The main role of DNA is to direct the manufacture of proteins, which regulate everything that occurs in the cell. However, DNA does not directly manufacture proteins. Instead, DNA passes its information to a molecule called ribonucleic acid (RNA). These RNA molecules carry out the instructions in DNA and produce the proteins that determine the structure and function of cells. Know the difference well for the AP Biology exam.
DNA --> RNA --> Proteins
By directing the manufacture of proteins, DNA serves as the cell’s blueprint. It is passed from cell to cell by replicating itself in a process called DNA replication.
As the DNA molecule is twisted over on itself, the first step in the replication process is to unwind the double helix by breaking the hydrogen bonds. An enzyme called helicase carries out this process. The exposed DNA strands then form a y-shaped replication fork.
Each strand can serve as a template for the synthesis of another strand. Replication beings as specific sites called origins of replication. Because the DNA helix twists and rotates during replication, enzymes called DNA topoisomerases cut and rejoin the helix to prevent tangling. An enzyme called DNA polymerase adds nucleotides alongside the existing DNA strand using complimentary base pairing.
DNA polymerase can only add nucleotides to the 3’ end of an existing strand. Therefore, to start replication at the 5’ end, DNA polymerase must add nucleotides to an RNA primase (a short strand of RNA nucleotides).
One strand is called the leading strand, and is made continuously. This means DNA polymerase steadily adds the nucleotides one after another. The other strand is called the lagging strand, and is made discontinuously. Unlike the leading strand, the lagging strand is made in short pieces of nucleotides known as Okazaki fragments. This is because nucleotides are added in the 3’ to 5’ direction. However, when the DNA double helix is unzipped, one strand remains in the 5’ to 3’ direction.
The DNA polymerase does not function in this direction, which means the Okazaki fragments need to be added in pieces. These fragments are then linked together by the enzyme DNA ligase to produce a continuous strand. Finally, hydrogen bonds form between the new base pairs, leaving two identical copies of the original DNA molecule.
After DNA replication. We don’t end up with two entirely new molecules. Each new molecule has half of the original molecule. Because half of the original molecule is conserved in each new molecule, it is said to be semiconservative.
Many enzymes and proteins are involved in DNA replication. Below is a list of the ones you’ll need to know for the AP biology Exam.
- Helicase – unwinds the double helix into two strands
- Polymerase – binds the Okazaki fragments
- Topoisomerase – cuts and rejoins the helix in order to prevent tangling
- RNA primase – catalyzes the synthesis of RNA primers