DNA replication

• DNA replication occurs before the cell divides during the synthesis phase of the cell cycle so each daughter cell has a copy of the daughter cell after division
• The passing of DNA and genetic information is known as genetic continuity
• The first step of DNA replication is the double helix unwinding. DNA helicase breaks the hydrogen bonds between complementary base pairs
• The unwound region of DNA is called a replication fork. DNA gets unwound in one direction only, meaning the replication fork moves continuously in a unilateral direction
• The entire DNA does not unwind simultaneously. DNA unwinding occurs a region at a time to ensure stability
• DNA replication is semi-conservative
• The original strands of DNA act as a template for the synthesis of new strands of DNA
• Each DNA molecule is made up of one parent strand and one new daughter strand
• The unwound strands of DNA are referred to as the parent strands
• Free floating nucleotides in the nucleus are attracted to these parental strands of DNA
• Condensation reactions occur to complete DNA replication. DNA polymerase catalyses the reactions by reading the nucleotides and enabling them to join. DNA ligase is responsible for the actual condensation reaction
• In a double helix, the two strands are antiparallel
• DNA polymerase catalyses the addition of free nucleotides
• In the nucleus, there are free nucleotides. Complementary free nucleotides will attach to the template strand by hydrogen bonding
• DNA polymerase catalyses the formation of phosphodiester bonds between nucleotides, forming a complementary strand alongside the template parent strand
• DNA is made differently on the two strands at a replication fork. This is because DNA is only synthesized in the 5' to 3' direction
• The leading strand runs towards the fork. Synthesis can happen continuously, as helicase ‘unzips’ the DNA
• The lagging strand runs away from the fork. Synthesis occurs in small chunks called Okazaki fragments. This is because the DNA polymerise starts at the point the DNA has been ‘unzipped’ to and then synthesises along the chain until it gets to the point it finished previously. It then goes back to the beginning, where more DNA has been ‘unzipped’
• DNA ligase is needed to join lagging strand segments together by forming phosphodiester bonds, to form a continuous complementary DNA strand
• DNA polymerase ‘proofreads’ the complementary strand as it moves along the DNA. If it detects a mismatch, it can ‘snip out’ the wrong nucleotide and replace it with the right one
• DNA polymerase has an accuracy rate of about 99%, which means that mistakes do occur every once in a while. A mistake results in a change to the DNA base sequence, which is known as a mutation. DNA mutations can have detrimental effects to the organism, since an altered base sequence can change the sequence of amino acids in a protein, causing it to fold differently and possibly lose its function
• Watson and Crick were two scientists who worked together to confirm the double-helix structure of DNA in 1953. They also came up with the idea of semi-conservative replication. However, this was just a theory
• It used to be thought DNA replicated conservatively - the original strands stay together and the new repeated molecule would be made out of two brand new strands
• To prove semi-conservative replication, bacteria are grown in a broth containing the heavy (15N) nitrogen isotope. Since DNA contains nitrogen in its bases, as the bacteria replicated, they used nitrogen from the broth to make new DNA nucleotides. After some time, the culture of bacteria had DNA containing only heavy (15N) nitrogen.The bacteria containing only 15N DNA was then taken out of the 15N broth and added to a broth containing only the lighter 14N nitrogen. The bacteria were left for enough time for one round of DNA replication to occur. They were then centrifuged
• If conservative DNA replication had occurred in the replication experiment, the original template DNA molecules would only contain the heavier nitrogen and would settle at the bottom of the tube, whilst the new DNA molecules would only contain the lighter nitrogen and would settle at the top of the tube
• If semi-conservative replication had occurred in the replication experminent, all the DNA molecules would now contain both the heavy 15N and light 14N nitrogen and would therefore settle in the middle of the tube. This is what they did