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DNA Polymerase, the main enzyme involved in DNA synthesis requires a template (the opposite strand of DNA), and a primer (the -OH group on the THIRD carbon (not the second) of deoxyribose). This -OH group is located on the previous nucleotide. The phospate group ( -PO4 ) of the incoming nucleotide will be added to the ( -OH ) group of the previous nucleotide.
During DNA synthesis, the double strands of DNA separate, so that each of the old strands can serve as template for each of the new halves. When synthesis is finished, 2 new identical strands of DNA will exist, each one consisting of one old strand and one newly synthesized strand.
So now for your answer. Since the DNA polymerase needs to "see" a 3 prime hydroxyl group ( -OH ) in order to proceed adding new bases along the template you might ask, "How does synthesis start in the first place?" The answer comes from RNA polymerase (the enzyme responsible for synthesizing RNA from a DNA template). At the start of DNA replication, RNA polymerase creates a short temporary primer on the DNA template. Like DNA polymerase, the RNA polymerase requires a template, BUT unlike DNA polymerase, the RNA does NOT need to see that -OH group of the previous nucleotide. After a short RNA primer sequence has been created on the DNA template strand, the RNA polymerase falls off and DNA polymerase can add onto the hydroxyl group (-OH) of the RNA primer and start synthesizing from there. The RNA is removed shortly after the DNA synthesis starts.
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In short, DNA polymerase cannot make strands from scratch. But RNA polymerase can do it. so RNA polymerase synthesizes RNA primer which then acts as a substrate for DNA polymerase to start the synthesis of new strands. these primers are removed later on.
The main difference between DNA and RNA is the sugar present in the molecules. While the sugar present in an RNA molecule is ribose, the sugar present in a molecule of DNA is deoxyribose. Deoxyribose is the same as ribose, except that the former has one more OH.
DNA does not usually exist as a single molecule, but instead as a tightly-associated pair of molecules. These two long strands entwine like vines, in the shape of a double helix. This arrangement of DNA strands is called antiparallel. The asymmetric ends of DNA strands are referred to as the 5′ (five prime) and 3′ (three prime) ends. One of the major differences between DNA and RNA is the sugar, with 2-deoxyribose being replaced by the alternative pentose sugar ribose in RNA. The four bases found in DNA are adenine (abbreviated A), cytosine (C), guanine (G) and thymine (T). A fifth pyrimidine base, called uracil (U), usually takes the place of thymine in RNA and differs from thymine by lacking a methyl group on its ring.
DNA is self-replicating.RNA is synthesized from DNA when needed.