2. DNA Replication

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4. Replication of DNA Before a cell can divide by mitosis or meiosis, it must first make a copy of its chromosomes The DNA in the chromosomes is copied in a process called DNA replication. Without DNA replication, new cells would have only half the DNA of their parents. DNA Replication

5. Steps of Replication Double helix “unzips” Free bases bind to their complementary partners on each side of the DNA double helix. Two identical copies are made each one with half of the original strand; semi-conservative replication.

6. Copying DNA Original DNA Original DNA Strand Free Nucleotides New DNA molecule New DNA Strand New DNA molecule

7. Replication of DNA DNA Replication Movie

8. Copying DNA DNA is copied prior to cell division. It is important that the new copies are exactly like the original molecules.

9. 2005-2006 Replication: 1st step Unwind DNA helicase enzyme unwinds part of DNA helix stabilized by single-stranded binding proteins single-stranded binding proteins

10. 2005-2006 Replication: 2nd step Bring in new nucleotides to match up to template strands But … We ’ re missing something! What? Where ’ s the ENERGY for the bonding!

11. 2005-2006 energy Energy of Replication Where does the energy for the bonding come from? ATPADPAMP You remember ATP! Is that the only energy molecule? GTPTTPCTPGMPTMPCMP

12. 2005-2006 Energy of Replication The nucleotides arrive as nucleosides DNA bases with P–P–P DNA bases arrive with their own energy source for bonding bonded by DNA polymerase III ATPGTPTTPCTP

13. DNA P III energy energy energy Replication Adding bases can only add nucleotides to 3 end of a growing DNA strand strand grow 5'  3’ energy 5' 3' 5' B.Y.O. ENERGY leading strand

14. ligase energy 5' 3' 5' leading strand 3'5' 3' 5' lagging strand

15. DNA Replication Protein binds to DNA Enzyme begins breaking the hydrogen bonds between bases. Enxyme (DNA polymerase) binds to each of the separated parent strands. DNA polymerase moves along the strand and replicates it.

16. DNA Replication The strand only grows from the 5’ to 3’ direction. The strand that is replicated continuously is called the “leading strand” DNA ligase creates sections of the other strand that are replicated separately, also in the 5’ to 3’ direction. The chain that is filled in with fragments is called the “Lagging strand”

17. DNA Replication Each fragment that is replicated is called an Okazaki fragment. Okazaki fragments are joined when the ligase is removed and replaced by appropriate bases.

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19. Red DNA or Blue DNA? Semi-Conservative!

20. Replication Bubbles The average human chromosome contains 150 x 10 6 nucleotide pairs which are copied at about 50 base pairs per second. It would take a month to replicate if replication only occurred at one site. Many places on the DNA are replicated at the same time.

21. Replication Bubbles Each replication site is called a “replication Bubble” It takes about an hour to replicate a human chromosome

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