DNA Replication

The original DNA strand to be copied is called the parent strand, or %%parent DNA%%. The parent strand is composed of the bases Adenine, Thymine, Cytosine, and Guanine, along with a deoxyribose sugar and three phosphates, forming a %%nucleotide triphosphate%%. The bases %%Adenine and Thymine%% always pair because of 2 perfectly aligned hydrogen bonds, and the bases %%Cytosine and Guanine%% always pair because of 3 perfectly aligned hydrogen bonds. This property of selective pairing is called %%complimentary%%. The backbone of the DNA is formed with a %%phosphate%% and sugar, and the other two phosphates of the triphosphate are removed. The carbons of the sugar align the strand in a %%5’ to 3’%% direction. The two strands of DNA run antiparallel to each other, meaning one strand rings 5’ to 3’, where the other runs 3’ to 5’.

DNA replication forms 2 daughter strands. Helicase, an enzyme, “unzips” the parent DNA, creating 2 exposed strands. At the replication fork, the single-stranded binding proteins support the unzipping strand and DNA gyrase prevents the strand from supercoiling. To create the daughter strands, DNA Polymerase adds nucleoside triphosphate to an exposed strand. DNA Polymerase only adds in the 3’ to 5’ direction, and is supported or held in place by the %%beta clamp%%, and %%clamp loaders%%. The strand which runs in the 3’ to 5’ direction is called the %%leading strand%%, as it is directly fed into the polymerase. In the opposite direction the exposed 5’ to 3’ strand is uniquely copied. This strand is called the %%lagging strand%% and needs to be copied in segments. As the parent strand unzips, a %%primase%% attaches to the 3’ end of the exposed strand and loops it back to the polymerase. Now with a 3’ end, it can be fed into the polymerase to be copied. This process has to be repeated many times with the segments of DNA, or %%Okazaki fragments%%. %%DNA ligase%% is an enzyme that bonds the Okazaki fragments, and then the polymerase %%proofreads%% the new strands for any errors.