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3 dna replication starts at the origin of replication

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Unformatted text preview: eplication. What are the enzymes involved, and their role in the process? 3 ● DNA replication starts at the origin of replication, where the origin binding proteins open the DNA helix. ● The helicase then unwinds the DNA and continues to break hydrogen bonds at the replication forks. Single-strand DNA binding proteins keep the strands from rejoining, and topoisomerase relieves the induced strain in the unwound portion of the double helix. ● Primase adds an initial RNA primer to the template. ● DNA polymerase begins adding nucleotides to the 3′ end. The leading strand is synthesized continuously in the 5′ to 3′ direction. The lagging strand is synthesized in Okazaki fragments started at RNA primers. ● The RNA primers are removed by DNA polymerase and replaced with DNA (also in the 5′ to 3′ direction) and the DNA ligase joins the backbone of the fragments to form a complete strand. DNA replication gets complicated because the two strands of a double-stranded DNA molecule are antiparallel, and DNA polymerase can only move in a 5’ 3’ direction. Along one of the template strands, DNA polymerase can synthesize a complementary strand continuously by elongating the new DNA in the 5’ to 3’ direction. This is called the leading strand and it requires only one RNA primer to be made to start replication. However, to elongate the other strand in the 5’ to 3’ direction, DNA polymerase must elongate the new DNA strand in a direction opposite to the movement of the replication fork. This strand is known as the lagging strand. As the replication form proceeds and exposes a new section of the lagging strand template, RNA Primase must put down a new primer for DNA Polymerase to elongate. In this way, the lagging strand is completed in a discontinuous manner. The synthesized DNA fragments on the lagging strand are called Okazaki fragments. DNA polymerase is responsible for removing the RNA primers and replacing them with DNA. DNA ligase then seals the gaps between the Okazaki fragments to complete the newly synthesized lagging strand. What does this mean? How does the problem get solved? This means that the chain gets shorter every time, so DNA polymerase adds non-coding DNA called telomeres. How does DNA polymerase manage to replicate 3 billion base pairs of DNA in a timely manner, with minimal mistakes? DNA polymerase proofreads its own work. If it finds a nucleotide that doesn't obey base pairing rules, it removes and replaces the nucleotide. How does DNA replication result in two identical molecules of DNA? The parent DNA serves as a template for daughter DNA What are telomeres? What is their function? How and when/where are they replaced? The linear nature of eukaryotic DNA presents a challenge not typically found in bacterial DNA replication: DNA polymerase cannot add the final sequence of DNA to the 5′ end of the lagging daughter strand. A solution is required or genetic information would be lost with each round of replication. For this reason, eukaryotic chromosomal DNA has special sequences at their ends called telomeres. Telomeres contain repeating sequences of bases that do not code for proteins; they serve to protect the genetic information contained at the ends of eukaryotic chromosomes. Telomerases...
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