3.4 & 7.2 DNA Replication

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

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 Thymineand 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’ directioni.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]