Semi-Conservative DNA Replication (CIE A Level Biology)

• 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 hydrogen bonds between the base pairs on the two antiparallel polynucleotide DNA strands are broken
• This ‘unzips’ or unwinds the DNA double helix 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 – the original strand and the new strand then join together to form a 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 was shown to be the method of replication by Meselson and Stahl in 1958. They used E. coli (a bacteria) and two nitrogen isotopes, a heavy form ¹⁵N and the ‘normal’ form ¹⁴N, to demonstrate how the density of DNA changes over generations as the ¹⁵N isotope was replaced with the ¹⁴N isotope

Semi-conservative replication of DNA

DNA Polymerase

• In the nucleus, there are free nucleotides to which two extra phosphates have been added (these free nucleotides with three phosphate groups 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

Leading & lagging strands

• 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
• This template strand that the DNA polymerase attaches to is known as the leading strand
• 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 (called Okazaki fragments)
• 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

The synthesis of the complementary strands occurs slightly differently on the leading and lagging template strands of the original DNA molecule that is being replicated