LECTURE                                        DNA REPLICATION

In this lecture you will learn:


DNA is the master copy of all the genes that build and run each cell.  Every cell in the body has its own copy of DNA.  When it is time for a cell to split into daughter cells, a new copy of DNA must first be made and that process of duplicating the DNA is called REPLICATION

DNA REPLICATION MOVIE
 
THE ENZYMES OF DNA REPLICATION
Replication occurs at multiple sites of double DNA strands
1. Topoisomerase initiates the unwinding of the DNA supercoil by briefly cutting a strand so the DNA can unwind. 
2. Helicase unwinds the double strand and melts the hydrogen bonds holding the bases together (needs ATP to function)
3. Primase primes DNA replication with a small RNA primer that has an - OH for DNA polymerase to attach. 

The RNA primer is later removed by RNase and the gap is filled in by DNA  polymerase I.

4. DNA polymerase 
(III- major polymerase)
 It binds to one side of the DNA and guides nucleotides to bond with their complementary
 base pair 

It mediates formation of the covalent phosphodiester bond on the 5' end

It needs a primer (the RNA primase) with a 3'OH group onto which

It has proofreading and nuclease activity  to cut away mistakes

5. Ligase is an enzyme that can catalyze  a phosphodiester bond to fill in a  gap left when the RNA primer is removed and filled in. 
2 diagrams of DNA replication

 
 
ENZYMES

Enzymes are proteins that are organic catalysts.  They basically they make reactions occur that would not occur fast enough normally.  They do this by lowering the energy of activation.  Enzymes often have co-enzymes or factors.

WATCH ANIMATION OF HOW ENZYMES WORK HERE

Enzymes bring substrates together by binding to one or more of the reactants to an active site thru hydrogen bonds.  The shape of the active site just fits the shape of the substrate, somewhat like a lock fits a key.  The enzymes often undergo a conformational change when they mediate the reaction. 
The reaction can be affected by temp and/or pH as enzymes have an optimal temp and pH at which they work the best.  They also have temp and pH where they wont work.

While enzymes mediate the reactions, they are not themselves changed by the reaction.

hold down shift and then reload to see animation


REVIEW QUESTIONS
What is the purpose of replication?
Describe the 5 steps involved in DNA replication.
What are the characteristics of enzymes?