the structure of DNA
the DNA is a very large single molecule, and hennce it is described as a macromolecule. it is composed of two complemennntary strands wound around each other in a double helix 
each sinngle DNA strand is composed of repeating nucleotides which are made of three components , phosphate,sugar (pentose) arranged lenghtwise and a nitrogennous base attached to the sugar inwards which extends to join (by hydrogen bond) the complementary nitrogenous base from the other strand thus the two strands together make a ladder-like arrangement 
okk....but its only dna
hmmmm i know but plzzz
i don't know abt rna
ok...but for dat some1 else should also i can mark best

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RNA is Ribonucleic acid.  RNA is a very essential part of the cell.  It is very similar to the structure of DNA.  RNA is also called a macro-molecule like DNA because it is a long chain of organic compounds. 

RNA plays a very important role in the coding, decoding, expression (transmission) of the genes to next generation.

Structure : see diagrams for easy understanding.

   Unlike DNA, it is only one stranded helical chain structure. It bends and folds on to itself at the ends.  It contains a chain of nucleotides, which are compounds that carry energy, involve in the metabolism, and do cell signalling.  RNA contains Riboses (simple sugars) having hydroxides in the structure, placed differently in deoxyribose (deoxy sugars) in DNA. 

   A nucleotide RNA contains ribose sugar with 5 carbons.  The first carbon is connected to base from adenine, guanine, uracil and cytosine.  To the 3rd and 5th carbons phosphates are attached.  At the second carbon there is the Hydroxide group.
DNA: Deoxyribonucleic acid:           see diagram for structure.

         It contains the genetic instructions needed for functioning of organisms.  It is a double helix with two polymer molecules forming strands with each other.  These helical strands are called the backbone.  The back bone contains deoxyribose sugars with 5 carbons.  The carbons are attached to phosphates on one side and to bases in between the two strands. The bases are Adenine , Thymine, Cytosine, Guanine.  Two bases form hydrogen bonds between them and join the two strands of the double helix.

click on thank you. I hope this is simple and easily understood.