1. DNA Replication

2. DNA replication
“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”
James Watson
Francis Crick
1953
The greatest understatement in biology!

3. Watson and Crick … and others…
1953 | 1962
Watson and Crick … and others…
Discovered & published in 1953
Nobel Prize in 1962:
Watson, Crick, Wilkins

4. 1953 | 1962
Maurice Wilkins… and…

5. Rosalind Franklin ( )
A chemist by training, Franklin had made original and essential contributions to the understanding of the structure of graphite and other carbon compounds even before her appointment to King's College. Unfortunately, her reputation did not precede her. James Watson's unflattering portrayal of Franklin in his account of the discovery of DNA's structure, entitled "The Double Helix," depicts Franklin as an underling of Maurice Wilkins, when in fact Wilkins and Franklin were peers in the Randall laboratory. And it was Franklin alone whom Randall had given the task of elucidating DNA's structure. The technique with which Rosalind Franklin set out to do this is called X-ray crystallography. With this technique, the locations of atoms in any crystal can be precisely mapped by looking at the image of the crystal under an X-ray beam. By the early 1950s, scientists were just learning how to use this technique to study biological molecules. Rosalind Franklin applied her chemist's expertise to the unwieldy DNA molecule. After complicated analysis, she discovered (and was the first to state) that the sugar-phosphate backbone of DNA lies on the outside of the molecule. She also elucidated the basic helical structure of the molecule.
After Randall presented Franklin's data and her unpublished conclusions at a routine seminar, her work was provided - without Randall's knowledge - to her competitors at Cambridge University, Watson and Crick. The scientists used her data and that of other scientists to build their ultimately correct and detailed description of DNA's structure in Franklin was not bitter, but pleased, and set out to publish a corroborating report of the Watson-Crick model. Her career was eventually cut short by illness. It is a tremendous shame that Franklin did not receive due credit for her essential role in this discovery, either during her lifetime or after her untimely death at age 37 due to cancer.

6. HOW IS DNA COPIED? The structure of DNA explains how it can be copied.
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HOW IS DNA COPIED?
The structure of DNA explains how it can be copied.
Each strand has all the info needed to construct
the __________other half.
If strands are separated,
_____________ rules allow
you to fill in the complementary bases.
matching
base-pairing

7. Matching halves? Why is this a good system?
Copying DNA
Replication
2 strands of DNA helix are complementary
have one, can build other
have one, can rebuild the whole
when cells divide, they must duplicate DNA exactly for the new “daughter” cells
Why is this a good system?
Matching halves? Why is this a good system?

8. Copying DNA Replication of DNA
base pairing allows each strand to serve as a template for a new strand
new strand is 1/2 parent template & 1/2 new DNA

9. Anti-parallel strands
Nucleotides in DNA backbone are bonded from phosphate to sugar between 3 & 5 carbons
DNA molecule has “direction”
complementary strand runs in opposite direction 5 3 3 5

10. Figure 12–11 DNA Replication
Section 12-2
Original strand
DNA polymerase
New strand
Growth
DNA polymerase
Growth
Replication fork
Replication fork
Nitrogenous bases
New strand
Original strand
Sites where strand separation and
replication occur are called _____________
replication forks

11. REPLICATION STEPS __________ “unzip” molecule by breaking Helicase
_______________ that hold the strands
together and unwind it.
2. _______________ joins nucleotides
using original strand as template and
______________for errors.
3. Copying happens in ________ directions
along the two strands & in __________
places at once.
Helicase
Hydrogen bonds
DNA polymerase
spell checks
opposite
multiple

12. Replication: 1st step Unwind DNA helicase enzyme
unwinds part of DNA helix
helicase
replication fork

13. Where’s the ENERGY for the bonding! We’re missing something!
Replication: 2nd step
Build daughter DNA strand
add new complementary bases
DNA polymerase
Where’s the ENERGY for the bonding!
But…
We’re missing something!
What?
DNA
Polymerase III

14. Leading & Lagging strands
Okazaki
Leading & Lagging strands  5 5 5 3 5 3 5 3
Lagging strand 3
growing
replication fork 3 5
Leading strand  3 5
Lagging strand
Fragments 3
DNA polymerase III
Leading strand
continuous synthesis

15. Synthesis of the New DNA Strands
The Leading Strand is synthesized as a single strand from the point of origin toward the opening replication fork

16. Synthesis of the New DNA Strands
The Lagging Strand is synthesized in the OPPOSITE direction of the leading strand.
This strand is made in MANY short segments It is replicated from the replication fork toward the origin

17. Replication fork / Replication bubble5 3 3 5
DNA polymerase III
leading strand 5 3 5 3 5 5 3
lagging strand 5 3 5 3 5 3 5
lagging strand
leading strand
growing
replication fork
growing
replication fork 5
leading strand
lagging strand 3 5 5 5

18. What does it really look like?1 2 3 4

19. DNA Replication
Semiconservative replication – each new strand of DNA has one strand from the parent and one new strand.
The separation of the parent strands and pairing of free nucleotides continues at multiple locations until the entire DNA molecule has been replicated. Since the two original parent strands serve as a template for building the complementary strands, each new double helix of DNA contains one strand from the original molecule and one newly created strand.
This process is referred to as semiconservative replication, and is illustrated in the diagram shown here.

20. REPLICATION
ANIMATION
See a video clip about DNA REPLICATION (12B)

21. ACTIVITY
BE A DNA MOLECULE