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Structure & Arrangement
Crick and Watson published their landmark paper on the structure of DNA in 1953 and it gave rise to an entirely new dimension in the field of science. DNA or deoxyribonucleic acid has got many facets and to understand them is very crucial for scientists and researchers. One of the facets is the antiparallel arrangement of DNA strands. Antiparallel means the two DNA strands are arranged in the opposite direction.
A single DNA strand has two ends – one end has 5’-OH group - which is where a free phosphate group is attached to deoxyribose sugar. At the other end there is a 3’-OH group which is where a free hydroxyl group is attached to a deoxyribose sugar. The DNA chains are so arranged that you can never have two 5’ (five prime) or 3’ (three prime) at one end. This arrangement is called antiparallel, the strands run in opposite directions to each other.
In the case of circular DNA strands, which are mostly found in bacteria or viruses – though the structure is circular, there occurs some kind of polarity in the individual strand. When looking at how the DNA structure is written up in text books you'll note that on the left the strand runs in the 5’ – 3’ direction and the right side strand runs in the 3’ – 5’ direction.
In addition, there are certain conventions while writing down the base sequence of DNA. It is usual to represent a polynucleotide structure by simply writing the bases of component nucleotides. It is not always necessary to write both strands -- you can write one strand only, as the complementary sequence is automatically specified. So, in most the cases, the structure of a gene is represented by a single base sequence. Conventionally, you need to specify the 5’ end to the left hand side and 3’ end to the right hand side of the sequence.
5’ CATAGA 3’
3’ GTATCT 5’