Proposed Models of DNA Replication. Conservative Model. Semi-Conservative Model. Dispersive model

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Griffin Skinner
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1 5.2 DNA Replication

2 Cell Cycle Life cycle of a cell Cells can reproduce Daughter cells receive an exact copy of DNA from parent cell DNA replication happens during the S phase

3 Proposed Models of DNA Replication Conservative Model Semi-Conservative Model Dispersive model

4 1. Conservative Model Parental DNA makes two new daughter strands After first replication cycle: New daughter strands join to form double helix Parental DNA remain as original double helix Newly formed DNA shown as blue

5 2. Semi-conservative Model Parental DNA makes two new daughter strands After first replication cycle: Each new daughter strand binds to a parental strand to form double helix Newly formed DNA shown as blue

6 3. Dispersive Model During replication, parental DNA broken into small fragments After first replication cycle: Each small fragment binds to pieces of the newly copied DNA to form double helix Newly formed DNA shown as blue

7 Proposed Models of DNA Replication Conservative Model Results One new molecule and one old molecule Semi-Conservative Model Two hybrid molecules of one old and one new strands Dispersive model Two hybrid molecules of mixture of old and new strands

8 Experiment to Determine Replication Model Matthew Meselson and Franklin Stahl Distinguish between parent and daughter strands of DNA by using two isotopes of nitrogen: light 14 N heavy 15 N

9 Experiment to Determine Replication Model Tag DNA with isotopes Separate content by centrifuge DNA containing the denser isotope 15 N forms a band near the bottom of the test tube DNA containing the lighter isotope 14 N forms a band near the top of the test tube

20 minutes (one cycle) After 40 minutes (two cycles)

10 Experiment to Determine Replication Model E. Coli grown in 15 N medium Transfer to 14 N medium After 20 minutes (one cycle) After 40 minutes (two cycles) Take a sample mix DNA with cesium chloride centrifuge density gradient

11 And the Winner is... Semi-Conservative Model

12 DNA Replication 3 main phases: 1. Initiation double helix is unwounded; base pairs are exposed 2. Elongation parental DNA is used as template to assemble the new strand; final DNA has one parental and one new strand 3. Termination the two new DNA strands are separated

13 Phase 1: Initiation Specific nucleotide sequence indicates the origin of replication Circular prokaryotic DNA has 1 origin linear eukaryotic DNA often has thousands

14 Phase 1: Initiation 1. Initiator proteins bind to DNA at the origin of replication 2. Helicase (enzyme) unwinds double helix by breaking the hydrogen bonds that link the complementary base pairs. 3. Single-strand-binding proteins stabilize the unwound strands. 4. Topoisomerase II (enzyme) relieves strain on the double helix that is generated from unwinding. A replication bubble forms with a Y-shaped replication fork at each end

15 Phase 1: Initiation Replication begin at many origin of replications. The replication bubbles expand laterally as DNA replication continues on both strands. All of the replication bubbles eventually fuse together.

16 Phase 2: Elongation (Priming) DNA polymerase cannot start incorporating nucleotides on its own. Needs an existing 3 end of a nucleic acid. RNA primase lays down RNA primer onto template strand The primer (10-60 nucleotides long) provides that 3 end.

17 Phase 2: Elongation DNA polymerase III then binds to parental strand adds complementary nucleotides using parental DNA as a template 5 to 3 direction towards the replication fork

18 Phase 2: Elongation Free bases are floating in the nucleoplasm as deoxyribonucleoside triphosphates. The energy required for DNA synthesis is provided by hydrolyzing the bond between the 1st and 2nd phosphates of the deoxyribonucleoside.

19 Phase 2: Elongation Elongation proceed in two direction, outwards from the origin of replication. The two DNA strands are antiparallel. DNA polymerase III only replicates in the 5 to 3 direction. Elongation is semi-discontinuous

Phase 2: Elongation (Semi-discontinuous) Leading strand uses the 3 to 5 template strand as its guide. Is built continuously towards the replication fork.

20 Phase 2: Elongation (Semi-discontinuous) Leading strand uses the 3 to 5 template strand as its guide. Is built continuously towards the replication fork. Lagging strand uses the 5 to 3 template strand as its guide. Is built discontinuously in short fragments. RNA primase constantly adds new RNA primers along the template strand. The fragments are known as Okazaki fragments.

21 Phase 2: Elongation (Semi-discontinuous) DNA polymerase I Removes the RNA primers Replaces them with the proper deoxyribonucleosides DNA ligase Joins the Okazaki fragments together (phosphodiester bonds)

22 Phase 3: Termination Eventually two replication forks will fuse together and form a continuous strand of newly synthesized DNA. The two new daughter DNA molecules will contain a newly synthesized copy of DNA and the parental DNA.

23 Important Enzymes in DNA Replication

Covalently bonded sugar-phosphate backbone with relatively strong bonds keeps the nucleotides in the backbone connected in the correct sequence.

Covalently bonded sugar-phosphate backbone with relatively strong bonds keeps the nucleotides in the backbone connected in the correct sequence. Unit 14: DNA Replication Study Guide U7.1.1: DNA structure suggested a mechanism for DNA replication (Oxford Biology Course Companion page 347). 1. Outline the features of DNA structure that suggested