Nucleotides Okazaki fragments

The lagging strand is synthesized discontinuously as a series of small fragments (about 1,000 nucleotides long) known as Okazaki fragments. Each Okazaki fragment is initiated by the synthesis of an RNA primer by primase, and then Completed by the synthesis of DNA using DNA polymerase III. Each fr ment is made in the 5 - 3 direction. 

The replisome promotes rapid DNA synthesis, adding -1,000 nucleotides/s to each strand (leading and lagging). Once an Okazaki fragment has been completed, its RNA primer is removed and replaced with DNA by DNA polymerase I, and the remaining nick is sealed by DNA ligase (Fig. 25-15). 

DNA polymerase then incorporates a dNMP onto the 3" end of the primer and initiates lagging strand synthesis. The polymerase extends the primer for about 1,000 nucleotides until it comes in contact with the 5 end of the preceding primer. These short segments of RNA/DNA are known as Okazaki fragments (Fig 11.24). 

This dilemma was resolved by Reiji Okazaki, who found that a significant proportion of newly synthesized DNA exists as small fragments. These units of about a thousand nucleotides (called Okazaki fragments) are present briefly in the vicinity of the replication fork (Figure 28.21). 

The answer is d. (Murray, pp 435—451. Scriver, pp 3—45. Sack, pp 1-40. Wilson, pp 101-120.) DNA synthesis cannot occur until an RNA primer is made. A specific type of RNA polymerase called primase synthesizes a short stretch of RNA of about five nucleotides that is complementary to the template DNA strand in duplex DNA near the replication fork. This function cannot be carried out by DNA polymerase. In contrast, both DNA polymerase and RNA polymerase work in the 5 to 3 direction and add nucleoside monophosphates from nucleotide triphosphates to the growing polynucleotide chains of DNA or RNA. Only DNA polymerase edits as it synthesizes DNA and fills the gap between Okazaki fragments. 

Okazaki fragment Short segments of DNA (approximately 1000 nucleotides in prokaryotes, 100 to 200 nucleotides in eukaryotes) synthesized on the lagging strand during DNA replication. As the replication fork opens, the ribonucleic acid (RNA) polymerase primase synthesizes a short RNA primer, which is extended by DNA polymerase until it reaches the... 

Okazaki and his colleagues provided the experimental evidence for discontinuous DNA synthesis.) Subsequently, these pieces (now called Okazaki fragments) are covalently linked together by DNA ligase. (In prokaryotes such as E. coli, Okazaki fragments possess from 1000 to 2000 nucleotides.)... 

Okazaki fragments. From 100 to 200 nucleotides long, the Okazaki fragments of eukaryotes are significantly shorter than those in prokaryotes. 

DNA ligase (Figure 24.24) - Covalently closes nicks in double-stranded DNA. The nick must contain 3 hydroxyl and 5 phosphoryl termini and the nucleotides being linked must be adjacent in a duplex structure and properly base-paired. DNA ligase functions to seal Okazaki fragments in the lagging strand of DNA replication. 

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