp strand continuously, moving toward the replication fork Lagging strand: DNA polymerase must work in the opposite direction, away from the fork, synthesizing Okazaki fragments  Joined together by DNA ligase

Initiation DNA poly cannot initiate rep, since they can only add nucleotides to the 3’ end And RNA primer is required, synthesized by Primase  One for the leading strand  Multiple for lagging strand, each Okazaki fragment must be primed separately

DNA Replication “Machine” Various proteins participate in rep  DNA poly, helicase, ligase, etc They form a single, large complex or machine

Proofreading and Repairing DNA DNA poly proofread newly synthesized DNA an replace any incorrect nucleotides Excision repair: enzymes cut out and replace damaged stretches of DNA

Replicating the Ends of DNA Molecules The ends of euk chromosomal DNA get shorter with each round of replication So, the ends are made of repeating nucleotide sequences called telomeres  Postpone the erosion of genes near the ends of DNA molecules In germ, cancer, stem cells:  Telomerase catalyzed the lengthening of telomeres to prevent the loss of essential genes

Teach it to a 7 th Grader Draw a flow chart that uses analogies to explain the roles of each protein/enzyme involved Must have drawings and explanations Analogy example:  Helicase : Pair of scissors