1. AP Biology Discussion Notes
Monday 01/30/2017

2. Goals for Today
Be able to identify & describe the players & processes in DNA replication
Be able to identify two limitations of DNA polymerases and describe three problems that derive from these limitations.

3. 1/30 Question of the Day
DNA POLYMERASES are the enzymes responsible for building the new polymer of DNA. Identify one limitation of DNA Polymerases aside from their directional limitation (they can only read __’ -> __’ & build __’ -> __’) and describe how this problem is solved.

4. The Solution….
Primase – an enzyme that adds a short RNA primer to begin initial nucleotide strand

5. Overall directions of replication
Figure 16.17
Overview
Leading strand
Origin of replication
Lagging strand
Leading strand
Lagging strand
Overall directions of replication
Leading strand 5
DNA pol III 3
Primer
Primase 3 5 3
Parental DNA
DNA pol III
Lagging strand
Figure A summary of bacterial DNA replication. 5
DNA pol I
DNA ligase 4 3 5 3 2 1 3 5

6. Be able to….. Identify in which direction the leading strand is built.
Identify in which direction the lagging strand (aka Okazaki fragments) is built.
Identify and describe the role of each enzyme.

7. Overall directions of replication
Figure 16.15a
Replication Bubble
Overview
Leading strand
Lagging strand
Origin of replication
Primer
Leading strand
Lagging strand
Have students Draw this!
Figure Synthesis of the leading strand during DNA replication.
Overall directions of replication

8. Identify which enzyme/molecule(s) are responsible for:
Breaking H-bonds that hold two DNA strands together.
Keeping the DNA single stranded once it is broken apart
Relieves strain caused by pulling DNA strands apart
Building RNA primer so that DNA polymerase can start its job.
Adding DNA Nucleotides to build the new DNA polymer
Deleting sections of RNA and completing DNA nucleotides
Linking the covalent backbone of fragments together.

9. Replication Bubble Overview Leading strand Lagging strand
Figure 16.15a
Replication Bubble
Overview
Leading strand
Lagging strand
Origin of replication
Primer
Have students Draw this!
Figure Synthesis of the leading strand during DNA replication.

10. Questions?

11. 3rd Problem with DNA Polymerase

12. Replicating the Ends of DNA Molecules – the problem…
The usual replication machinery provides no way to complete the 5 ends, so repeated rounds of replication produce shorter DNA molecules with uneven ends
This is not a problem for prokaryotes, most of which have circular chromosomes

13. Ends of parental DNA strands Lagging strand 3
Figure 16.20a 5
Leading strand
Ends of parental DNA strands
Lagging strand 3
Last fragment
Next-to-last fragment
RNA primer
Lagging strand 5 3
Parental strand
Figure Shortening of the ends of linear DNA molecules.
Removal of primers and replacement with DNA where a 3 end is available 5 3

14. Second round of replication
Figure 16.20b 5 3
Second round of replication 5
New leading strand 3
New lagging strand 5 3
Figure Shortening of the ends of linear DNA molecules.
Further rounds of replication
Shorter and shorter daughter molecules

15. Replicating the Ends of DNA Molecules – the Solution…
Eukaryotic chromosomal DNA molecules have special nucleotide sequences at their ends called telomeres
Telomeres do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules
It has been proposed that the shortening of telomeres is connected to aging

16. If chromosomes of germ cells became shorter in every cell cycle, essential genes would eventually be missing from the gametes they produce
An enzyme called telomerase catalyzes the lengthening of telomeres in germ cells

17. The shortening of telomeres might protect cells from cancerous growth by limiting the number of cell divisions
There is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist

18. Questions?

19. DNA Assignment
DUE – Wednesday– February 1st!