Reproduction is an important process for propagation of living organism. To facilitate reproduction the genetic material in the cell of the organism needs to be multiplied so that the genetic characters can be passed off into the offspring. The genetic material of most organisms is usually DNA while RNA acts as a messenger.
The molecular structure of DNA was described by Watson and Crick. DNA has a double-stranded helical structure with the 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 case of DNA) and phosphate group. The nitrogenous base is bonded to the pentose sugar by a 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 does have a free phosphate at the 5′ end and 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 case of 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 while is bonded to cytosine. Guanine is bonded to cytosine via 3 hydrogen bonds. in this way a purine is always bonded to a 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.
Before cell division the cell makes 2 copies of the DNA to be distributed in the two daughter cells.
As each base can only bind to its complement base, each strand of DNA serves as a template and can code for the other strand. DNA replication is completed in 3 steps.
DNA synthesis starts at specific points within the DNA strand known as Origins. A protein complex of initiator proteins forms around this origin where this replication begins and this site is known as replication fork. The enzyme DNA helicase unwinds the two strands by hydrolyzing the ATP which forms the bonds between the base pairs(thus breaking the bonds) so that each strand can be used as a template for replication. It is energetically expensive to unwind the entire length of DNA so only small parts 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 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 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 a 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 till 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 as each DNA has one strand belonging to the parent and one new strand.