The Peculiarities Of DNA Replication Process

Introduction

Cell division is important for an organism's growth. However, DNA must be replicated before the cell divides. DNA is the hereditary molecule that stores instructions to make proteins.1 Since DNA is the hereditary material, it must be copied from a cell to its daughter cell and this is done by the process of DNA replication. DNA replication is a process of copying DNA to produce two identical copies of the original DNA.2 It occurs in both eukaryotic cells and prokaryotes cells during S phase which is a phase of the cell cycle that occurs during interphase stage.

DNA structure

DNA is made of two strands coiled around each other to form spiral antiparallel structure known as the double helix. Each strand consists of four bases adenine (A), cytosine (C), guanine (G) and thymine (T). Adenine is linked with thymine and cytosines with guanines. The backbone of the strands is a repeated pattern of 5-carbon sugar with phosphate groups.3 (figure 1).Figure 2 results of Meselson-Stahl experiment.

Semi-conservation model was supported by Meselson-Stahl experiment. E-coli bacteria was grown in the presence of 15N isotopes then changed to 14N .DNA was collected from two generations of E-coil .The result for the first generation showed that one band of DNA have equal quantity of 14N and 15N .The result for the second generation showed two bands of DNA , one has equal quantity of 14N and 15N ,but the other has only 14N .The result for two generation showed that all original DNA have heavy nitrogen and any new resulting DNA have a light nitrogen .6 (figure 2).7

DNA replication process

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DNA replication is a complex process. There are proteins and enzymes that are involved in this process to ensure complete replication of DNA molecule without any mistake.8

1. The first step is separating the two strands of DNA and this is done by Helicase enzyme. The initiating point where the splitting happens is known as origins of replication. The structure that is created is called Replication Fork and it is a Y shaped structure. Single-strand binding proteins coat the strands to prevent them from reforming.8,9(figure 3).10
2. Next, primers- which are made by primase enzyme -marks the starting point for DNA synthesis. Another enzyme that binds to the primer is called DNA polymerase. It reads the sequence and adds nucleotides that are complementary to those in the template. However, DNA polymerase can only add DNA bases in one direction from 5'-3 'on to 3 ' end.8.9(see figure 3).10Figure 3 DNA replication process and the enzymes involved.
3. One of the new strands is made continuously and is called leading strand. In leading strand, DNA polymerase adds bases in 5'-3 ' continuously. On the other hand, the other new strand is called lagging strand. It cannot be read by DNA polymerase continuously because it runs in the opposite direction. In the lagging strand, there are more RNA primers added, so DNA polymerase can add bases from 5'-3 ' in small sections forming gabs called Okazaki Fragments.8,9(figure 3).10
4. Next, the exonuclease enzyme removes all RNA primers from both of the strands and another DNA polymerase fills the gaps that are left. Then, ligase enzyme seals the gabs between Okazaki fragment to form a continuous strand.8,9(figure 3).10
5. The next step is Termination. When DNA reaches the end and RNA primers are removed. It is impossible for DNA polymerase to fill the gabs without primers . As a result, the end of the parental strand is not replicated. These ends -of chromosomes of linearDNA which contains repeated sequences- are called telomeres, so in each DNA replication cycle, a part of the telomere is removed.8,9

Importance of DNA replication

The purpose of DNA replication is creating copies of DNA molecule. This process replaces damaged and dead cells and form the needed gametes for fertility .11DNA replication is an important process during cell division. It is responsible for creating another copy of DNA Which goes to one of the two daughter cells. Without replication, there will not be enough genetic information that help to make orders for making proteins that our body needs.12The rate of DNA replication is different from one cell to another. Some of cells are dividing continuously so they need to replicate their DNA continuously. Some cells are dividing in slower rate, and they don't need to DNA replication. 12

Errors in DNA replication and DNA Repair

The aim of replication is producing identical copies of the original DNA, errors in the process make this impossible and lead to mutations. Some mutations are fatal to cells, but many are not and cause changes in genetic material. Errors in the DNA replication can be fixed by some mechanisms. One mechanism is done by DNA polymerases called proofreading. If there is a nucleotide linked to a wrong base, DNA polymerases get rid of it and replace.

DNA replication Model

The components of DNA replication model are scissors, tape, and pipe cleaners. First, to make the sides, we take two strands of one color and two strands of another color. Second, we take one of each color and twist them to get two long strands. Third, to make the inside, we take one strand of four different colors and cut each of them into five small strands to get 20 small strands. assign two colors to one strand and two other colors to the other strand. Then we tape the long strands together. Next, we turned our model and compressed it. we are going to repeat the steps again to make two more molecules that represent the two daughter DNA molecules. Next, we are going to cut the first DNA till we reach the middle, and we get Y shaped structure. Then we are going to assign the two daughter strands to the first DNA molecule. Finally, we are going to represent the enzymes that are involved in DNA replication by their symbols in the chart. (Figure 11).20

Conclusion

DNA replication is a process of making two identical copies from one DNA molecule. It has been approved that DNA replicated semi-conservatively. That means the two strands of the parental DNA separate and act as a template for making a new complementary strand. Cellular proofreading and error-checking procedures helps making a perfect replication. DNA replication can be done in vitro by the process of Polymerase chain reaction. It is a way to isolate pieces of DNA region and make copies of it through repeated cycles of heating and cooling to DNA synthesizing.

References

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5. Khan Academy. 2019. Mode of DNA replication: Meselson-Stahl experiment (article) | Khan Academy. [ONLINE] Available at: https://www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-replication/a/mode-of-dna-replication-meselson-stahl-experiment. [Accessed 16 February 2019].
6. Semi-Conservative DNA Replication | Learn Science at Scitable. 2019. Semi-Conservative DNA Replication | Learn Science at Scitable. [ONLINE] Available at: https://www.nature.com/scitable/topicpage/semi-conservative-dna-replication-meselson-and-stahl-semi-conservative-dna-replication-meselson-and-stahl-421. [Accessed 17 February 2019].
7. Semi-Conservative | BioNinja. 2019. Semi-Conservative | BioNinja. [ONLINE] Available at: http://ib.bioninja.com.au/standard-level/topic-2-molecular-biology/27-dna-replication-transcri/semi-conservative.html. [Accessed 16 February 2019].
8. Khan Academy. 2019. Molecular mechanism of DNA replication (article) | Khan Academy. [ONLINE] Available at: https://www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-replication/a/molecular-mechanism-of-dna-replication. [Accessed 16 February 2019].
9. Steps of DNA Replication. 2019. Steps of DNA Replication. [ONLINE] Available at: http://www.dnareplication.info/stepsofdnareplication.php. [Accessed 16 February 2019].
10. Encyclopedia Britannica. 2019. Replication | genetics | Britannica.com. [ONLINE] Available at: https://www.britannica.com/science/replication/media/498384/110068. [Accessed 16 February 2019].