DNA or Deoxyribo nucleic acid is the hereditary material in most of the living organisms and DNA replication is the biological process that produces two identical copies of DNA from one original DNA.
DNA replication occurs in both prokaryotes and eukaryotes in the similar steps where DNA unwinding is done with the help of an enzyme DNA helicase and manufacturing of new DNA strands is accomplished by enzymes known as polymerases. Both of the organisms follow semi-conservative replication where individual strands of DNA are manufactured in the different directions. And both of them begin new DNA strands with a small primer of RNA. However, there are differences between prokaryotic and eukaryotic DNA replication which we’ll understand further.
There is only one point of origin in prokaryotic cells when replication occurs in the cell cytoplasm. Here, replication takes place in the two opposing directions at the same time and prokaryotic cells have one or two types of polymerases. Replication occurs much faster in prokaryotes as compared to eukaryotes. It is finished off in 40 minutes in some bacteria and as they have circular chromosomes they have no ends to synthesize like telomeres in eukaryotes.
Average eukaryotic cell has a bigger DNA than a prokaryotic cell, which is 25 times larger. Eukaryotic cells have multiple points of origin and use unidirectional replication within the cell nucleus.These have four or more polymerases enzymes to help during DNA replication. Eukaryotes may take up to 400 hrs for replication and they have a distinct process for replicating the telomeres present at the ends of their chromosomes. Here, the cell undergoes DNA replication during the S-phase of the cell cycle.
DNA replication occurs when a cell needs DNA before its division so that the new daughter cells can also get a copy of DNA. Specifically, in a eukaryotic cell, it occurs before mitosis or meiosis during interphase.
DNA codes for the traits of living organisms and DNA replication means making more DNA; let’s explore the steps involved in the process of DNA replication.
Initiation (The Origin)
DNA replication starts at a point called Origin and it is identified by certain DNA sequences.
Here at the origin, Helicase starts unzipping and unwinds the DNA. Now, the strands are separated and single-stranded binding proteins (SSB) helps in keeping them separated. As a result, a DNA replication fork is formed.
Topoisomerase prevents the DNA from supercoiling (which is over-winding of the DNA while we need DNA to be separated for replication to occur).
Now, primase comes in and plays its role in making RNA primers on both the strands. It is very helpful for DNA polymerase to know where to start its action.
As DNA strands are antiparallel to each other, i.e. one will be 3’ to 5’ and the other 5’ to 3’, DNA Polymerase III bind to primer and builds the new strand in the 5’ to 3’ direction, i.e. it adds new bases to the 3’ end on the new strand.
One of the strands where it builds the new strand continuously is called leading strand and the other strand is known as the lagging strand as it has to wait for the original strand to unwind to start replication and then add bases in the 3’ end. Fragments result from the lagging strand where primase has to act again and again, and these are known as Okazaki fragments.
Ligase helps in sealing these okazaki fragments together.
Termination of DNA Replication
Now, at the end of replication, two identical double helix DNA molecules are formed from one original DNA molecule. The whole process is semi-conservative because each of the two copies contain one original strand and one newly made strand.
DNA Replication Diagram
The steps in DNA replication explained above can be more clear with the help of the DNA replication diagram shown below.
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DNA replication enzymes have the ability to quicken reactions and build up or break down the items that they act upon. Below listed are the enzymes involved in DNA replication.
DNA Polymerase III: It is known as the builder. This enzyme replicates DNA molecules actually building a new strand of DNA. It also has proofreading ability to code the correct gene by matching the correct DNA bases, thereby forming the right protein.
Helicase: It is also known as the unzipping or unwinding enzyme as it unzips the two strands of DNA. While unzipping, it breaks through the hydrogen bonds holding the DNA bases together.
Primase: It is called the initializer and without it, it becomes difficult for DNA polymerase to figure out where to start with. Primase makes the primer so that it becomes easier for DNA polymerase to figure out where to start the work. And this primer is made of RNA.
RNase H (DNA Polymerase I): It removes primer as DNA polymerase III approaches it and is especially important on lagging strands.
DNA Ligase: It is known as the gluer. Because it helps DNA strands combine together.
Understanding the DNA replication has resulted in various life saving medical treatments where one can stop DNA replication in harmful cells like pathogenic bacteria or human cancer cells.
1. What is DNA replication?
Ans. DNA replication is the process that takes place in prokaryotes and eukaryotes which results in the formation of two identical copies from one original DNA. It is a semi-conservative process i.e. each of the new DNA copies contains one strand from the original DNA and one new strand.
2. Explain the mechanism of DNA replication.
Ans. The DNA replication occurs with the help of three stages, namely initiation, elongation and termination. DNA synthesis starts at initiation points called ‘origins’ which are specific coding regions. There are a number of origin sites and when replication of DNA begins, it forms the shape of a fork and therefore called as DNA replication forks. DNA helicase is the enzyme that unwinds the double helix and exposes the two individual strands. These two templates are used for replication. DNA primase enzyme synthesises a small RNA primer that acts as a kick-starter for DNA polymerase. DNA polymerase creates the new strands of DNA and helps in its expansion. The leading strand is newly formed in a 5’ to 3’ direction for one of the templates that existed in 3’ to 5’ direction. The other lagging strand will be synthesized in 3’ to 5’ direction from the 5’ to 3’ direction template. Since for lagging strand, continuous DNA synthesis is not possible, DNA synthesis occurs in fragments where RNA primers are added to exposed bases everytime and these fragments are called Okazaki fragments. THe expansion of the new DNA strands continues until there is either no more template left to replicate at the chromosomal end or two DNA forks meet and subsequently terminate.