Replication of DNA (College Board AP Biology)

• DNA replication occurs in preparation for mitosis (cell division)
• In mitosis, 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

An Overview of DNA Replication

• 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
  • This process is catalyzed by the enzyme DNA helicase
• 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 semiconservative replication because half of the original DNA molecule is kept (conserved) in each of the two new DNA molecules

Semiconservative Replication of DNA Diagram

Semiconservative replication of DNA

DNA Polymerase

• RNA primers mark the position for the DNA polymerase to bind
• 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 synthesizes new DNA strands from the two template strands
  • It does this by catalyzing dehydration synthesis 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

Function of Helicase and DNA Polymerase Diagram

Helicase and DNA polymerase work together to replicate each strand of DNA

Action of DNA Polymerase Diagram

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

Leading & lagging strands

• DNA polymerase can only build the new strand in one direction (5’ to 3’ direction)
  • pronounced "five prime to three prime direction"
  • The point where the strands separate is called the replication fork
  • A different enzyme, topoisomerase, assists by relaxing the supercoiling of DNA just ahead of the replication fork
• 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 synthesize 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 the lagging 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 synthesize the lagging DNA strand in short segments (called Okazaki fragments)
• Another enzyme, DNA ligase, is needed to join these lagging strand segments together to form a continuous complementary DNA strand
• DNA ligase does this by catalyzing the formation of phosphodiester bonds between the segments to create a continuous sugar/phosphate backbone

Leading & Lagging Strand Formation Diagram

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