Simple Explanation of DNA Replication

Introduction to DNA/RNA

DNA or Deoxyribose Nucleic Acid is the motherboard of every living thing. It is a double helix structure comprised of a Phosphate and sugar (Deoxyribose) backbone that is connected by 5′-3′ phosphodiester bonds. Attached to the backbone is the main genetic material that codes for the synthesis of proteins and other structures. These important molecules are called nitrogenous bases (Adenine, Cytosine, Guanine, Thymine, and Uracil in RNA) and are attached to the sugar molecule via 1′ Glycosidic linkage. Together the entire structure of sugar, nitrogenous base, and phosphate create a nucleic acid.

So how does DNA code for so many things? Even though DNA is extremely small and can not be seen by the human eye, it is still extremely massive in the microscopic world. If you were to stretch all the DNA in one cell and lay it across a table, it would reach a length of 6 feet or 2 meters! Within this extreme length are millions of Nitrogenous base sequences that code for something different.

The language of genetics is read in sequences of three. So, if you had an Adenine, Thymine, and Guanine next to each other the ribosome would read ATG. ATG is also the code for “start”. This is how proteins know where to start replicating DNA, instead of trying to start at a random place. Likewise, there are “stop” points. These are marked by the codes TAA, TAG, and TGA. Everything between these start and stop points codes for amino acids that will eventually make proteins. Sequences such as TAT code for Tyrosine and GGT code for Glycine.

As seen just from this introduction of DNA, one can understand its importance to all living things. One can also understand why replicating an entire genome of DNA might be complicated and extremely important to get correctly. If a singular nitrogenous base is not copied exactly the same it can cause proteins to denature and even cause diseases such as Tay-Sachs disease.

Unwinding (Step 1)

The first step of replicating DNA is to unwind it. DNA is a double helix with two backbones. For replication to occur DNA must be open and nitrogenous bases must be available for enzymes to analyze and copy them.

The first thing to do is to make the helix into a straight ladder. The enzyme that does this is Topoisomerase. Following Topoisomerase is Helicase. The job of helicase is to splice the ladder structure in half. It does this by breaking the hydrogen bonds that form between opposite nitrogenous bases. After the DNA has been unwound and spliced it is ready for the next step

Elongation (Step 2)

After unwinding is done, there are now two free strands of spliced DNA called the leading and lagging strands. The leading strand is the DNA that has a free 3′ carbon on its furthest, first spliced, sugar molecule. The lagging strand is the opposite strand of DNA that has a free 5′ carbon on its furthest sugar molecule.

The reason this is important is that the enzymes that replicate the free DNA can only synthesize in the 5′->3′ along the 3′->5′ template. This means that a continuous replication strand can occur on the leading strand while an inconsistent replication strand can occur on the lagging strand.

© 2006 Nature Publishing Group Bell, S. D. Molecular biology: Prime-time progress. Nature 439, 542-543 (2006). All rights reserved. 

The enzyme that replicates DNA on the leading and lagging strand is called Polymerase III. The only difference between them is the creation of Okazaki fragments on the lagging strand. These are short segments of synthesized DNA created by the discontinuous synthesis of DNA on the lagging strand. These fragments will be connected during proofreading when DNA ligase fills in the empty areas.

Termination (Step 3)

Replication does not just occur in a single location in eukaryotic cells. Thus, when a replication fork reaches another replication fork, elongation will end. This will end the replication process of DNA.

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