DNA Replication Most cells in living organism undergo DNA replication within their nucleus which occurs before cell division. Just like any other chemical reactions within cell, replication is also a chemical reaction involving some enzymes as follows.
- DNA polymerase functions in new DNA elongation process in replication.
- Helices functions is to unwind double helix DNA in replication.
- DNA primase functions to assembly RNA nucleotides to form primary RNA. Primary RNA is the first strand in elongation process.
- DNA ligase functions to assembly DNA fragments into growing strands.
- DNA nuclease functions to cut DNA strand, such as when damage occurs in certain part of DNA strand.
There are three hypotheses on DNA replication that explain how double helix DNA makes the copy of itself as follows.
- Conservative explains that the two strands of DNA generate a completely new strands.
- Semi-conservative explains that both of DNA strands unwind and each strand produces a new DNA strand that is the complementary sequence to the parental DNA.
- Dispersive explains that the two DNA molecules with section of both old and new DNA interspersed along each strand. Therefore, the new strands are a mix of old and new strands.
DNA replication Animation model. (a) Hypotheses conservative, (b) hypotheses semi-conservative, dan (c) hypotheses dispersive.
In 1950s, Franklin Stahl and Matthew Meselson tested the three hypotheses of DNA replication by using bacteria E.coli. Their Experiment result showed that only semi-conservative hypothesis was considered to be the right one to explain DNA replication, while conservative and dispersive hypothesis were not proved yet.
DNA Animation replication. Base added in new nucleotide strands.
DNA replication begins with the unwinding of two polynucleotide chains, each of which functions as template. To synthesize a new complementary chain, helicase enzyme forms replication bubbles. A single strand bond protein will make the both. strands in each bubble stays separated. Replic
ation bubbles in eukaryote can reach hundreds to thousands along the DNA molecule.
At the end of each bubble there is the replication fork, which is a Y form area. Fragment, which is in the form of DNA nucleotides, one by one, is formed and added to new nucleotide until it makes a long nucleotide chain. The fragment addition is catalyzed by DNA polymerase, whereas the attachment of each fragment along DNA strand is done by DNA ligase. After the formation of new DNA strands is done, each of the two old strands will wind again.