The Structure of DNA Questions to Ponder 1) How is the genetic info copied? 2) How does DNA store the genetic information? 3) How is the genetic info passed from generation to generation?
The Structure of a Single DNA Strand Sugar-phosphate backbone Repeating unit: Nucleotide Nitrogenous bases: purines & pyrimidines 5 and 3 ends
The Double Helix Base-Pairing
Uniform Width of the DNA Double Helix
The Double Helix Base-Pairing
Density Gradient Centrifugation, the Dead Sea, and Meselson and Stahl s Experiment dh 2 O 25% saturated NaCl 50% saturated NaCl 100% (saturated) NaCl
Possible Models of DNA Replication Meselson & Stahl s Experiment Predicted:
DNA Replication Overview
Origins of Replication - Eukaryotes may have hundreds or even thousands of replication origins replication begins at specific sites where parent strands separate to form replication bubbles replication bubbles expand laterally as DNA replication proceeds replication bubbles fuse and synthesis of the daughter strands is complete sister chromatids!!!
The Cast of Characters
The Role of DNA Polymerase Elongation of new DNA at a replication fork: Is catalyzed by enzymes called DNA polymerases, which add nucleotides to the end of a growing strand
How does the antiparallel structure of the double helix affect replication? DNA polymerases add nucleotides only to the free end of a growing strand As a consequence: Along one template strand of DNA, the leading strand: DNA polymerase III can synthesize a complementary strand continuously, moving toward the replication fork To elongate the other new strand of DNA, the lagging strand DNA polymerase III must work in the direction away from the replication fork The lagging strand is synthesized as a series of segments called Okazaki fragments, which are then joined together by DNA ligase
Priming Replication DNA Polymerases can not initiate replication! They can only add nucleotides to the 3 end Need to have a primer in place! RNA Only one primer is needed for replication of the leading strand! For the lagging strand, each Okazaki fragment must have its own primer!
Synthesis of Leading and Lagging Strands During DNA Replication DNA Pol III elongates DNA strands only 5 3 One new strand, the leading strand, can elongate continuously as the replication fork progresses. The lagging strand must grow in an overall direction by addition of short segments, Okazaki fragments, that grow (numbered here in the order they were made). DNA ligase joins Okazaki fragments by forming a bond between their free ends. This results in a continuous strand.
Summary of DNA Replication Helicase unwinds the parental double helix. The leading strand is synthesized continuously in the direction by DNA pol III. DNA ligase bonds the end to the end of the first fragment. Molecules of singlestrand binding protein stabilize the unwound template strands. Primase begins synthesis of RNA primer for fifth Okazaki fragment. DNA pol I removes the primer from the end of the second fragment, replacing it with DNA nucleotides that it adds one by one to the end of the third fragment.
4 5 6 1 Primase joins RNA nucleotides into a primer. 3 Template strand After reaching the next RNA primer (not shown), DNA Pol III falls off. After the second fragment is primed. DNA Pol III adds DNA nucleotides until it reaches the first primer and falls off. DNA Pol 1 replaces the RNA with DNA, adding to the end of fragment 2. DNA ligase forms a bond between the newest DNA and the adjacent DNA of fragment 1. Lagging Strand Synthesis 2 2 2 RNA primer 1 1 1 1 1 Okazaki fragment 2 DNA Pol III adds DNA nucleotides to the primer, forming an Okazaki fragment. 7 The lagging strand in this region is now complete. Overall direction of replication
Summary of DNA Replication Helicase unwinds the parental double helix. The leading strand is synthesized continuously in the direction by DNA pol III. DNA ligase bonds the end to the end of the first fragment. Molecules of singlestrand binding protein stabilize the unwound template strands. Primase begins synthesis of RNA primer for fifth Okazaki fragment. DNA pol I removes the primer from the end of the second fragment, replacing it with DNA nucleotides that it adds one by one to the end of the third fragment.
http://www.ted.com/talks/drew_berry_anim ations_of_unseeable_biology.html http://www.ted.com/talks/david_bolinsky_a nimates_a_cell.html
The Cast of Characters
What is a Clone? A genetically identical copy of an organism or DNA segment produced through the use of biological technologies. Types of Cloning Techniques: 1. Recombinant DNA technology 2. Therapeutic cloning 3. Reproductive cloning
The Polymerase Chain Reaction (PCR) Producing identical copies of a DNA segment (typically smaller than a gene) Taq polymerase Thermas aquaticus