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copyright cmassengale

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1 copyright cmassengale
DNA Replication copyright cmassengale

2 Anti-Parallel Strands of DNA
On the left is the DNA double helix. When the helix is unwound, a ladder configuration shows that the uprights are composed of sugar and phosphate molecules and the rungs are complementary bases. Notice that the bases in DNA pair in such a way that the phosphate-sugar groups are oriented in different directions. This means that the strands of DNA end up running antiparallel to one another, with the 3’ end of one strand opposite the 5’ end of the other strand.

3 copyright cmassengale
Replication Facts DNA has to be copied exactly before a cell divides DNA is copied during the S or synthesis phase of interphase New cells will need identical DNA strands copyright cmassengale

4 Synthesis Phase (S phase)
S phase during interphase of the cell cycle Nucleus of eukaryotes Mitosis -prophase -metaphase -anaphase -telophase G1 G2 S phase interphase DNA replication takes place in the S phase. copyright cmassengale

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DNA Replication Begins at Origins of Replication Two strands open forming Replication Forks (Y-shaped region) New strands grow at the forks Replication Fork Parental DNA Molecule 3’ 5’ copyright cmassengale

6 copyright cmassengale
DNA Replication As the 2 DNA strands open at the origin, Replication Bubbles form Prokaryotes (bacteria) have a single bubble Eukaryotic chromosomes have MANY bubbles Bubbles copyright cmassengale

7 Synthesis of the New DNA Strands
The Leading Strand is synthesized as a single strand from the point of origin toward the opening replication fork RNA Primer DNA Polymerase Nucleotides 3’ 5’ copyright cmassengale

8 Synthesis of the New DNA Strands
The Lagging Strand is synthesized discontinuously against overall direction of replication This strand is made in MANY short segments It is replicated from the replication fork toward the origin RNA Primer Leading Strand DNA Polymerase 5’ 3’ Lagging Strand 5’ 3’ copyright cmassengale

9 Lagging Strand Segments
Okazaki Fragments - series of short segments on the lagging strand Must be joined together by an enzyme Lagging Strand RNA Primer DNA Polymerase 3’ 5’ Okazaki Fragment copyright cmassengale

10 Joining of Okazaki Fragments
The enzyme Ligase joins the Okazaki fragments together to make one strand Lagging Strand Okazaki Fragment 2 DNA ligase Okazaki Fragment 1 5’ 3’ copyright cmassengale

11 Replication of Strands
Replication Fork Point of Origin copyright cmassengale

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Proofreading New DNA DNA polymerase initially makes about 1 in 10,000 base pairing errors Enzymes proofread and correct these mistakes The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors copyright cmassengale

13 Semiconservative Model of Replication
New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNA DNA Template New DNA Parental DNA copyright cmassengale

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DNA Damage & Repair Chemicals & ultraviolet radiation damage the DNA in our body cells Cells must continuously repair DAMAGED DNA Excision repair occurs when any of over 50 repair enzymes remove damaged parts of DNA DNA polymerase and DNA ligase replace and bond the new nucleotides together copyright cmassengale

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