BS - Photopigments - Week 7 Flashcards
Describe a rhodopsin molecule, what is contained within it, and what it is derived from.
It is a 7 a-helical transmembrane protein, that forms a cylinder, with a retinal molecule within.
Retinal is a vitamin A derivative.
Describe the chromophore of rhodopsin.
11-cis-retinal
What percentage of rhodopsin is lipid, and why?
50%, important for membrane fluidity.
What chromophore is shared by all mammalian retinas?
11-cis-retinal
What is chromophore absorption determined by?
The opsin amino acid sequence.
What morphological and histochemical differences are there between L and M cones, and what percentage identity do they have?
No known morphological or histochemical differences.
They are 98% identical.
Describe the cones in order of density.
L>M>S
Which cone type is infrequent at the fovea?
S cones.
What family of protein are opsins categorised as?
G-protein coupled receptors.
True or false
The gene for all three cone types are found on the same chromosome.
False, S cones are found on #7, L and M cones are found on #X
Describe the three major events for trichromacy.
1 - gene duplication and spectral tuning for a new wavelength
2 - expression of the new pigment in a new set of photoreceptors
3 - neural wiring to discriminate new outputs
Where do photoreceptor genes arise from?
From a single ancestor.
Duplication and sequence diversification leads to more pigments.
Rhodopsin shares high percentage identity with which pigment? What does this indicate?
Rhodopsin and blue pigments show common amino acid sequences.
Indicates blue pigments evolved first.
How did a third opsin locus arise, and in what species?
Old world primates.
Believed to be due to an unbalanced crossover event.
When a third opsin locus arose, was the locus control unit copied over?
No.
During recombination, what drives alignment?
DNA sequence similarity.
How do incorrect offset alignments occur during recombination?
Up/Downstream repetitive sequence similarities
What kind of mutations can an incorrect offset alignment during recombination result in?
Insertional and deletional mutations.
Define intragenic recombination.
When half of one gene is recombined into half of another gene.
By how many amino acids do L and M cones differ?
15
What was responsible for the conversion of the second M pigment gene copy to L?
Point mutations
True or false
Everyone has one copy of the L and M pigment genes.
Consider the answer, does this have any effect?
False, only 25% have one of each.
the rest have more than one copy of the M pigment gene.
Has no effect, only the first gene copy of each pigment type is expressed,
What is the ratio of L:M cones.
Varies between 1.5:1 to 9:1.
Consider the genes for the pigments. To which pigment is there a bias towards and why?
Heavy bias to L cones, possibly due to its proximity to the LCR region. It is closer than the M copies.
Is the neural wiring able to accommodate to unfamiliar additional pigment genes?
Yes, due to neural plasticity.
Define protanopia, deuteranopia, and tritanopia.
Protanopia - red deficit
Deuteranopia - green deficit
Tritanopia - blue deficit
Distinguish between di/monochromacy and anomalous trichromacy.
Di/monochromacy - complete lack of a given cone(s)
Anomalous trichromacy - mild deficiency in a given cone(s)
What is anomalous trichromacy/dichromacy commonly caused by?
Intragenic recombination
What is typically the main cause intragenic recombination?
Name 3 less common causes.
Chromosomal misalignment during recombination Less common causes: -Point mutation -Deletion -Single L/M hybrid gene in a protanope
