12.9__Major_Enzymes.pdf - 12.9: Major Enzymes The process...

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8/21/202012.9.112.9: Major EnzymesThe process ofDNA replicationis catalyzed by a type of enzyme calledDNA polymerase(polymeaning many,mermeaning pieces, and –asemeaning enzyme; so an enzyme that attachesmany pieces of DNA). Observe Figure 1: the double helix of the original DNA molecule separates (blue) and new strands are made to match the separated strands. The result will be two DNAmolecules, each containing an old and a new strand. Therefore, DNA replication is called semiconservative. The termsemiconservativerefers to the fact that half of the original molecule (oneof the two strands in the double helix) is “conserved” in the new molecule. The original strand is referred to as thetemplate strandbecause it provides the information, or template, for thenewly synthesized strand.3′ synthesis shown, no enzymes in diagram. ” width=”1024″ height=”508″> Figure 1. By Madprime(wikipedia) (DNA replication split horizontal) CC BY-SA 2.0Figure 2. Primer and TemplateDNA replicationrelies on the double-stranded nature of the molecule. One double stranded DNA molecule, when replicated, will become two double-stranded molecules, each containing oneoriginal strand and one newly synthesized strand. You remember that the two strands of DNA run antiparallel: one from the 5′ to the 3′, and the other from the 3′ to the 5′. The synthesis of thenew DNA strand can only happen in one direction: from the 5′ to the 3′ end. In other words, the new bases are always added to the 3′ end of the newly synthesized DNA strand. So if the newnucleotide is always added to the 3′ end of an existing nucleotide, where does thefirstnucleotide come from? In fact,DNA polymeraseneeds an “anchor” to start adding nucleotides: a shortsequence of DNA or RNA that is complementary to the template strand will work to provide a free 3′ end. This sequence is called aprimer(Figure 2).How doesDNA polymeraseknow in what order to add nucleotides? Specific base pairing in DNA is the key to copying the DNA: if you know the sequence of one strand, you can use basepairing rules to build the other strand. Bases form pairs (base pairs) in a very specific way. Figure 3 shows howA(adenine) pairs withT(thymine) andG(guanine) pairs withC(cytosine). It isimportant to remember that this binding is specific:Tpairs withA, but not withC. The molecular recognition occurs because of the ability of bases to form specific hydrogen bonds: atomsalign just right to make hydrogen bonds possible. Also note that a larger base (purine,AorG) always pairs with a smaller base (pyrimidine,Cor

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