Review Questions DNA Replication 1. Explain Semi .

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Review QuestionsDNA Replication1. Explain semi-conservative replication.Prior to cell division, a cell must make a copy of its DNA to pass alongto the next generation. Copying DNA is called “replication”. Rather thanbuild a DNA molecule from scratch, the new DNA is composed of oneold DNA strand (used as the template) and one brand new strand.“Semi-conservative” means that half of the new DNA molecule is oldDNA.2. How can the speed of DNA replication increase while the rate ofreplication remains constant?The conundrum of DNA replication is that in humans the replicationenzymes can copy at a rate of 50 base pairs per second. That mayseem like a fast rate but there are 3.1 billion base pairs in the humangenome. At that rate, if the machinery started at one end of the DNAand replicated all the way down to the other end, it would take 2years to copy one DNA molecule. Replication occurs much faster thanthat. How? Well, the answer is that DNA replication starts at manyplaces along the molecule. These separate “origins of replication” form“replication bubbles”. Once a bubble forms, replication moves in bothdirections. These expanding bubbles of replication will eventually meetand the whole genome will be copied in a matter of hours rather thanyears.

3. Explain the process of DNA replication.Unwinding and Unzipping the Double Helix. The two strands in a DNAmolecule are connected by hydrogen bonds between thecomplementary bases. An enzyme called “helicase” travels along theDNA unwinding and breaking the hydrogen bonds between the twostrands. An enzyme called “topoisomerase” travels just ahead ofhelicase on the replication fork to prevent the DNA from winding tootight. Once the DNA strands are separated, “single strand bindingproteins” hold the strands in place so they can be replicated efficiently.Replicating the Leading Strand. The leading strand is built by using thelagging strand of the old molecule as the template. Replication of theleading strand begins with the enzyme “primase”. Primase adds asmall complementary piece of RNA ( 10 nucleotides), called an “RNAprimer”, near the end of the lagging strand. The RNA primer gives anenzyme called “DNA polymerase III” something to attach a DNAnucleotide to. A jury-rigged design, RNA primers are necessarybecause DNA polymerase III can only attach nucleotides to the 3’ endof a pre-existing nucleotide. At the start of a DNA molecule there is nopre-existing nucleotide, so DNA polymerase III can’t work. Another

limitation is that DNA polymerase III can only build a strand in the 5’ 3’ direction. Luckily, in building the lagging strand, the DNApolymerase III builds the leading strand in the direction of thereplication fork (5’ 3’) The RNA primer is eventually replaced by DNAby another enzyme “DNA polymerase I”.Replicating the Lagging Strand. Replication of the lagging strand is abit more complicated. Just like the replication of the leading strand,primase first has to install an RNA primer. DNA polymerase III, asexpected, builds the new strand in the 5’ 3’ direction. However,since the template strand of the lagging strand is anti-parallel to thetemplate strand of the leading strand, DNA polymerase III builds thestrand away from the replication fork. Another jury-rigged design. Tosolve this problem, the lagging strand is built in a series of shortsegments known as Okazaki fragments (1,000 to 2,000 bases). Andeven though individual fragments are built in a direction away from thereplication, each new one starts closer to the fork. “One step forward,two steps back.” The details are as follows: (1) primase installs anRNA primer. (2) DNA polymerase III attaches a nucleotide to the 3’ endof the last RNA nucleotide in the primer and begins building the strandin the 5’-3’ direction (away from the fork). (3) After building an Okazakifragment of 1000 to 2000 bases, DNA polymerase III stops. (4) DNApolymerase I replaces the RNA primer with DNA. (5) Primase backs upand installs a new primer one Okazaki fragment closer to thereplication fork. (6) DNA polymerase III attaches the first nucleotide ofthe Okazaki fragment to the RNA primer and then builds the fragment.(7) DNA polymerase I replaces the RNA primer. (8) Ligase, anotherenzyme, connects the two new Okazaki fragments together.

4. Explain how DNA erosion occurs and how it is prevented.In the replication of the leading strand, primase installs an RNA primerat the beginning of the DNA molecule. DNA polymerase I removesand, in fact, does not replace the RNA primer. As a result, the newDNA molecule is going to be a tad shorter. Each time the DNAreplicates, it gets shorter and shorter until the erosion removes animportant gene and the cell dies. A typical cell can usually divide amaximum of about 50 times before croaking.

5. What about stem cells? They aren’t limited to 50 divisions. Stemcells can divide indefinitely. What makes a stem cell impervious toerosion?The answer is the enzyme telomerase. After each replication,telomerase adds extra bits of DNA onto the ends of the strands (knownas telomeres) to offset the loss from erosion.

DNA Replication 1. Explain semi-conservative replication. Prior to cell division, a cell must make a copy of its DNA to pass along to the next generation. Copying DNA is called “replication”. Rather than build a DNA molecule from scratch, the new DNA is composed of one old DNA strand (used as the template) and one brand new strand.

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