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12.3 DNA Replication THINK ABOUT IT Before a cell divides, its DNA must first be copied. How might the double-helix structure of DNA make that possible? Copying the Code What role does DNA polymerase play in copying DNA? DNA polymerase is an __________________ that joins individual nucleotides to produce a new strand of DNA. Base pairing in the double helix explained how DNA could be copied, or ____________________, because each base on one-strand pairs with only one base on the opposite strand. Each strand of the double helix has all the information needed to reconstruct the other half by the mechanism of _____________ __________________. Because each strand can be used to make the other strand, the strands are said to be _______________________. The Replication Process Before a cell divides, it duplicates its DNA in a ________________ process called replication. This process ensures that each resulting cell has the _____________ complete set of DNA molecules. During replication, the DNA molecule __________________ into two strands and then produces two ________ complementary strands following the rules of base pairing. Each original strand of the double helix of DNA serves as a template, or _______________, for the new strand. The two strands of the double helix _____________________, or “unzip,” allowing two replication forks to form. As each new strand forms, _______ bases are added following the rules of base pairing. If the base on the old strand is _______________, then _______________ is added to the newly forming strand. Likewise, guanine is always paired to _____________. The _________________ of replication is ___________ DNA molecules ________________ to each other and to the original molecule. Each DNA molecule resulting from replication has one original strand and one new strand. The Role of Enzymes DNA replication is carried out by a series of ________________. They first “unzip” a molecule of DNA by breaking the _______________ bonds between base pairs and unwinding the two strands of the molecule. Each strand then serves as a template for the attachment of complementary bases. The principal enzyme involved in DNA replication is called ______ _____________________. DNA polymerase is an enzyme that ___________ individual __________________ to produce a new strand of DNA. DNA polymerase also “proofreads” each new DNA strand, ensuring that each molecule is a perfect copy of the original. Telomeres The tips of chromosomes are known as _____________________. The ends of DNA molecules, located at the telomeres, are particularly difficult to copy. Over time, DNA may actually be ______________ from telomeres each time a chromosome is ___________________. An enzyme called ______________ compensates for this problem by adding short, repeated DNA sequences to telomeres, _______________________ the chromosomes slightly and making it less likely that important gene sequences will be lost from the telomeres during replication. Replication in Living Cells How does DNA replication differ in prokaryotic cells and eukaryotic cells? Replication in most prokaryotic cells starts from a ___________ point and proceeds in ______ directions until the entire chromosome is copied. In eukaryotic cells, replication may begin at _____________ or even __________________ of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied. The cells of most prokaryotes have a ________, circular DNA molecule in the cytoplasm, containing nearly all the cell’s genetic information. Eukaryotic cells, on the other hand, can have up to 1000 times _______ _________ . Nearly all of the DNA of eukaryotic cells is found in the ______________. Prokaryotic DNA Replication In most prokaryotes, DNA replication does not start until regulatory _____________ bind to a single starting point on the ___________. This triggers the beginning of DNA replication. Replication in most prokaryotic cells starts from a single point and proceeds in two directions until the entire chromosome is copied. Often, the two chromosomes produced by replication are attached to different points inside the cell membrane and are separated when the cell splits to form two new cells. Eukaryotic DNA Replication Eukaryotic chromosomes are generally much _____________ than those of prokaryotes. In eukaryotic cells, replication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied. The two copies of DNA produced by replication in each chromosome remain closely associated until the cell enters _______________ of mitosis. At that point, the chromosomes condense, and the two _________________ in each chromosome become clearly visible. They separate from each other in anaphase of ______________, producing two cells, each with a complete ___________________ ________________ coded in DNA. ASSIGNMENT OPTIONS – choose one: A.)Complete a circle of learning for the following sections for “Copying the Code” and “Replication in Living Things” by summarizing the key ideas found in the subsections. Remember the key questions as you summarize. OR B.) Summarize all sections by writing post-it notes with key ideas. Place the post-it notes over the section once it’s summarized. Copying the Code The Replication Process The Role of Enzymes Telomeres Replication in Living Cells Prokaryotic DNA Replication Eukaryotic DNA Replication CIRCLES OF LEARNING Note which section you are writing about next to each circle. Add more to the back if needed. Prokaryotic DNA Replication Vs. Eukaryotic DNA Replication