The sequence AAUAAA is a signal for cleavage to generate a 3’ end of mRNA that is polyadenylated. Addition of poly(A) to nuclear RNA can be prevented by the analog 3’–deoxyadenosine known as cordycepin

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#Unit 1. Molecules and their Interaction Relevant to Biology

This results in a strong hydrogen bond that has the nearly optimum N…O distance of 2.8 A.

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#Unit 1. Molecules and their Interaction Relevant to Biology

In a typical a-helix of 12 (or n) residues, there are 8 (or n-4) hydrogen bonds.

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#Unit 1. Molecules and their Interaction Relevant to Biology

The first 4 amide hydrogens and the last 4 carbonyl oxygens cannot participate in helix H bonds.

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#Unit 1. Molecules and their Interaction Relevant to Biology

Right-handed helical conformation has 3.6 residues per turn with height of 5.4A

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#Unit 1. Molecules and their Interaction Relevant to Biology

The diameter of a typical α helix is about 12 Å, which matches the 12 Å wide and 6–8 Å deep major groove of B-form DNA. 

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#Unit 1. Molecules and their Interaction Relevant to Biology

If the α helix lies parallel to the direction of the major groove, the straight α helix can contact 4–6 bp before the bases curve out of the plane of the amino acids in the α helix.