DNA replication

• • In order for cell division to occur, the two strands of DNA making up the double helix need to separate

  • Each strand then acts as a template for a new double stranded DNA molecule.

  
• To separate the strands, hydrogen bonds between complementary base pairs must be broken. This is done by an enzyme- DNA helicase

  
• “Unzipping” is complete. There are now two strands of DNA. Free nucleotides will then pair up with the exposed complementary base from the strand.

  
• Once nucleotides have paired with their complementary base they need to be joined together. This is done by an enzyme- DNA Polymerase

  This forms phosphodiester bonds between adjacent nucleotides. Hydrogen bonds then form between the complementary base pairs of the template and new DNA strands

  
• This process is known as semi conservative replication. Two new molecules of DNA are produced and each consists of one old strand of DNA and one new strand

  
• DNA Polymerase starts building at the 5’ end, The daughter strand which goes in the 5’ to 3’ direction can be made continuously because the DNA Polymerase can move continuously in this direction and follow the replication fork. This is the leading strand. The other strand is known as the lagging strand as it runs from 3’ to 5’.
• the lagging strand -

  DNA polymerase binds and moves up away from the unzipping DNA

  DNA polymerase stops when it reaches the DNA that has been added already

  DNA polymerase moves back to where DNA has just unzipped and make another fragment

  These fragments (Okazaki fragments) are later joined together to make a full chain 
• As a strand is copied bases can be matched incorrectly

  This leads to random and spontaneous changes called mutations