DNA Replication 2007-2008

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!

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

1953 | 1962 Maurice Wilkins… and…

Rosalind Franklin (1920-1958) 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 After Randall presented Franklin's data and her unpublished conclusions at a routine seminar, her work was provided - without Randall's knowledge - to her competitors at Cambridge University, Watson and Crick.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 1953. 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.

HOW IS DNA COPIED? The structure of DNA explains how it can be copied. Image from: http://evolution.berkeley.edu/evosite/evo101/images/dna_bases.gif 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

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?

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

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

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

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

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

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

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

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

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

Replication fork / Replication bubble 5 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

What does it really look like? 1 2 3 4

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.

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

ACTIVITY BE A DNA MOLECULE