Overdominance the heterozygote has higher fitness than the  fitnesses of the two homozygotes

Underdominance, in which the heterozygote has lower fitness than both homozygotes.

Directional selection leads to fixation of the favored allele and elimination of the other allele, as  long as no other evolutionary forces act on the population.

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#Unit 1. Molecules and their Interaction Relevant to Biology

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#Unit 1. Molecules and their Interaction Relevant to Biology

In fig, Phi is fixed at 0º. Clashes occur at ALL Psi angles. Phi = 0º is physically

impossible.

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#Unit 1. Molecules and their Interaction Relevant to Biology

The Ramachandran Principle is that alpha helices, beta strands, and

turns are the most likely conformations for a polypeptide chain to 

adopt, because most other conformations are impossible due to

steric collisions between atoms. 

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#Unit 1. Molecules and their Interaction Relevant to Biology

Beta Hairpin

Two anti-parallel beta strands connected by a loop / turn, look like hairpin

Can occur independently in isolation

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#Unit 1. Molecules and their Interaction Relevant to Biology

                                                                        Helix-turn-Helix

Two helices are connected by a small loop or turn

These helices are stabilized by non-polar contacts between the helices

Mostly found in DNA binding proteins, directly involved in DNA binding

Generally consists of 20 amino acids

Second helix is involved in DNA binding, enters in major groove of the target site

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                                                               Helix-turn-Helix