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What 5'&3'means?
DNA is usually a double-helix and has two strands running in opposite directions. (There are some examples of viral DNA which are single-stranded). Each chain is a polymer of subunits called nucleotides (hence the name polynucleotide).


Each strand has a backbone made up of (deoxy-ribose) sugar molecules linked together by phosphate groups. The 3' C of a sugar molecule is connected through a phosphate group to the 5' C of the next sugar. This linkage is also called 3'-5' phosphodiester linkage. All DNA strands are read from the 5' to the 3' end where the 5' end terminates in a phosphate group and the 3' end terminates in a sugar molecule.
DNA and RNA are synthesized in cells by DNA polymerases and RNA polymerases. Short fragments of nucleic acids also are commonly produced without enzymes by oligonucleotide synthesizers. This leads to formation of the so-called "sugar-phosphate backbone", from which the bases project.

A key feature of all nucleic acids is that they have two distinctive ends: the 5' (5-prime) and 3' (3-prime) ends. This terminology refers to the 5' and 3' carbons on the sugar. For both DNA  and RNA, the 5' end bears a phosphate, and the 3' end a hydroxyl group.

Another important concept in nucleic acid structure is that DNA and RNA polymerases add nucleotides to the 3' end of the previously incorporated base. Another way to put this is that nucleic acids are synthesized in a 5' to 3' direction.



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