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16-20 Study Guide

16-20 Study Guide - 1 Explain how a nucleoside triphosphate...

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1. Explain how a nucleoside triphosphate is added to a growing polymer of DNA. Specifically, where do the new nucleotides attach & in what direction is the chain synthesized The replication process begins somewhere in the middle of the DNA molecule, at places known as origins of replication. There, hydrogen bonds are broken, & the paired bases are separated. The helix begins to pull apart (the unwinding is facilitated by the enzyme helicase). Each replication origin contains two replication complexes, which move in opposite directions & create two Y- shaped replication forks as unwinding continues. The main function of DNA polymerase, the primary enzyme responsible for replication, is to add the 5’ phosphate of a new nucleotide to an existing 3’ OH group. Since DNA polymerase can only add new bases to the 3’ end of a growing strand, DNA strands always grow from their 5’ to their 3’ ends. This means that while replication can occur in the 5’ to 3’ direction on one of them, it must occur in the 3’ to 5’ direction on the other. 2. Describe the final structure of DNA. Include its geometric form, what holds it together, & some idea of its length. The configuration of the two strands of DNA wound around each other is that of the famous double helix. Within the double helix, the two strands of DNA run in opposite directions. They are referred to as antiparallel. One chain runs from 5’ to 3’ & the other from 3’ to 5’. Adenine & thymine always form two hydrogen bonds, while guanine & cytosine form three hydrogen bonds. The two strands are complementary (whatever nucleotides are in one strand, they rigidly fix the sequence of nucleotides in the other strand). The hydrogen bonds just mentioned serve the purpose of holding together the long DNA molecule which, in turn, can be thousands of base-pairs long. 3. Using the terms hydrogen bonding , pyrimidine , purine , & the letters G , A , C , & T , explain the manner in which nitrogen bases fit together in the completed DNA molecule. Watson discovered that adenine (a purine) & thymine (a pyrimidine) form two hydrogen bonds, while guanine (a purine) & cytosine (a pyrimidine) from three hydrogen bonds. This was the only way that the DNA molecule could fit together. Hydrogen bonds therefore hold together the two complementary strands that form a DNA molecule. 8. What is gene replacement therapy, & how might it be accomplished? (350) Gene replacement therapy is the injection of copies of a normal gene into cells that lack the normal gene. The starting point for gene replacement therapy is the genomic library, where the gene is still complete (promoters & introns included). In mice, when faulty eggs taken from females were microinjected with copies of a normal allele, 20% of them incorporated at least one copy of the gene in one of their chromosomes. After fertilization & reimplantation in a surrogate mother, the altered embryos grew into normal mice. One possibility for gene replacement therapy in humans is the replacement of defective genes in somatic cells. An example is the treatment of immune deficiencies.
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