Explain the process of DNA replication in eukaryotes.

Explanation

(i) Replication starts at a specific site on a DNA sequence known as the origin of replication. There are more than one origin replication. of replication in eukaryotes.
(ii) DNA replication in eukaryotes starts with the assembly of a prereplication complex (preRC) consisting of 14 different proteins.)
(Iii) The origin of replication in yeast is called as ARS sites (Autonomously Replicating Sequences).

(iv) Replication fork is the site (point of unwinding) of separation of parental DNA strands where new daughter strands are formed. Multiple replication forks are found in eukaryotes.
(v) The enzyme helicases are involved in unwinding of DNA by breaking hydrogen bonds holding the two strands of DNA and replication protein A (RPA) prevents the separated polynucleotide strand from getting reattached.
(vi) Topoisomcrasc is an enzyme which breaks DNAs covalent bonds and removes positive supercoiling ahead of replication fork.
(vii) DNA replication is initiated by an enzyme DNA polymerase `alpha` / primase which synthesizes short stretch of RNA primers on both leading strand (continuous DNA strand) and lagging strands (discontinuous DNA strand).
(viii) Primers are needed because DNA polymerase requires a free 3' OH to initiate synthesis. DNA polymerase covalently connects the nucleotides at the growing end of the new DNA strand.
(ix) DNA Pol `alpha` (alpha), DNA Pol `delta` (delta) and DNA Pol `epsilon` (Epsilon) are the 3 enzymes involved in nuclear DNA replication.
(x) DNA Pol `alpha` - Synthesizes short primers of RNA.
(xi) DNA Pol `delta` - Main replicating enzyme of cell nucleus.
(xii) DNA Pol `epsilon` - Extend the DNA Strands in replication fork.
(xiii) DNA synthesis takes place in 5' `rarr` 3' direction and it is semidiscontinuous. When DNA is synthesized in 5' `rarr` 3' direction, only in the free 3' end (OH end) DNA is elongated.
(xiv) In discontinuous strand the new DNA strand is synthesized in short pieces called Okazaki fragments. The Okazaki fragments are united by ligase and is also called Lagging strand where the replication direction is 5' `rarr` 3' which is opposite to the direction of fork movement.
(xv) The continuous strand is called Leading strand where the replication direction is 5' `rarr` 3' which is same to the direction to that of the replication fork movement.
(xvi) DNA ligase joins any nicks in the DNA by forming a phosphodiester bond between 3' hydroxyl and 5' phosphate group.