The polyadenylation reaction passes through two stages. 

First, a rather short oligo(A) sequence (10 residues) is added to the 3’ end. This reaction is absolutely dependent on the AAUAAA sequence and poly(A) polymerase performs it under the direction of the specificity factor. 

In the second phase, the nuclear poly(A) binding protein (PABP II) binds the oligo(A) tail to allow extension of the poly(A) tail to the full 200 residue length.

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#Unit 13. Methods in Biology

DIC microscopy easily defines the outlines of large organelles, such as the nucleus and vacuole.

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#Unit 13. Methods in Biology

Formaldehyde a common fixative, crosslinks amino groups on adjacent molecules

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#Unit 13. Methods in Biology

Histological samples are often stained with hematoxylin and eosin (“H&E stain”).

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#Unit 13. Methods in Biology

Hematoxylin binds to basic amino acids (lysine and arginine).

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#Unit 13. Methods in Biology

Eosin binds to acidic molecules (DNA and side chains of aspartate and glutamate).

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#Unit 13. Methods in Biology

Darkfield microscopy is used for very thin spirochetes, such as Treponema pallidum, the causative agent of syphilis.