1. 3.4 & 7.2 DNA Replication Pp 16 – 19 & 60 - 61Pp 58 – 60 & 197 -200

2. How it works Prior to division: cell has to duplicate its DNA DNA replication occurs during S phase of interphase in preparation for mitosis cell division DNA replication is semi – conservative i.e. 50% of daughter DNA molecule is new strand while 50% is old (original) strand Helicase splits double strand DNA polymerase binds nucleotides to create strand 2 new strands are formed Strands are identical to original due to complementary base pairing

5. significance of complementary base pairing Adenine forms hydrogen bonds with Thymine and Guanine form hydrogen bonds with Cytosine complementary base pairing ensures proper bases are incorporated into the new DNA strand This ensures conservation of the base sequence by making identical copies of the DNA strand

6. Semi-conservative DNA Replication

8. Direction of DNA Replication DNA replication occurs in a 5’ to 3’ direction!

9. Enzymes drive replication Helicase:- uncoils DNA & splits the complimentary strands DNA polymerase III:- adds nucleotides in 5’ → 3’ direction (leading strand) – movement towards helicase (replication fork) RNA primase:- adds short RNA primer strand to DNA (primer = allows DNA polymerase to start on lagging strand) DNA polymerase I:- removes RNA primer & replaces it with DNA leaving gaps between Okazaki fragments (i.e. short lengths of DNA formed between RNA primers) DNA ligase:- seals up gap (creates sugar-P bond) between Okazaki fragments

10. Okazaki fragments In eukaryotes, replication is initiated at many points concurrently. On lagging strand, DNA replication occurs away from the replication fork, forming Okazaki fragments (i.e. short lengths of DNA formed between RNA primers)

11. Process of DNA replication DNA replication occurs during S phase of interphase in preparation cell division DNA replication is semi-conservative i.e. the new DNA molecule has one new strand & one old strand Enzyme helicase unwinds the double helix & separates the strands by breaking the hydrogen bonds between the two complimentary strands each strand of parent DNA is used as template for synthesis of a new strand synthesis of a new strand is continuous on leading strand but not continuous on lagging strand Okazaki fragments (small strands of DNA) are formed on lagging strand synthesis of new DNA strand occurs in 5’ → 3’ direction i.e. new nucleotides are added to 3’ of the growing strand RNA primer synthesized on parent DNA using RNA primase to act as the starting point of synthesing a new strand DNA polymerase III adds the nucleotides to the 3 end according to complementary base pairing adenine pairs with thymine and cytosine pairs with guanine DNA polymerase I removes the RNA primers and replaces them with DNA DNA ligase joins Okazaki fragments as deoxynucleoside triphosphate joins with growing DNA chain, two phosphates are broken off releasing energy required for bond formation

14. Revision Questions Explain the significance of complementary base pairing during DNA replication. [3] State a role for each of five different named enzymes in DNA replication. [5] Explain the process of DNA replication. [8]