DNA Replication 06/25/12

Building DNA! Twizzlers are the phosphate backbone + the sugars! Different colored gummy bears are the bases. – Red = A – Yellow = T – Green = G – Clear = C Toothpicks are the bonds. – A bonds with T. G with C.

DNA DNA is the molecule that codes for protein. – This protein is what builds us and enables us to live. It is the genetic material Last Friday, we talked about cells replicating. – In order for this to occur, our DNA needs to replicate. Today we are going to talk about how this happens

Recall The four bases of DNA are A, C, G, and T. – A = Adenine – G = Guanine – C = Cytosine – T = Thiamine. A always bonds with T G always bonds with C

DNA Structure 5’ end and 3’ end are now important to us! Hydrogen Bonds: Weaker than bonds between atoms.

DNA Replication The first thing that needs to happen is the “unwinding” of DNA. – In other words, splitting apart the two strands. This is done by the enzyme helicase. The place where helicase is splitting the two strands is called the replication fork.

DNA Replication Now that the two strands are separated, free nucleotides floating around can be added to each strand, forming two DNA molecules.

How does this happen? An enzyme – Polymerase This enzyme adds free nucleotides to the new strands in a 5’ to 3’ manner. – This gets VERY COMPLICATED so really pay attention. Not only does DNA Polymerase add nucleotides, it edits misplaced ones.

Leading Strand Lagging Strand

Leading Strand Polymerase starts on the 3’ end of the original strand. Thus, the new strand will be made 5’ to 3’. Synthesized continuously Just focus on the bottom one

Lagging strand The open end of the original strand is the 5’ end which is the 3’ end for the new strand. – Polymerase CANNOT synthesize from 3’ to 5’. – Thus, polymerase must jump ahead and synthesize “backwards”

Lagging Strand All of the fragments on this strand are “sealed together” by Ligase.

Semi-conservative Replication Conservative means the absence of change. In this case, we are referring to DNA strands. – As you can see, one strand is ALWAYS copied from another strand. So each DNA molecule has one original strand (from parent DNA) and one new strand. – This is called semiconservative replication