DNA replication

• DNA replication

  occurs before cells divide to provide a copy for the new cell.

  The process of DNA replication is called semi-conservative replication

  each new DNA molecule that it produces one strand from the parental DNA and one strand from the newly synthesised DNA.
• Steps of semi conservative replication 1

  DNA helicase breaks the hydrogen bonds between the complementary base pairs between the 2 strands within a double helix. This causes the DNA double helix to unwind
• Steps of semi conservative replication 2

  Each polynucleotide strand acts as a template.

  Free floating DNA nucleotides are attracted to their complementary base pairs on the template strands of parental DNA.
• Steps of semi conservative replication 3

  The adjacent nucleotides join together by DNA polymerase using a condensation reaction which forms a phosphodiester bond
• Steps of dna replication 4

  The two sets of daughter DNA contains one strand of the parental DNA and one newly synthesised.
• Action of DNA polymerase

  Each end of DNA strand is slightly different in its structure

  -one end is called 3' and other 5
• During dna replication-5' and 3'

  During DNA replication active site of DNA polymerase is only complementary to 3' end of newly forming DNA strand so the enzyme can only add nucleotides to the new strand at 3' end →meaning new strand is made a 5' to 3' direction and DNA polymerase moves down the template strand in 3'to 5' direction because the strands in the double helix are antiparallel the dna polymerase working on one of the template strands moves in the opposite direction to DNA polymerase working on other template strand
• Evidence for semi conservative replication

  Watson and crick determined structure of DNA

  Also came up with theory of semi conservative DNA replication

  Wasn't until meselson and stahl experiment a few years later that this theory was validated before that people were unsure whether it was conservative or semi conservative

  If it was semi conservative the original dna strand would stay together and the new dna molecules would contain 2 new strands