Question

DNA replication

Answer 1

The process of synthesis of a two identical replicas of DNA molecule from an existing DNA molecule is termed as DNA replication. DNA replication is the basis for biological inheritence and information progression.DNA has a double helical structure with each strand complementary to the other strand and running antiparallel to each other. During replication, this double helical structure unwinds by means of some enzymes such as helicase and then DNA polymerase is required to add new nucleotides on each strand to produce two DNA molecules. This method of DNA replication is termed as "Semi-Conservative" since, in the newly synthesized DNA molecule, one strand is from the old DNA molecule and the other newly synthesized complementary to it.

In a cell, DNA replication begins at specific points known as "Origin of replication" or "Ori". Unwinding of DNA at the origin and synthesis of new strands, accommodated by an enzyme known as helicase, results in replication forks growing bi-directionally from the origin. DNA polymerase is the family of main enzyme the carries out the DNA replication and is involved in maintaining the fidelity of DNA replication by proof-reading and editing. DNA polymerase adds a new strand of DNA by extending the 3' end of an existing nucleotide chain, adding new nucleotides matched to the template strand one at a time via the creation of phosphodiester bonds.

The process of DNA replication could be divided into three steps as follows:

1. Initiation- The first step in DNA replication is to find the origin of replication in DNA. Initiator proteins such as DnaA in E.coli or origin recognition complex in yeast target the Ori and recruit other proteins to form the pre-replication complex, which unwinds the double-stranded DNA.
2. Elongation- DNA polymerase synthesizes the new DNA strand in 5'--->3' direction. Since, the strands run antiparallel to each other, this means that on strand the DNA synthesis is continuous in nature; this is called as "leading strand" and on the other strand, the DNA synthesis is discontinuos; this is called as lagging strand and the DNA fragments thus synthesized are termed as "Okazaki fragments".

On the leading strand, an RNA primer is added at the 3' end of the template which provides a free 3'-OH group for addition of new nucleotides by DNA polymerase. On the lagging strand, there are multiple RNA primers since the DNA is synthesized in form of Okazaki fragments. After completion of replication, these primers are removed by RNAse activity and the gaps are sealed by another DNA polymerase.When this is complete, a single nick on the leading strand and several nicks on the lagging strand can be found. Ligase works to fill these nicks in, thus completing the newly replicated DNA molecule.

3. Termination- Termination requires that the progress of the DNA replication fork must stop or be blocked. Termination at a specific locus, when it occurs, involves the interaction between two components: (1) a termination site sequence in the DNA, and (2) a protein which binds to this sequence to physically stop DNA replication. In various bacterial species, this is named the DNA replication terminus site-binding protein, or Ter protein.

The replication process is very stringently regulated in both eukaryotes and prokaryotes in the cell cycle.