DNA Replication

ScienceTopping | Sept 3, 2021

DNA replication is the second phase in cell cycle. Replication of DNA allows genetic materials to be passed on from one generation to the next.

We inherit characteristics or traits from our parents, whereby physical appearance or behavior may have a quirk of similarity. This is why DNA replication is essential in inheritance.

Scientifically, DNA replication makes a complete copy from the original DNA, ending up with two DNAs synthesised from one DNA.

Regarding the way how replication works, it is ambiguous for many scientists in the olden days. Thus, three models have been proposed to suggest the mode of replication, namely

Conservative Replication
Semi-Coservative Replication
Dispersive Replication

Which is the correct model?

The truth is finally revealed in 1953. After Watson and Crick discovered the double helix structure of DNA, they finally proposed the semi-conservative method as the way how DNA replication works. Their proposal is later verified by Stahl and Meselson through their experiment using the bacteria, Escherichia Coli (E. coli) and nitrogen isotopes, N-14 and N-15. The gist of the experiment is the comparison between percentage of E. coli bacteria containing N-14 isotopes and N-15 isotopes. With the relative data in hand, the concept of semi-conservative model is proven crystal-clear.

DNA replication can be analysed stage-by-stage. Remember, DNA is visible under standard light microscope, while its double helix structure can only be revealed under electron microscope.

Process of DNA Replication

One DNA molecule is replicated into two DNA molecule.

IMAGE BY OPENSTAX, CC BY 4.0/WIKIMEDIA COMMONS

Stage 1

The Hydrogen bonds between two polypeptide strands of DNA are broken by helicase enzyme in the nuclues.

Stage 2

The double helix structure of DNA then unwinds, producing two polypeptide strands.

Stage 3

In the nuclues, two phosphorus molecules bind to free nucleotides, activating the free nucleotides.

Stage 4

Free activated nucleotides bind to the complementary binding sites of the two polypeptide strands.

Stage 5

The two extra phosphorus are detached from each nucleotide once binding occurs.

Stage 6

DNA Polymerase joins the nucleotides to the polypeptide strands from 5' end to 3' end via complementary base pairing.

Stage 7

Hydrogen bonds are formed between free nucleotides and each original polypeptide strand. Phosphodiester bonds are formed between phosphate group and adjacent nucleotide's pentose sugar.

Stage 8

Two complete DNA molecules are synthesised from one DNA molecule, thus doubling the DNA molecules in the nucleus.

The new DNA molecule consists of a new polypeptide strand and an original parent polypeptide strand. This is the concept of semi-conservative replication.

In human, there are 46 chromosomes, each with one DNA molecule. By first doubling the number of DNA molecules to 92 during DNA replication, meiosis can then occur, halving it to 46 DNA molecules in each daughter cell. This ensures that the correct chromosomal number is inherited from one generation to the next.

If not, human will never be human again.

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