B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition), страница 94

The mechanism depends on an enzyme calledDNA primase, which uses ribonucleoside triphosphates to synthesize shortRNA primers on the lagging strand (Figure 5–10). In eukaryotes, these primersare about 10 nucleotides long and are made at intervals of 100–200 nucleotides onthe lagging strand.The chemical structure of RNA was introduced in Chapter 1 and is describedin detail in Chapter 6. Here, we note only that RNA is very similar in structure toDNA. A strand of RNA can form base pairs with a strand of DNA, generating aDNA–RNA hybrid double helix if the two nucleotide sequences are complementary. Thus, the same templating principle used for DNA synthesis guides the synthesis of RNA primers.

Because an RNA primer contains a properly base-pairednucleotide with a 3ʹ-OH group at one end, it can be elongated by the DNA polymerase at this end to begin an Okazaki fragment. The synthesis of each Okazakifragment ends when this DNA polymerase runs into the RNA primer attached tothe 5ʹ end of the previous fragment. To produce a continuous DNA chain from themany DNA fragments made on the lagging strand, a special DNA repair systemacts quickly to erase the old RNA primer and replace it with DNA. An enzymecalled DNA ligase then joins the 3ʹ end of the new DNA fragment to the 5ʹ end ofthe previous one to complete the process (Figure 5–11 and Figure 5–12).Why might an erasable RNA primer be preferred to a DNA primer that wouldnot need to be erased? The argument that a self-correcting polymerase cannotstart chains de novo also implies the converse: an enzyme that starts chains anewcannot be efficient at self-correction.

Thus, any enzyme that primes the synthesisof Okazaki fragments will of necessity make a relatively inaccurate copy (at leastone error in 105). Even if the copies retained in the final product constituted aslittle as 5% of the total genome (for example, 10 nucleotides per 200-nucleotideDNA fragment), the resulting increase in the overall mutation rate would be enormous. It therefore seems likely that the use of RNA rather than DNA for primingbrings a powerful advantage to the cell: the ribonucleotides in the primer automatically mark these sequences as “suspect copy” to be efficiently removed andreplaced.Figure 5–11 The synthesis of one of many DNA fragments on thelagging strand. In eukaryotes, RNA primers are made at intervals spacedby about 200 nucleotides on the lagging strand, and each RNA primer isapproximately 10 nucleotides long.

This primer is erased by a special DNArepair enzyme (an RNAse H) that recognizes an RNA strand in an RNA/DNAhelix and fragments it; this leaves gaps that are filled in by DNA polymeraseand DNA ligase.2455′3′3′HO5′3′RNA primerDNA primase3′HO5′5′3′MBoC6 m5.11/5.10RNAprimer3′5′laggingstrandtemplate3′5′5′new RNA primersynthesis by DNAprimase3′5′3′DNA polymerase adds to newRNA primer to start newOkazaki fragment5′ 3′5′3′DNA polymerase finishesDNA fragment3′5′5′3′old RNA primer erasedand replaced by DNA3′5′5′3′sealing by DNA ligasejoins new Okazaki fragmentto the growing chain3′5′5′3′246Chapter 5: DNA Replication, Repair, and Recombination5′ phosphate3′OH5′3′A PP P5′3′A PA P P PSTEP 1STEP 2ATPusedAMPreleasedSpecial Proteins Help to Open Up the DNA Double Helix in Frontof the Replication ForkFor DNA synthesis to proceed, the DNA double helix must be opened up (“melted”)ahead of the replication fork so that the incoming deoxyribonucleoside triphosphates can form base pairs with the template strands.

However, the DNA doublehelix is very stable under physiological conditions; the base pairs are locked inplace so strongly that it requires temperatures approaching that of boiling water toseparate the two strands in a test tube. For this reason, two additional types of replication proteins—DNA helicases and single-strand DNA-binding proteins—areneeded to open the double helix and provide the appropriate single-strand DNAtemplates for the DNA polymerase to copy.DNA helicases were first isolated as proteins that hydrolyze ATP when they arebound to single strands of DNA.

As described in Chapter 3, the hydrolysis of ATPcan change the shape of a protein molecule in a cyclical manner that allows theprotein to perform mechanical work. DNA helicases use this principle to propelMBoC6 m5.13/5.12themselves rapidly along a DNA single strand. When they encounter a region ofdouble helix, they continue to move along their strand, thereby prying apart<b>Текст обрезан, так как является слишком большим</b>.