DNA is the basic unit of inheritance. It is like a storehouse with all the genetic information. Cells undergo division and form new cells as they wear out. In this process of cell division DNA also divides and replicates. This is to ensure that the daughter cells that are formed also have the same genetic information as the original cells. Replication of DNA is necessary to maintain genetic information and the smooth functioning of every living organism.
DNA is a double helix structure, which is made up of a backbone of alternating phosphate and deoxyribose sugars. It also consists of 4 nitrogen bases Adenine, Guanine, Thymine and Cytosine. These nitrogen bases on each strand form hydrogen bonds with complementary bases on the other sides. Adenine forms a double bond with Thymine and Cytosine forms a triple bond with Guanine.
The two strands of DNA have a direction, a 3' hydroxy end (called the 3 prime end) and 5' phosphate end (called the 5 prime end). The 3' end of one strand associates with the 5' of the other strand. Thus one strand runs from 3' to 5' and the other runs from 5' to 3'. These strands are therefore termed as antiparallel. A complete turn of the DNA accommodates 10 nucleotides (A nucleotide comprises the phosphate, sugar and nitrogen base). The distance between two adjacent nitrogen bases in a DNA double helix is 3.4 angstroms. In order to learn how does DNA replicate, it is necessary to know the basic structure of DNA in order to understand the DNA replication steps. DNA replication is a very crucial process and involves a lot of complex reactions catalyzed by specialized replication enzymes.
The first step of DNA replication is the unwinding of the DNA that takes place. The hydrogen bonds between the nitrogen bases are broken by the enzyme known as helicase. The point or position where this unwinding occurs is known as the origin of replication. Once the unwinding is complete the structure formed is known as a replication fork.
The strand synthesis in DNA replication takes place in the 5'→3' direction. This synthesis takes place due to DNA polymerase enzymes which only operate in the 5'→3' end. The DNA polymerase does not synthesize an entirely new strand. It can extend an already existing strand. For this purpose short DNA or RNA fragments known as primers pair with the template strands. Upon pairing, the DNA polymerase can act. It extends the 3' end of these primers, by attaching an incoming 5' complementary nucleotide to it.
The leading strand runs in the 3' → 5' direction. This allows the enzyme, DNA polymerase to extend the complementary strand in the 5' → 3' direction. Synthesis of this strand is continuous and therefore this strand is known as the leading strand.
The lagging strand runs in the 5' →3' direction. As a result, the synthesis of the antiparallel complementary strand by DNA polymerase becomes difficult, as it can only synthesize strands in the 5' → 3' direction. The synthesis of this strand is staggered and therefore known as the lagging strand. In the synthesis of this strand, an enzyme known as primase adds short RNA segments that are identified by the DNA polymerase and extended, which are then known as Okazaki fragments. The isoenzyme of DNA polymerase then replaces the RNA molecules with DNA. Due to these short synthesized fragments of the complementary strands, there are gaps in this strand. These fragments are joined by the enzyme ligase.
As replication needs to be an extremely accurate process, to maintain the correct sequence of bases, the replication step also involves a proof-reading step that makes the process of replication less prone to errors. Another DNA replication enzyme which is the DNA polymerase with an exonuclease activity (in the 3'→ 5' direction) proof reads the sequence to correct any errors in the pairing of bases.
Termination of Replication
Prokaryotic and eukaryotic replication differs. Prokaryotic DNA is usually circular and therefore termination of replication occurs when two replication forks meet. Eukaryotic DNA initiates at various points in a chromosome forming bubbles. Due to the linearity of the chromosome the replication is very complex and cannot go on till the end of the chromosome. This is known as telomere shortening. To sort this, an enzyme known as telomerase adds a repetitive sequence towards the end of the chromosome. The untimely and uncontrolled activation of this enzyme leads to formation of cancer cells. DNA replication is then followed by DNA transcription and DNA translation for the synthesis of various proteins in the body.
These are the steps involved in DNA replication. The whole process utilizes energy in the form of ATP that is used in the formation of phosphodiester bonds and unwinding of DNA. Hope this article has helped you in understanding how does DNA replicate and why is it important in the survival of living organisms.