Okazaki fragment is –
Correct Answer: Synthesised by DNA polymerase d in eukaryotes
Description: Ans. is 'b' i.e., Synthesised by DNA polymerase d in eukaryotes * Replication is a process by which two dsDNAs are synthesized from a single dsDNA. It is done by DNA polymerases. * DNA polymerases can synthesize a new strand only in 5'3' direction. Since they can synthesize a new strand complementary and antiparallel to the template strand, they want the template strand to be in 3'5' direction to synthesize a new strand continuously. * During replication, when the parent strand unwinds, there are two strands - one in 3'5' direction and the other one in 5'3' direction. * The 3'5' strand is treated as leading strand (as against a 3'5' strand, a 5'3' new strand can be synthesized continuously). 5'3' strand will be treated as lagging strand as against a 5'3; strand there is no DNA polymerase to synthesize a new strand in 3'5' direction. So across this 5'3' strand, the DNA polymerases synthesize from the angle of separation. * At the angle of separation, replication fork is formed. Replication fork is made up of Helicase (to cause unwinding of the two strands to enable replication to happen along the whole of DNA), RNA primase (to synthesis the RNA primer that is necessary for DNA polymerase to elongate the DNA strands), DNA polymerase d (to elongate the DNA strands) and single-stranded binding proteins (to prevent the two strands to reanneal). * First RNA primer synthesizes the RNA primer and then DNA polymerase a elongates the first 10 nucleotides of the DNA strand and then it is taken over by DNA polymerase d, which synthesizes 100 to 150 nucleotides. These short fragments with RNA primer attached to DNA fragments synthesized along lagging strand are called OKAZAKI FRAGMENTS. So Okazaki fragments are synthesized by RNA primer, DNA polymerase a and DNA polymerase d. * DNA polymerase d continues the synthesis of DNA strand until it comes across another RNA primer (which is synthesized by the previous replication fork). Here it creates a flap at the 5' end of the RNA primer and that is removed by Flap endonuclease FEN1. * After the RNA primer is removed, DNA polymerase d fills the gap and DNA ligase unites the ends. * In prokaryotes, the process is almost the same, except that the enzymes are different. DNA helicase causes unwinding. RNA primase synthesizes RNA primer. DNA polymerase III synthesizes the DNA fragment. Hence Okazaki fragments are synthesized by RNA primase and DNA polymerase III in prokaryotes. * DNA polymerase III synthesizes DNA fragments until it visualizes an RNA primer (synthesized by previous replication fork). DNA polymerase III recruits DNA polymerase I, which uses its 5*3* exonuclease activity to remove the RNA primer and DNA polymerase I fills the gap and DNA ligase unites the ends. FUNCTION PROKARYOTIC ENZYME EUKARYOTIC ENZYME Leading strand synthesis DNA polymerase III DNA polymerase d Okazaki fragment synthesis RNA primase + DNA polymerase III RNA primase + DNA polymerase a + DNA polymerase d Removal of RNA primer and gap-filling DNA polymerase I DNA polymerase d + Flap Endonuclease I Proofreading and repair DNA polymerase II DNA polymerase e and b Mitochondrial DNA synthesis - DNA polymerase g To summarise, Okazaki fragments are 1) Synthesised along lagging strand not along leading strand 2) Synthesised by DNA polymerase d in eukaryotes and by DNA polymerase III in prokaryotes 3) Joined together by DNA ligase not by Flap endonuclease I 4) Flap Endonuclease I helps in removing RNA primer in eukaryotes
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