THE VARIETY OF LIVING ORGANISMS

The Cell Cycle

The Cell Cycle

The cell cycle is the process in which genetic information is copied and then passed on to daughter cells. Unlike in meiosis, the resulting cells are genetically identical to the original or parent cell.

DNA replication

In order for all the daughter cells to gain an exact copy of the parent genes, the DNA first needs to replicate. This is the interphase part of the cell cycle. The type of division involved is called semi-conservative: the DNA molecule of the parents splits into its two strands and each strand is copied.

The process goes as follows:

  • The enzyme DNA topoisomerase nicks one DNA strand causing the molecule to uncoil.
  • This creates a replication fork. (Note that only one section of DNA is replicated at a time; the DNA molecule doesn’t completely unravel).
  • Then the enzyme DNA helicase breaks the hydrogen bonds holding the two strands together thereby allowing them to separate.
  • The bases are now exposed.
  • Another enzyme, DNA polymerase, now makes its way along these exposed bases and forms a complementary strand as it moves along the sequence. As it reads the strand from its 3′ end to its 5′ end it creates the new strand from its 5′ end to its 3′ end. There’s not just one enzyme of DNA polymerase acting on the strand but several, each working on a separate piece.
  • In front of each DNA polymerase enzyme is an enzyme of RNA polymerase which creates an RNA primer to guide the DNA polymerase.
  • The new DNA segments are bonded together by the enzyme DNA ligase.
  • Once joined the daughter molecules can then coil into the double helix structure.