The Concepting Of Cells And Consequences Of DNA Symcules

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1. DNA replication begins with the unzipping of the parent molecule as the hydrogen bonds between the base pairs break. SSB proteins prevent reanealing of single stranded DNA. Many molecules of SSB bind cooperatively to single stranded DNA, stabilizing separated strands and preventing renaturation. The sequence of bases on each of the separated strands serves as a template to guide the insertion of a complementary set of bases on the strand that is being synthesized. The new strands are assembled from deoxynucleoside triphosphates. Each incoming nucleotide is covalently linked to the 3' carbon atom as the second and third phosphates are removed together as a molecule of pyrophosphate. Primase synthesizes a short RNA primer, which remains base-paired to template DNA that is extended at 3’ end by DNA polymerase. The nucleotides are assembled in the order that complements the order of bases on the strand serving as the template. Thus each C on the template guides the insertion of a G on the new strand, each G a C, and so on. When the process is complete, two DNA molecules have been formed identical to each other and to the parent molecule. A portion of the double helix is unwound by a helicase. A molecule of a DNA polymerase binds to one strand of the DNA and begins moving along it in the 3' to 5' direction, using it as a template for assembling a leading strand of nucleotides and reforming a double helix. In eukaryotes, this molecule is called DNA polymerase delta. Because DNA synthesis can only occur 5' to 3', a molecule of a second type of DNA polymerase (epsilon in eukaryotes) binds to the other template strand as the double helix opens. This molecule synthesizes discontinuous segments of polynucleotides (called Okazaki fragments). Another enzyme, DNA ligase I then stitches these together into the lagging strand. 2. When a mitotic checkpoint is weakened, nondisjunction, or the failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division, usually resulting in an abnormal distribution of chromosomes in the daughter nuclei, typically occurs due to the fact that the checkpoints usually stop or delay cell division until all components are ready to proceed to the next phase. When a checkpoint is weakened a cell can fail to realize a chromosome pair has yet to be lined up and proceed to the next stage. In this case, most chromosomes would separate normally, while others could fail to separate at all. This would generate a daughter cell lacking a copy (monosomy, 2n-1) and a daughter cell with an extra copy (trisomy, 2n+1). 3. The
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