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DNA Replication

DNA Replication. Cell Cycle. Two stages in Cell Cycle: Growth stage: Interphase Division stage: Mitosis Cytokinesis Leads to the formation of two IDENTICAL daughter cells. DNA Replication. During Interphase DNA within the nucleus duplicates.

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DNA Replication

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  1. DNA Replication

  2. Cell Cycle Two stages in Cell Cycle: • Growth stage: • Interphase • Division stage: • Mitosis • Cytokinesis Leads to the formation of two IDENTICAL daughter cells.

  3. DNA Replication • During Interphase DNA within the nucleus duplicates. This happens by breaking H-bonds between complimentary base pairs, which allows for the DNA helix to unwind. Once unwound, each single DNA strand acts as a template to build a complementary strand of DNA (two identical strands).

  4. Semi-conservative Replication • Each new DNA molecule is made of one parent strand and one newly synthesized strand.

  5. Initializing DNA Replication • Replication begins when proteins bind at a specific site on the DNA knows as the replication origin. There are multiple origins of replication in eukaryotes.

  6. Separating the Strands • DNA helicase: unwinds the two DNA strands by breaking the H-bonds between complimentary base pairs. • Single-stranded binding proteins (SSBs): bind to the exposed DNA single strands and block H-bonding. • DNA gyrase: an enzyme that relieves any tension brought about by the unwinding of the DNA. Replication Fork

  7. Region of Replication • Replication fork: the junction where two strands of DNA are disrupted. • Replication bubble: the region where two replication forks are in close proximity producing a bubble in the replicating DNA.

  8. Building the Complementary Strands • Requires a RNA (ribonucleic acid) primer of 10-60 base pairs to be annealed to the template strand; synthesized by an enzyme called primase. • DNA polymerase III: the enzyme that builds the complementary strand using the template strand as a guide (builds DNA in the 5’ to 3’ direction)

  9. Leading Strand • DNA is always synthesized by DNA polymerase III in the 5’ to 3’ direction. • Since DNA is anti-parallel, only one complementary strand can be built continuously by adding on to the 3’ end. • Leading strand: uses the 3’-5’ as the template strand and is built toward the replication fork.

  10. Lagging Strand • Lagging strand: uses the 5’-3’ as the template strand and is built in short fragments called okazaki fragments in the direction OPPOSITE of the replication fork. • RNA primers are continuously added to the 5’ end. • Built in short segments of about 100-200 nucleotides.

  11. Creating One Strand from Okazaki Fragments • DNA polymerase I: removes RNA primers and replaces them with appropriate deoxyribonucleotides. Nicks are left between fragments. • DNA ligase: joins okazaki fragments from the lagging strand into one strand by creating phosophodiester bonds. • As the complementary strand is synthesized it automatically twists into a helix with the template strand.

  12. DNA Repair • DNA polymerase III and I act as proofreaders of the newly synthesized strand. • Act as exonucleases: when mistakes occur (nucleotide pairing) they are excised immediately. • Prevents mistakes from being copied in future replications.

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