The current accepted model for replication is semi-conservative replication. This involves replicating all DNA before mitosis or meiosis occurs
Definitions
Complementary base pairing is where the bonding of the bases on one strand corresponds with the bases on the oppposite strand.
DNA polymerase forms covalent bonds between the nucleotides on the newly forming DNA, a phosphodiester bond is formed in a condensation reaction. It only works in the [3' --> 5'] direction.
The leading strand is a strand on DNA that runs [3' --> 5'], DNA polymerase attaches to the strand and follows the helicase is one go.
The lagging strand runs [5' --> 3'] - therefore DNA polymerase runs along it in sections.
Okazki fragments are the sections of DNA formed when polymerase completes a section (working backwards) and then it falls off. Another DNA polymerase joins to the strand and it continues.
The template strand is the strand of DNA which the nucleotides are attached to
RNA primase primes the DNA strands so they can join together
DNA ligase joins the two strands of DNA together
The process
- A section of DNA is unraveled by DNA helicase - it breaks the hydrogen bonds between the complementary bases to separate them into two strands (this is also known as a replication fork)
- Free nucleotides float to the DNA through diffusion and bond to their complementary base pair
- DNA polymerase joins together the sugar-phosphate backbone of the base pairs - this is done through condensation reactions
- The product is two separate identical strands of DNA
Semi-conservative replication
This is the theory that when DNA is replicated, 50% is new and 50% is old - therefore one strand is from the original DNA and the other is newly created. It was discovered by the Meselson-Stahl experiment.
E. coli was taken and grown on 15N (nitrogen that is slightly radioactive and heavier than normal nitrogen) for generations. This new nitrogen was then incorporated into the nitrogenous bases. Eventually, all 'normal nitrogen' (14N) was removed from the nitrogenous bases.
After this, the E. coli was grown in a medium that didn't contain any 15N. When the DNA replicated, two slightly lighter strands were produced. As more replications occured, the strands continued to get lighter.