1. There’s one last thing you need to know about DNA structure…

2. The DNA molecule is a double helix.

3. Double Helix Two strands of nucleotides twisted around each other. T N

4. Single helix
Double helix

5. DNA is twisted because the covalent bonds in the backbone are bent.

6. Bent bonds = spiral shape

7. Now that we know the shape of DNA, we can think of replication in a new way:

8. DNA Replication
Process that separates the double helix and then copies each strand. S D H C S

9. DNA replication occurs before a cell divides to ensure that both daughter cells can keep making the same protein. B D D C P
Insulin

10. DNA replication is performed by enzymes.

11. Replication occurs in 3 basic steps.

12. 1. First, the cell un-twists the double helix.

13. Why is the enzyme called “helicase?”
2. Next, an enzyme called helicase breaks the hydrogen bonds; this allows the two strands to separate. E H H S
Why is the enzyme called “helicase?”
Helicase separates the double helix
Helicase

14. DNA Polymerase polymerizes nucleotides
3. Finally, another enzyme called DNA polymerase uses the old strand as a template to build a complementary new strand. E D P S C
Guide
DNA Polymerase
DNA Polymerase polymerizes nucleotides
Why is the enzyme called “polymerase?”
DNA Polymerase

15. The final result is two identical DNA molecules…

16. …each with one old strand and one new strand.

17. REPLICATION 2.
DNA polymerase 4.
Template strand
Free
nucleotides 3. 5.
New strand
Helicase
New 6.
strand
Thymine 9. 8. 7.
DNA polymerase
Template strand

18. DNA replication is a nearly perfect process for 2 reasons.

19. 1. DNA polymerase “knows” which nucleotides to add because it follows base-pairing rules.

20. 2. DNA polymerase also “proofreads” each strand, making sure there are no errors.

21. Since bonds change during DNA replication, it is considered a chemical reaction.

22. However, the reaction has high activation energy…

24. …to prevent the DNA molecule from changing accidentally.

25. DNA replication will only occur if enzymes are present to lower the A
DNA replication will only occur if enzymes are present to lower the A.E. of the reaction. R E L