The Cell Cycle


The Cell Cycle

Replication of DNA

  • The cells that make up organisms are always derived from exisiting cells by the process of division.
  • Cell division occurs in two main stages:
  • Nuclear division - this is the process by which the nucleus divides. This is done by either meiosis or mitosis.
  • Cell division - follows nuclear division and is the process by which the whole cell divides.
  • Before a nucleus divides its DNA must be replicated; this ensures that all daughter cells have the same genetic information to produce the enzymes and other proteins that they need.
  • DNA replication is very precise as the new cells are more or less identical to the original one.
  • The process of DNA replication is known as semi-conservative replication.

Semi-conservative replication:

For this to take place there are four main requirements:

  • The four types of nucleotides, each with their bases of adenine, guanine, thymine and cytosine.
  • Both strands of the DNA molecule must act as a template for the attachment of these nucleotides.
  • The enzyme DNA polymerase is needed to catalyse the reaction.
  • A source of chemical energy is required to drive the process.

Process of semi-conservative replication:

  • A representative portion of DNA, which is about to undergo the replication.
  • The enzyme DNA helicase breakes the hydrogen bonds linking the base pairs of DNA.
  • As a result the double-helix separates into its two strands and unwinds.
  • Each exposed polynucleotide strand then acts as a template to which complementary nucleotides are attracted.
  • Energy is used to activate these nucleotides.
  • The activated nucleotides are joined together by the enzyme DNA polymerase to form the 'missing' polynucelotide strand on each of the two original polynucleotide strands of DNA.
  • Two identical DNA strands are produced, both containing one original polynucelotide strand of DNA and one new polynucleotide strand. This is why the process is called 'semi-conservative' as it uses the orignial strands.

Nuclear division can take place my either mitosis of meiosis:

  • Mitosis produced two daughter nuclei that have the same number of chromosomes as the parent cell and each other.
  • Meiosis produces four daughter nuclei each with half the number of chromosomes of the parent cell.


  • Mitosis is the division of the nucleus of a cell that results in each of the daughter cells having an exact copy of the DNA of the parent cell; except in the rare event of mutation.
  • The genetic make-up is identical to that of the parent nucleus.
  • Mitosis is always proceeded by a period during which the cell is not dividing; this period is called interphase and is the period of considerable cellular activity that includes the replication of DNA.
  • Mitosis is divided up into five stages:
  • Interphase -


No comments have yet been made