Structure of  Nucleotides and DNA

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Note that the deoxyribose has a 3' and 5' hydroxyl group these landmarks are used to orient the direction of the DNA strands.

Deoxyribose is a 5 carbon sugar.

 

                                                                                                                                                            Note this 5 nucleotide (TAAGC) single strand of DNA has a 5' and a 3' end.

Phosophodiester links join adjacent bases.
The twisting that creates a helix is caused by longitudinal HYDROGEN bonds between bases.
The Helix twists through 360* every 3.4nm 
 

Note that when the 2 sides of the DNA helix are paired up they are antiparallel the
3'-5' ends are oriented in opposite directions.
                                                                                                     The Double Helix

The helix makes a complete turn every 3.4 nm . 
Hydrogen bonds between the base pairs hold each strand of the helix together laterally and also create a torque in the molecule that causes a twisting helical shape to appear.  
The structure of DNA allows for duplication by formation of a complementary half. The H bonds between the 2 halves of the DNA strands are strong enough to hold together and yet supple enough to be opened for transcription and duplication.
The DNA molecule is elastic and supple yet can retain its integrity for thousands of years in the suitable conditions.
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