The Central Dogma Theory 2: DNA Replication
In central dogma theory 1, we explained how the deoxy ribonucleic acid occupies the central role in the transfer of genetic information within the biological system. For a molecule to effectively occupy this position, it must have some special characteristics which include being cosmopolitan in all cells, being able to store biologically useful information without changing much, ability to code and express information for the control of functions of living cells, inherent capacity to show modifications according to differences in organisms and most importantly, ability to make a precise copy of itself and then pass over the copies to successive cells or generations. In addition, such molecule must be able to undergo mutation as well as recombination and both must be stable as well as inheritable.
The most important of all the attributes that make the DNA suitable as the main genetic material in living organisms is the ability to make copies of itself, that is, replication. Growth and reproduction cannot be possible without cell division, and when cells divide, the daughter cells need copies of their own genetic materials. It becomes pertinent for the DNA in the parent cell to double up in order to share into two daughter cells. Replication is therefore an important part of the cell cycle prior to the actual cell division.
How does the DNA replicate?
Basically, there are several steps in the process of DNA replication. Each step is an important process in preserving the integrity of the DNA itself.
Formation of replication fork
The first step in the replication process is the unwinding of the double helix structure. Recall that the two strands are associated with each other by weak hydrogen bond. The unwinding starts by the action of DNA helicase enzyme which serves to disrupt the hydrogen bond between base pairs to create what is known as replication fork. The replication fork serves as the template for replication to begin and the actual unwinding is done by the enzyme known as DNA gyrase. The directionality of the DNA strands makes the replication fork to be bidirectional with the 3' - 5' forming the leading strand and the 5' - 3' the lagging strand.
Binding of primers
Immediately after unwinding of the DNA strands is binding of a short ribonucleic acid strand known as primer to the leading DNA strand. The primers are synthesize by an enzyme known as the DNA primase enzyme. The leading strand is quite easy to replicate and it is continuous unlike the lagging strand which is replicated in discountinous fragment also known as okazaki fragments. So the primer binds to the leading strand at the 3' end while it binds to the lagging strand at multiple sites.
Elongation of nucleotide chains
Chains of bases are created by an enzyme known as DNA polymerase which binds to the DNA strand at the site of the primer and adds base pairs complementary to the DNA strand in a process known as elongation. There are about five different types of DNA polymerase with some involving in binding to DNA strand at the site of the primer while others are involved in a process of proofreading the newly synthesized chains fro errors and necessary repairs. Elongation of the leading strand is continuous while the lagging strands are synthesized in fragments which are later joined by another enzyme known as DNA ligase.
Termination of chains
At the end of the elongation process, all RNA primers are removed from the original DNA strands by an enzyme known as exonuclease while another exonuclease replaces the primers with relevant purine or pyrimidine bases. A third type of exonuclease proofreads the newly formed DNA strands and makes the necessary corrections. The lagging strand synthesized in discontinuous fragments are joined to form a continuous one by DNA ligase. Telomeraze, a special type of DNA polymerase synthesizes telomeres at the end of each newly formed DNA strands which act as protective cap at each end to prevent them from fusing. Thereafter, the newly synthesized strands coils around their respective parent strands to give a two double helix DNA consisting of one old strand and one new one. This method of replication of DNA is called 'semi-conservative replication method'.
Replication is an important process in the central dogma theory through which the DNA produces a carbon copy of itself. It is an important process before cell division, the latter being very necessary for growth and reproduction. The processes involved include formation of replication fork, binding of primers, elongation of chains and termination of chains after which the each newly synthesized strand coils in an helical manner to their respective parent strands in a process known as semi-conservative replication.