Explain why replication machinery is incapable of completely replicating the end

Question

Explain why replication machinery is incapable of completely replicating the ends of the chromosomes. What is the practical effect of this? How do eukaryotic cells get around this problem? Explain why replication machinery is incapable of completely replicating the ends of the chromosomes. What is the practical effect of this? How do eukaryotic cells get around this problem? Explain why replication machinery is incapable of completely replicating the ends of the chromosomes. What is the practical effect of this? How do eukaryotic cells get around this problem?

Explanation / Answer

The major function of deoxyribonucleic acid (DNA) polymerases is to replicate the genome and thus to allow transmission of genetic information from one generation to the next. In eukaryotic cells, this takes place during a discrete period (S phase) of interphase, and replicated chromosomes are subsequently segregated to daughter cells during mitosis. As well as replicating DNA, polymerases help to maintain the integrity of the genome by participating in various modes of DNA repair. DNA polymerases are also required for the replication of eukaryotic viruses. Some viruses, such as SV40, use host polymerases while others, such as adenovirus, encode their own polymerase; discussion of viral polymerases is outside the scope of this review. See also: Cell Cycle The basic catalytic reaction of DNA polymerases is to effect semiconservative replication of DNA, using a singlestranded DNA chain as a template and four deoxynucleotides (deoxythymidine triphosphate (dTTP), deoxycytidine triphosphate (dCTP), deoxyguanosine triphosphate (dGTP), deoxyadenosine triphosphate (dATP)) as precursors for DNA synthesis (Figure 1). The enzyme assembles the precursor nucleotides on the template to form a complementary DNA strand, selecting the incoming nucleotide using the base pair rules A .T and G . C. To start synthesis on a singlestranded DNA molecule, DNA polymerases need a primer. This is a length of ribonucleic acid (RNA) or DNA that is annealed to the single-stranded template. The primer provides a 3’-OH that can be extended by the polymerase; this configuration of the primer is important because polymerases can only extend a new chain in the 5’ to 3’ direction. If the DNA template to be replicated is double-stranded, polymerase action still needs a primer and, in addition, requires other enzymes to unwind the double helix. See also: DNA Helicases; DNA Polymerase Fidelity Mechanisms; Polymerase Processivity: Measurement and Mechanisms.

Primer Template 5’ 3’ – – A T T A G C – GATT 3’ OH

Primer Template 5’ 3’ – – A T T A G C C G – ATT 3’ OH +dCTP +PPi (a) Polymerase Template Polymerase Template Nx +dNTP Polymerase Template Nx Polymerase Template Nx+1 +PPi dNTP

(a) Basic mechanism catalysed by DNA polymerases. Polymerization of nucleotides on the single-stranded template requires a primer (RNA or DNA) which provides a 3’-OH group to which the incoming nucleotide is joined and the direction of synthesis is thus 5’ to 3’. Only nucleotides that correctly base pair with the templates strand (according to A . T, G .C rules) are incorporated. One molecule of pyrophosphate (PPi ) is produced per nucleotide incorporated. (b) Steps in the polymerization reaction. The order of the reaction is binding to the template–primer, followed by binding of the dNTP. dNTP is cleaved at the a/b bond to give dNMP that is added onto the chain, PPi is released, and the polymerase translocates along to the next 3’ terminus or dissociates.

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