DNA Replication

What is DNA replication? When does it happen? DNA replication is the process by which the DNA molecule duplicates itself to create identical copies during the S phase prior to cell division – either mitosis or meiosis. DNA replication is the process by which the DNA molecule duplicates itself to create identical copies during the S phase prior to cell division – either mitosis or meiosis.

Semi-Conservative Replication DNA replication is also called semi-conservative replication DNA replication is also called semi-conservative replication One DNA molecule is used as a template to produce two new, identical molecules. Thus, each new DNA molecule consists of one “parent” strand and one newly synthesized strand.

Steps in DNA Synthesis Anneal -the pairing of complementary strands of DNA through hydrogen bonding. Anneal -the pairing of complementary strands of DNA through hydrogen bonding. DNA replication requires a number of different steps, each associated with a different enzyme. DNA replication requires a number of different steps, each associated with a different enzyme. The rules of complementary base pairing (A-T, G-C) helps explain how DNA is replicated prior to cell division. The rules of complementary base pairing (A-T, G-C) helps explain how DNA is replicated prior to cell division.

Ensures that DNA REPLICATION occurs rapidly

1. Gyrase - Relieves pressure in coil caused by unwinding - Initiates unwinding of DNA 2. Helicase - Unwinds DNA at replication fork by breaking the H bonds 3. SSB - Destabilizes helix - Facilitates unwinding by blocking H-bonding - Stability of replication fork 4. RNA Primer, Primase - Initiates complementary chain growing 5&7 DNA Polymerase - Constructs growing complementary chain (5’  3’) 6. DNA Ligase - joins the gaps in the Okazaki fragments Okazaki Fragment - Short fragment of DNA that is the result of the synthesis of the lagging strand Laggin g Strand - Uses 5’  3’ templat e as a guide Leading Strand - Uses 3’  5’ templat e as a guide

DNA polymerase III Enzyme/ProteinEffect DNA gyraseRelieves any tension from the unwinding of the double helix Single-stranded binding proteins (SSB)Keep separated strand of DNA apart by blocking hydrogen bonding. helicaseBreaks the hydrogen bonds between the base pairs primaseSynthesizes RNA primers that will be used by DNA polymerase as a starting point to build the new complementary strands Cannot initiate a new complementary strand by itself Requires an initial starting 3’ end to start elongation. Synthesize DNA in the 5’ to 3’ direction. Add free deoxyribonucletides triphosphates to a 3’ end of the elongation strand. Uses the energy from breaking the bonds b/w first and second phosphate to drive the condensation reaction that adds the complementary nucleotide. Act as quality control checker by proofreading the new strands of DNA DNA polymerase IRemoves RNA primers from the leading strand and replaces them with the appropriate DNA nucleotides to one end of the growing complementary strand of nucleotides and builds the DNA in a 5’ to 3’ direction exonucleaseRemove sections of DNA that are incorrectly paired to the complementary strand. This must be done quickly to avoid mistakes in the replication of DNA. ligaseJoins the gaps in the Okazaki fragments by binding the backbones (phosphate to sugar) of the fragments

A leading strand: replication proceeds continuously along it toward the replication fork. A leading strand: replication proceeds continuously along it toward the replication fork.

A lagging strand: A lagging strand: Composed of short segments of DNA, known as Okazaki fragments, is built discontinuously away from the replication fork. Composed of short segments of DNA, known as Okazaki fragments, is built discontinuously away from the replication fork. The process occurs in separate short segments because DNA polymerase I can only add nucleotides to the 3’ end of a growing DNA strand. The process occurs in separate short segments because DNA polymerase I can only add nucleotides to the 3’ end of a growing DNA strand. The overall direction of growth of the lagging strand must be from its 3’ end toward its 5’ end, which is opposite to the direction of nucleotide addition by DNA polymerases. The overall direction of growth of the lagging strand must be from its 3’ end toward its 5’ end, which is opposite to the direction of nucleotide addition by DNA polymerases.