Question

Define DNA replication. In which phase of the cell cycle does replication occur in eukaryotes? Describe briefly the mechanism of DNA replication.

Answer 1

DNA is usually described as Deoxyribose nucleic acid due to presence of deoxyribose sugar in it. DNA replication is usually described as a process in which it duplicates and produces two identical copies of itself. It occurs in all living organisms and is the most important process in biological inheritance. DNA carries information to pass from one generation to another. It acts as a blueprint for information carrier sources.

Complete answer:
S or synthesis phase is the phase in which synthesis and replication of DNA takes place. It is the phase in which a cell doubles its DNA. For example, if the initial amount of DNA is denoted by 2 C, then after the S phase the amount of DNA would become 4 C. The amount of nucleic acid, i.e., DNA (genetic material doubles but the number of chromosomes remains the same.
Process of DNA replication:-
origin of Replication : Replication begins at a particular region of DNA which is called origin of replication. It is because of the requirement of the origin of replication that a piece of DNA, if needed to be propagated during recombinant DNA procedures, requires a vector.
Activation of deoxyribonucleotides: Four types of deoxyribonucleotides, namely, dAMP, dGMP, dTMP and dCMP are activated by phosphate, energy and enzyme phosphorylase into triphosphate state.
Unwinding of helix: Unwinding of double helical parental molecule is brought about by enzyme helicase, which is ATP dependent. Unwinding of DNA molecules into two strands results in the formation of a Y-shaped structure, called replication fork.
Formation of primer strand: A new strand is now to be synthesised opposite to the parental strands DNA polymerase l is the true replicase in E. coli, which is incapable of initiating DNA synthesis, ie it is unable to deposit the first nucleotide in a daughter strand.
Elongation of new strand: The DNA dependent DNA polymerases catalysed polymerisation only in one direction, that is 5’---3’. This creates some additional complications at the replication fork. Consequently the replication is continuous on one template strand with polarity 3’--- 5’. It is now known as leading daughter strand.

Note: In eukaryotes, the replication of DNA takes place at S-phase of cell-cycle. The replication of DNA and cell division cycle should be highly coordinated. A failure in cell division after DNA replication results in polyploidy.