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The cell cycle

The DNA molecule has two functions:

  1. Carry recipes for proteins (genetic information).
  2. Carry the genetic information to the next generation.

By replicating, the genetic information may be carried to the next generation.

The cell cycle. DNA replication takes place during the S phase.The cell cycle. DNA replication takes place during the S phase.

In an individual (or part of an individual) that is growing, the cells find themselves in the cell cycle.


  • The cell is actively dividing, forming two new cells.


  • The phase in between two periods of mitosis

The interphase is divided in three sub-phases:

  • G1: Rapid growth, normal metabolic activity
  • (Sometimes also G0, if the cell is no longer actively dividing.)
  • S: Synthesis of new DNA
  • G2: Some growth, preparation for mitosis

The replication fork

DNA may be copied because the two strands are complementary to each other. Flash animation by prof. Giannini can be found here.

When DNA is replicated, a structure called a replication fork is formed.When DNA is replicated, a structure called a replication fork is formed.


  • Opens up the DNA molecule (like a "zipper").

SSBPs (single-strand binding proteins)

  • Stabilizing, prevent the single strands from joining again.


  • Lays down a small strand of RNA (primer).
  • The primer functions as a starting point for the DNA polymerase.

DNA polymerase

  • Synthesizes new DNA.
  • Nucleotides are base-paired with the nucleobases in the single DNA-strand (template).
  • Nucleotides are joined to form a new DNA-strand.

Leading and lagging strand

  • Leading strand: Synthesized in one fell swoop.
  • Lagging strand: Because of the chemical structure of DNA, this strand must be synthesized "backward". This leads to the formation of Okazaki fragments.

Final cleaning

  • RNA primers replaced by DNA.
  • Okazaki fragments ligated with help of a ligase.
  • Ready for mitosis!
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