A cycle of chromosome replication must be initiated at a fixed time of C + D =60 minutes before cell division.

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#Unit 4. Cell Communication and Cell Signaling

The heterotrimeric G protein coupled to rhodopsin, called transducin (Gt)

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#Unit 4. Cell Communication and Cell Signaling

In the absence of light,  bound 11-cis-retinal locks the rhodopsin into its inactive (R) conformation

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#Unit 4. Cell Communication and Cell Signaling

Absorption of light causes rapid photoisomerization of the bound cis-retinal to the all-trans isomer, triggering a conformational change in opsin that activate the Gαt.

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#Unit 4. Cell Communication and Cell Signaling

There are about 90 million rod cells in the human retina, and each contains about 4x107 molecules of rhodopsin that are very densely packed in the membrane disks.

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#Unit 4. Cell Communication and Cell Signaling

A single activated rhodopsin protein activates 

~13 transducin molecules in the few seconds before the covalent bond between the opsin protein and all-trans-retinal is spontaneously cleaved

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#Unit 4. Cell Communication and Cell Signaling

RPE65 is the enzyme in the pigment epithelial cells of the retina that catalyzes the isomerization of the intermediate esterified all-trans retinol to 11-cis-retinol.