DNA Replication

• what does DNA do before cell division ?

  copies itself so that each new cell has the full amount of DNA
• define semi- conservative replication

  half the strands in each new DNA molecule are from the original DNA molecule - meaning there's genetic continuity between generations of cells (cells produced by cell division inherit their genes from their parent cells)
• Explain the process of replication with the semi-conservative method:

  1. the enzyme DNA helicase breaks the hydrogen bonds between bases on the two polynucleotide DNA strands. This makes the helix unwind to form two single strands
  2. Each original single strand acts as a template for a new strand. Complimentary base pairing means that free-floating DNA nucleotides are attracted to their complementary exposed bases on each original template strand A with T and C with G

  
• Explain the process of replication with the semi-conservative method p2:

  3) condensation reactions join the nucleotides of the new strands together- catalysed by the enzyme DNA Polymerase. Hydrogen bonds form between the bases on the original and new strands

  4) Each new DNA molecule contains one strand from the original DNA molecule and one new strand

  
• Which direction does DNA polymerase build the new strand?

  • DNA polymerase can only build the new strand in one direction (5’ to 3’ direction)
  • As DNA is ‘unzipped’ from the 3’ towards the 5’ end, DNA polymerase will attach to the 3’ end of the original strand and move towards the replication fork (the point at which the DNA molecule is splitting into two template strands)
  • This means the DNA polymerase enzyme can synthesise the leading strand continuously

  
• When building new DNA strands what are the two strands called?

  • The template strand that the DNA polymerase attaches to is known as the leading strand - can synthesise continously
  • The other template strand created during DNA replication is known as the lagging strand
  • On this strand, DNA polymerase moves away from the replication fork (from the 5’ end to the 3’ end)
  • This means the DNA polymerase enzyme can only synthesise the lagging DNA strand in short segments

  
• Which enzyme helps join the lagging strand segments together?

  • A second enzyme known as DNA ligase is needed to join these lagging strand segments together to form a continuous complementary DNA strand
  • DNA ligase does this by catalysing the formation of phosphodiester bonds between the segments to create a continuous sugar-phosphate backbone

  
• Maths Skill : frequency of bases:

  • The bases in each of these pairs are said to be complementary to one another
  • This means that the frequency or number of adenine is equal to the frequency or number of thymine and the frequency or number of cytosine is equal to the frequency or number of guanine