DNA replication is an important part of reproduction. This occurs at the cellular level leading to the multiplication of the genetic material. The replication occurs in three basic steps as
This process helps to transfer the genetic characters from parents to offspring. The genetic material is usually the DNA, while RNA acts as a messenger.
Before you read further, you must know the basic structure of DNA. You can find the details at the bottom of this article.
As a part of the cell cycle, the cell makes 2 copies of the DNA. During cell division, these copies get distributed in the two daughter cells.
In the DNA strand, each base can only bind to its complementary base. So, each strand of DNA acts as a template and codes for the other strand. Thus DNA replication occurs and is completed in 3 steps.
DNA synthesis starts at specific points called “Origins,” which are located within the DNA strand.
Around this origin point, a protein complex of initiator proteins is formed. This is known as the replication fork and here, the process of replication begins.
An enzyme DNA helicase unwinds the two strands by hydrolyzing the ATP. This ATP forms the bonds between the base pairs (thus breaking the bonds). So each exposed strand acts as a template for replication.
It is energetically expensive to unwind the entire length of DNA. Hence, only small parts of it are opened each time and replicated.
DNA is extended by adding a free nucleotide triphosphate to the 3′ end of the chain.
DNA replication can occur only in one direction (but remember, these two strands are antiparallel).
Another enzyme called DNA Primase codes for a small RNA primer, which facilitates the activity of DNA polymerase.
The DNA polymerase attaches to the unwound strands of DNA, but this enzyme can only extend the primer from 5′ to 3′.
In this strand, the template is read from the 3′ to 5′ direction, and this is called the leading strand.
The other strand, which is in the direction of 5′ to 3′, the primer can’t be extended similarly as DNA polymerase acts in only one direction.
So, the DNA primer synthesizes an RNA primer for every 200 nucleotides, and the strand is copied downwards (5′ to 3′) in fragments.
These fragments are known as Okazaki fragments and later joined. This is called the lagging strand.
This extension of new DNA strands continues until there is no more template to copy.
Once the DNA synthesis is finished, the fragments of the lagging strand are joined by the enzyme DNA ligase.
These new strands of DNA are read by internal cell systems to check for errors and are stabilized to form new DNA.
Thus DNA replication is a form of semi-conservative replication because each DNA has one strand belonging to the parent and one new strand.
Basic Structure of DNA
Watson and Crick described the molecular structure of DNA.
DNA has a double-stranded helical structure with a backbone made of sugar and phosphate. The nitrogenous bases protrude outside.
DNA is a polymer of nucleotides; each nucleotide is made up of a nitrogenous base, a pentose sugar( ribose in RNA and deoxyribose in the case of DNA) and a phosphate group. The nitrogenous base is bonded to the pentose sugar by an N-glycosidic linkage to form a nucleoside.
The nucleoside is linked to a phosphate group. Two adjacent nucleotides are bonded together by 3′-5′ phosphodiester linkage. The polynucleotide chain will do have a free phosphate at the 5′ end and a free hydroxyl group of sugar at the 3′ end.
The nitrogenous bases are of two types- purine(adenine and guanine)and pyrimidine (cytosine and thymine in DNA and uracil in case of RNA).
RNA is single-stranded and there is no base pairing.
DNA is double-stranded and base pairing between particular nitrogenous bases of two different strands takes place. Adenine is always bound to thymine via 2 hydrogen bonds. Guanine is bonded to cytosine via 3 hydrogen bonds.
In this way, a purine is always bonded to pyrimidine, so the distance between the two strands remains almost constant.
The two chains of DNA have antiparallel polarity. If one strand has 5′ to 3′ polarity, the other has 3′ to 5′ polarity. The two chains are coiled in a right-handed fashion with 10 base pairs in each turn.
The central dogma of Molecular Biology given by Francis Crick forms the foundation for genetic studies. It states that genetic information starts flowing from DNA to RNA and then to protein. The proteins are responsible for conferring characters. DNA forms copies of itself by replication and codes for mRNA by transcription—the mRNA codes for the subsequent protein synthesis by translation.