DNA Structure and Replication

CENTRAL DOGMA Get out your macromolecule booklets, and get ready to tell me about the structure of DNA. –And put on your uniforms and Ids.

The structure of DNA DNA is a double stranded helix. The two strands are complimentary –Adenine bonds with Thymine –Cytosine bonds with Guanine A and G are purines T and C are pyrimidines. They are hydrogen bonded together. Note the sugar-phosphate backbone and the base pair rungs of the ladder. And 5’ and 3’ ends.

DNA Replication Replication is semi-conservative. The two strands separate, and each serves as a template for a new strand. This is possible because of complimentary base pairing Each new DNA molecule is 1/2 old and 1/2 new - semi conservative.

The details

Important Note Replication begins at an “origin of replication” a specific sequence of nucleotides that allows the binding of enzymes. EUKARYOTIC DNA has multiple origins of replication, allowing large amounts of DNA to be replicated quickly.

Enzymes of DNA Replication Helicase - At the origin of replication, helicase binds to the DNA strand and uncoils and separates the two strands. Single Strand Binding Proteins - prevent the two strands from going back together. Primase - puts down a short RNA primer to get the process started. DNA Polymerase III - Binds to the primer and adds new nucleotides in the 5’ to 3’ direction.

All of this occurs on the leading strand, where DNA polymerase III can simply bind add new nucleotides in the 5’ to 3’ direction until the end of the DNA molecule is reached. The lagging strand must be synthesized in short “Okazaki Fragments” because DNA polymerase III can only work in one direction.

More Enzymes DNA Polymerase I - replaces RNA nucleotides from the primer with DNA nucleotides. DNA Ligase - joins the gaps between Okazaki fragments

The whole thing