1. Its replication and its unending repair
DNA-
The "Stuff" of Life
Its replication and its unending repair

3. This is how the DNA molecule may be represented in a book
This is how the DNA molecule may be represented in a book. Notice the complementary base pairs.

4. DNA replication is semi-conservative :
(gives rise to 2 DNA molecules each with an old strand and a new strand) and not conservative (gives rise to 2 DNA molecules-one with 2 new strands and one with 2 old strands)

5. Model for DNA
Replication

6. -DNA is such a long molecule, replication must occur at the same time at a number of sites

7. This is a micrograph, three replication bubbles are visible along the DNA cultured Chinese hamster cells. The arrows indicate the directions of DNA replication at the two ends of the bubble.

8. The direction of DNA replication:
The are a number of enzymes involved in DNA synthesis.
-DNA polymerase can only add to the 3' end of a nucleotide. This means that synthesis can only occur from the 5'->3' direction.
-Nucleotides are always added on as triphosphates. When the nucleotides are added then two phosphates are cleaved off.

10. DNA replication
1. When it is time for the DNA to replicate, proteins attach to the places on the DNA molecule called origins of replication.
2. Enzymes unwind the DNA strand.
3. Single-stranded binding proteins stabilize the unwound parental DNA strand.
4. DNA polymerase starts base pairing DNA nucleotides to the original strands. The molecule is replicated in both directions. One side

13. DNA replication cont’d
is laid down continuously, the other side is laid down in discontinuous fragments because it can only grow from the 5' to the 3' direction. These fragments are called Okazaki fragments.
5. The Okazaki fragments are joined together by ligase.

17. Text representation of DNA synthesis

18. DNA mistakes
DNA polymerase can make mistakes apporximately 1 in every 1000 nucleotides. There are 6 billion nucleotides and therefore there should be approximately 1 million errors or mutations when DNA replicates but in reality there are about 3 errors or mutations in one replication.

19. How DNA is repaired
1. Repair enzymes rove up and down the DNA molecule looking for mistakes.
2. The incorrect DNA is removed by enzymes.
3. DNA polymerase then replaces them with the correct DNA nucleotide.
4. DNA ligase seals the new DNA sequence in place.

20. DNA repair