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Sagot :
DNA carries the information for making all of the cell's proteins.
These proteins implement all of the functions of a living organism and
determine the organism's characteristics. When the cell reproduces, it
has to pass all of this information on to the daughter cells.Before a cell can reproduce, it must first replicate,
or make a copy of, its DNA. Where DNA replication occurs depends upon
whether the cells is a prokaryote or a eukaryote (see the RNA sidebar on
the previous page for more about the types of cells). DNA replication
occurs in the cytoplasm of prokaryotes and in the nucleus of eukaryotes.
Regardless of where DNA replication occurs, the basic process is the
same.The structure of DNA lends itself easily to DNA replication. Each side of the double helix runs in opposite (anti-parallel)
directions. The beauty of this structure is that it can unzip down the
middle and each side can serve as a pattern or template for the other
side (called semi-conservative replication). However, DNA does not unzip entirely. It unzips in a small area called a replication fork, which then moves down the entire length of the molecule.Let's look at the details:An enzyme called DNA gyrase makes a nick in the double helix and each side separatesAn enzyme called helicase unwinds the double-stranded DNASeveral small proteins called single strand binding proteins (SSB) temporarily bind to each side and keep them separatedAn enzyme complex called DNA polymerase
"walks" down the DNA strands and adds new nucleotides to each strand.
The nucleotides pair with the complementary nucleotides on the existing
stand (A with T, G with C).A subunit of the DNA polymerase proofreads the new DNAAn enzyme called DNA ligase seals up the fragments into one long continuous strandThe new copies automatically wind up again
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