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Unit 14: Quest Number 2: DNA Replication, Mutations and Cancer Honors Biology 2011 Monkemeier The process of DNA replication is fundamentally similar for prokaryotes and eukaryotes. DNA replication is a semi-conservative process – this means that when the double helix is unwound, each original strand serves as a template or pattern for the newly synthesized strand. The replisome is a Major Components complex molecular machine that ◦ DNA Helicase carries out ◦ DNA Primase replication of DNA. The replisome ◦ DNA Polymerases contains all the necessary enzymes ◦ DNA ligases for replication. ◦ DNA Binding Proteins The process of Write these steps down: replication can be INITIATION: formation of thought of as having a replication fork beginning where the process starts; a ELONGATION: a number of middle where the enzymes work together to complementary accomplish the task of nucleotides are added assembling a new strand to the exposed parental strands; and TERMINATION: An end an end where the point to the replication process is terminated or stopped. DNA helicases unwind and unzip the DNA molecule forming a replication fork. A replication fork is a Y-shaped region where the new strands of DNA are elongating. The elongation process is semidiscontinuous. The leading strand of the DNA molecule (exposed 3’ end) is synthesized continuously in the direction of the opening replication fork. The lagging strand of the DNA molecule (exposed 5’ end) is synthesized discontinuously in the direction opposite to the opening of the replication fork. Remember - The two strands of the DNA double helix are antiparallel. This means that their sugar – phosphate backbones run in opposite directions! DNA polymerases add nucleotides only to the FREE 3’ end of a growing DNA strand, NEVER to the 5’ end. DNApolymerases synthesize ONLY in the direction 5’ to 3’. DNA Primase synthesizes a short complementary segment to the exposed 3’ end of the DNA strand. DNA polymerases extend the complementary segment toward the replication fork as it opens. The ability of the DNA polymerase to remain attached to the template (leading strand) is known as PROCESSIVITY. The synthesis of the lagging strand is described as discontinuous since it is synthesized in pieces that get glued together. DNA primase synthesizes a short segment of complementary DNA to the exposed 5’ end. DNA polymerases add complementary nucleotides in the direction away from the opening replication fork. DNA ligases glue the Okazaki Fragments together forming the new strand. Exposed 3’ end of the Exposed 5’ end of the original DNA molecule original DNA molecule Synthesized Synthesized continuously toward discontinuously away the opening replication from the opening fork. replication fork. The ability of a DNA The Okazaki fragments polymerase to remain attached to the leading are glued together by strand is known as DNA Ligases. processivity. Leading Strand Lagging Strand Prokaryotes have circular DNA so replication ends when the process comes around to the origin again. In eukaryotes, end points for each chromosome are indicated by telomeres. Telomeres are specialized structures on the ends of eukaryotic chromosomes that are composed of specific repeated DNA sequences. Telomeres are special nucleotide sequences on the ends of the linear eukaryotic chromosomes. The typical repeating unit in human telomeres is TTAGGG. The number of repetitions in a telomere varies between 100 and 1000. The telometric DNA protects an organism’s genes from being eroded through successive rounds of DNA replications. Remember that DNA polymerases can only add nucleotides in the 5’ to 3’ direction. The exposed 5’ end represents the lagging strand. The replisome does not contain the machinery to complete the 5’ ends of the daughter strands of the DNA. As a result, repeated replication produces shorter and shorter DNA molecules. Briefly describe the functions of the following enzymes during DNA replication: DNA helicases, DNA polymerases, and DNA ligases. Briefly provide an overview of DNA replication using the terms: initiation, elongation, termination. Distinguish between the leading strand and lagging strand during DNA replication?
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