DNA Replication Essential Question: How do enzymes help ensure DNA is copied correctly?

Why does DNA replicate? Cells copy genetic information before cell division so that each new cell has a complete set of DNA DNA replication occurs in the nucleus…WHY??

DNA Template

How does DNA replicate 1) UNWIND: Topoisomerase unwinds the coiled strands of DNA

2) UNZIP: DNA Helicase “unzips” the strands of DNA breaking the hydrogen bonds

3) HOLD OPEN: Single-Strand Binding proteins (SSBs) keep strands separated

4) BASE PAIRING: DNA polymerase III bonds free floating nucleotides with nucleotides from the parent strand using base pairing rules

Direction of Replication DNA polymerase can only build in the 5’ to 3’ direction RNA Primer DNA Polymerase Nucleotide 5’ 3’ Direction of Replication copyright cmassengale

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 copyright cmassengale

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

6) PROOFREAD: DNA Polymerase I proofreads new strand and backtracks to correct errors

5) JOINING NUCLEOTIDES: DNA ligase bonds those nucleotides and okazaki fragments together.

The result is 2 identical DNA molecules.

Each w/1 new strand & 1 old strand Called Semi-conservative replication

(Continuous Replication) (Place where replication begins) (Replication starts & stops)

DNA Replication Animation

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copyright cmassengale Question: What would be the complementary DNA strand for the following DNA sequence? DNA 5’-CGTATG-3’ copyright cmassengale

copyright cmassengale Answer: DNA 5’-CGTATG-3’ DNA 3’-GCATAC-5’ copyright cmassengale