DNA replication

  • DNA (deoxyribonucleic acid) is a very large molecule in chromosomes that carries the genetic code. Its shape is a double helix - two DNA strands twisted together.
  • Each DNA strand is made up of many nucleotides.  A nucleotide is made of three parts: Adeoxyribose sugar, a phosphate group, and a nitrogen base.
  • There are four different bases- adenine (A), thymine (T), cytosine (C) and guanine (G). The bases always pair  with the base pairing rule:
  •  Adenine with Thymine (A-T)
  • Cytosine with Guanine (C-G)



  • The bases are joined by hydrogen bonds, and these hold the two strands of DNA together.

DNA Replication

  •  DNA replication happens in interphase of the cell cycle.  A cell spends about 90% of its time in interphase, where it creates enzymes and gets nutrients needed for DNA replication and mitosis.
  • The cell cycles from interphase to mitosis to interphase to mitosis.
  • After nucleotides and enzymes needed are produced,  DNA replication occurs, followed by a second phase of cell growth and preparation for mitosis.
  • The purpose of DNA replication is to produce an exact copy of the DNA so that after cell division each of the two daughter cells has a copy of all the genetic material contained in the original cell.
  • The first step of DNA replication is the unwinding of the double-stranded DNA molecule by the enzyme helicase. The bases are then separated and the DNA polymerase enzyme begins to add nucleotides to the strands using new nucleotides. Nucleotides are joined using the base pair rule A-T, C-G. This produces two complete identical double-stranded DNA molecules which wind up again. 

  • This is semi-conservative replication as each of the two DNA molecules that are produced have one strand from original DNA and one strand produced by replication.
  • DNA replication is part of the cell cycle that occurs before mitosis. 
  • If a cell is in a rapidly growing area (meristem of plant), rate of replication is high. When a cell is in a slow growing area, rate is low. When the area is not growing, rate is very low as cell division is not occurring.

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