DNA Replication

Why? When? DNA replicates using a semi-conservative mechanism.

Leading strand- side of the DNA where replication is continuous. Lagging strand- side of DNA where replication occurs in segments, creating Okazaki fragments.

1. DNA helicase unwinds and then unzips the DNA, forming the replication fork. http://www.mcb.harvard.edu/Losick/images/TromboneFINALd.swf Occurs at the origin – region of DNA with many A-T pairs Binding proteins attach to keep the DNA from coming back together

2. RNA Primase attaches RNA primers to the DNA strands DNA nucleotides need something to attach to

3’ and 5’ explained The hydroxyl groups on the 5'- and 3'- carbons link to the phosphate groups to form the DNA backbone

3. Leading Strand - DNA polymerase III attaches free nucleotides to pair with the DNA template. Nucleotides are attached at the 3’ end of the sugar http://student.ccbcmd.edu/biotutorials/dna/dnareppr.html Continuous process

- Numerous RNA primers are attached 3. Lagging Strand – DNA polymerase III attaches nucleotides in segments = Okazaki fragments - Numerous RNA primers are attached http://www.mcb.harvard.edu/Losick/images/TromboneFINALd.swf

4. Exonucleases remove the RNA primers from the new DNA 5 4. Exonucleases remove the RNA primers from the new DNA 5. DNA Polymerase I attached DNA nucleotides to the areas where the RNA primers were attached

6. DNA Ligase creates the covalent bonds between the sugars and phosphate groups along the sides of the DNA molecule

Semi-conservative Each DNA molecule is half old DNA and half new DNA

The end result of replication is two identical DNA molecules, each composed of half old DNA and half new DNA.