• Before?a (parent) cell?divides, it needs to?copy the DNA?contained within it
  • Doubling the DNA ensures that the two new (daughter) cells produced will both receive?full copies of the parental DNA

   

• The DNA is copied via a process known as?semi-conservative replication?(semi = half)
  • The process is called this because?in each new DNA molecule?produced,?one?of the polynucleotide DNA strands (half?of the new DNA molecule) is?from the original DNA molecule?being copied
  • The other polynucleotide DNA strand (the other half of the new DNA molecule) has to be newly created by the cell
  • Therefore, the new DNA molecule has?conserved half?of the original DNA and then used this to create a new strand

   

The importance of retaining one original DNA strand

• Retaining one original DNA strand?ensures?there is?genetic continuity?(i.e. genetic information is conserved) between generations of cells
• In other words, it ensures that the new cells produced during cell division?inherit all their genes?from their parent cells
• This is important because cells in our body are?replaced regularly?and therefore we need the new cells to be able to do the same role as the old ones
  • Replication of DNA and cell division also occurs during?growth

   

Semi-conservative replication

• DNA replication occurs in preparation for?mitosis, when a parent cell divides to produce two genetically identical daughter cells – as each daughter cell contains the same number of chromosomes as the parent cell, the number of DNA molecules in the parent cell must be?doubled?before mitosis takes place
• DNA replication occurs during the?S phase?of the cell cycle (which occurs during?interphase, when a cell is?not dividing)
• The enzyme?helicase?unwinds?the DNA double helix by breaking the?hydrogen bonds?between the base pairs on the two antiparallel polynucleotide DNA strands to form two single polynucleotide DNA strands
• Each of these single polynucleotide DNA strands acts as a?template?for the formation of a?new strand?made from free nucleotides that are attracted to the exposed DNA bases by?base pairing
• The new nucleotides are then?joined together?by the enzyme?DNA polymerase?which catalyses?condensation reactions?to form a new strand
• The original strand and the new strand join together through hydrogen bonding between base pairs to form the new DNA molecule
• This method of replicating DNA is known as?semi-conservative replication?because?half of the original?DNA molecule is kept (conserved) in each of the two new DNA molecules

Semi-conservative replication of DNA

DNA polymerase

• In the nucleus, there are?free nucleotides?which contain three phosphate groups
  • These nucleotides are known as?nucleoside triphosphates?or?‘activated nucleotides’
  • The extra phosphates activate the nucleotides, enabling them to take part in DNA replication

   

• The bases of the free nucleoside triphosphates?align?with their?complementary bases?on each of the?template?DNA strands
• The enzyme?DNA polymerase?synthesises new DNA strands?from the two template strands
• It does this by?catalysing condensation reactions?between the?deoxyribose sugar and phosphate groups?of adjacent nucleotides within the new strands, creating the?sugar-phosphate backbone of the new DNA strands
• DNA polymerase cleaves (breaks off) the two extra phosphates and uses the?energy released?to create the?phosphodiester bonds?(between adjacent nucleotides)
• Hydrogen bonds?then form between the?complementary base pairs?of the template and new DNA strands

Nucleotides are bonded together by DNA polymerase to create the new complementary DNA strands

Mutations

• The replicated DNA molecules must be an?exact copy?of the parent DNA molecule, therefore the formation of the complementary strands must be a?highly accurate?process
• Although the process is astonishingly accurate considering it is happening?constantly?in cells and at a?considerable speed, occasional?mistakes?occur in the form of:
  • Bases being inserted into the complementary strand in the?wrong order
  • An?extra base?being inserted by accident
  • A base being?left out?by accident

   

• These mistakes in the process of semi-conservative replication of DNA result in the occurrence of?random,?spontaneous?mutations?(i.e.?errors?in the?genetic code)