1. DNA Replication
(In Vivo)
Escape Into The
Madness of
Perfection

2. I. Overview A. Conceptually simple, but the actual process:
1. is complex - the helical molecule of DNA must untwist while the two strands are copied simultaneously, requiring dozens of enzymes and other proteins.
2. is extremely rapid - approximately 500 nucleotides per sec. in prokaryotes vs. 50 per sec. in humans.
3. is accurate - only 1 in a billion nucleotides is incorrectly paired in higher order eukaryotes (human genome total: 6 X 109)
B. Although the process is fundamentally similar, more is known about the process in prokaryotes than in eukaryotes

3. II. Getting Started
A. Origins of Replication: a specific sequence of nucleotides where DNA replication begins
B. Specific proteins required to initiate replication
C. Dbl helix opens at the origin, and replication forks (y-shaped regions of replicating DNA) spread from the origin creating a replication bubble
D. Prokaryotic & viral DNA have only one origin vs. hundreds or thousands for eukaryotes
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Here
For An
Animation
Fig pg 295

4. III. Elongation A. Strand Separation
1. Gyrase - an enzyme which separates the dbl helix, forming a replication bubble
2. Helicase - an enzyme which unwinds the DNA by breaking H-bonds, exposing the template strands
B. Synthesis of New DNA Strands
1. Primase synthesizes small complementary strands of RNA, called primers (apx. 10 nucleotides) - which anneal to the single stranded DNA templates
a. replication must begin with a strand base-paired with the template
2. DNA polymerase adds nucleotides to the 3’ end of the primers. These new strands grow in their own 5' - 3' direction.

5. Fig pg 297

6. C. The anti-parallel nature of the two strands of DNA causes a problem since the overall direction of replication is the same for both strands (5' - 3').
1. one strand is replicated continuously: the strand which runs in the 3' - 5' direction (relative to the replication direction) - the new DNA strand synthesized from this strand is called the Leading Strand [animation]
2. the other strand (which runs in the 5' - 3' relative to the replication direction) is synthesized in fragments - this new strand is called the Lagging Strand

7. Fig. 16.13 pg 297 * captions are important

8. IV. Elongation of the Lagging Strand
A. Discontinuously synthesized against the overall direction of DNA replication
B. Produced as a series of short ( nucleotides in eukaryotes) segments called Okazaki fragments: each of which are synthesized in the expected 5' - 3' direction
C. Primers are replaced by DNA via another DNA polymerase, while the fragments are connected (ligated) by an enzyme called DNA ligase that catalyzes the formation of a covalent bond between the 3' end of each new fragment and the 5' end of the lagging strand [animation]

9. Overview of DNA Replication
[animation]
pg. 298

10. The End