This is the synthesis of DNA, a double stranded DNA molecule separates into two polynucleotide strands, each strand serves as templates for the synthesis of its complementary strand. The two polynucleotides strands separate easily because they are held together by hydrogen bonds which are weak. The enzyme helicase unzips breaks in hydrogen bonds; this is an active process requiring energy from ATP. Free nucleotide then come and aline in relation with each of the strands. The free nucleotide is held in place by enzyme DNA polymerase until each binds to a presiding nucleotide thus extending the new strands of DNA. These movements happen in the five prime to sprime dimension, the base pairing is complementary A with T and C with G.
In one strand, the DNA polymerase is move in the same direction as the helicaes that is 5 prime to 3 prime direction. These are called continuous replication. The copying of the other strand that is 3 prime to 5 prime end has to keep the being started again that is there is a small gap between 3 prime end and 5 prime end, DNA polymerase cannot joint the 3n prime ends of one newly synthesized piece of DNA to the 5 prime end of the next. An enzyme called DNA lipase closes the gap, this is termed discontinuous replication, two double helical molecules are this form, each containing a strand of the original (parent) DNA and a new strand which is complementary to it. This type of replication is known as semi conservative replication. These can be explain using zip fastener idea.
Another hypothesis is that the DNA replicates by conservative method which supposes that the double helix remains intact and in some way stimulates the synthesis of a double stranded helix and in some way stimulates the synthesis of the second double helix
Proof of the Semi Conservative method
In 1958, Meselson and Stahl examin the two main hypothesis. They argued the semi conservative hypothesis is correct. None of the products of the 1st generation should contain exactly the same molecule of the original double helix. One of the two strands will be form the parent DNA and the other strand will be new. If the conservative hypothesis is correct, one of the double helix will have exactly the same molecule as the parent on both strands
In the experiment, bacteria cell known as E.coli were grown for several generation in a heavier isotope of nitrogen (N-15). After the heavy nitrogen has been incorporated in cells DNA, it was then transferred into nitrogen 14 medium and left for some time. Sample of bacteria were taken at interval and the relative amount of the two kind nitrogen were examine. The normal N-14 was separated from the heavier one by means of extremely high speed centrifuge. The DNA was then detected by it absorption of U.V light. The result gave what was expected from a semi conservative replication
In the first generation all the DNA had destroy mid-way between that of heavy DNA and light DNA. In the second two types of DNA were detected: light DNA and the other contain equal amount of nitrogen 14 and 15
