AP Biology Notes- DNA Replication and DNA stuffs

• Structure:

  • DNA is a double helix composed of two complementary strands that run in opposite directions.

  • Each strand consists of nucleotides, which are made up of a sugar, a phosphate group, and a nitrogenous base.

  • The four types of nitrogenous bases are adenine (A), thymine (T), cytosine (C), and guanine (G).

  • Base pairing occurs between A and T, and between C and G, held together by hydrogen bonds.

• Nitrogen bases:

  • The pairs are: Adenine (A) to Thymine (T)- apples to trees, and Guanine (G) to Cytosine (C) - cars to garages

  • Adenine and Guanine are both purine meaning that they are double ringed

  • Cytosine and Thymine are both pyrimidine meaning they only have one ring

  • Although both nitrogen pairs bond through hydrogen bonds they create a different number of bonds; Adenine and Thymine form two hydrogen bonds, while Guanine and Cytosine form three hydrogen bonds

• All Together:

  • When put all together DNA has a phosphate group attached to the 5’ carbon of a DNA molecule on one side and the 3’ carbon of the next molecule in the chain

  • This works to give the DNA what is called directionality and is often how we refer to the strands as being antiparallel, meaning one strand runs 5’ to 3’ while the complementary strand runs 3’ to 5’

  • This antiparallel orientation is crucial for the replication process, as enzymes involved in DNA synthesis can only add nucleotides to the 3’ end of the growing strand

• DNA Replication:

  • Simply put, the parent DNA is used as a template for construction of new DNA, it is “unzipped” by the protein helicase and each side is used to create a new strand

  • After helicase unzips the DNA, a replication bubble is created where the DNA replication begins

  • DNA polymerase begins to build the new DNA from 5’ to 3’ end, the new strands are synthesized continuously on the leading strand and discontinuously on the lagging strand, forming Okazaki fragments

  • Before the DNA polymerase can begin to build the new strands however, RNA primer must first add a short sequence of RNA nucleotides to provide a starting point for DNA synthesis

  • These small RNA fragments are later removed by exonuclease and filled in DNA polymerase

  • Finally, ligase is the last step and is responsible for connecting the backbones between the okazaki fragments and the new sections of DNA that replaced the RNA fragments

And there you have it, DNA replication has finished and now we can move onto how this can be