BS - Photopigments - Week 7

Question 1

Describe a rhodopsin molecule, what is contained within it, and what it is derived from.

Answer 1

It is a 7 a-helical transmembrane protein, that forms a cylinder, with a retinal molecule within.
Retinal is a vitamin A derivative.

Question 2

Describe the chromophore of rhodopsin.

Answer 2

11-cis-retinal

Question 3

What percentage of rhodopsin is lipid, and why?

Answer 3

50%, important for membrane fluidity.

Question 4

What chromophore is shared by all mammalian retinas?

Answer 4

11-cis-retinal

Question 5

What is chromophore absorption determined by?

Answer 5

The opsin amino acid sequence.

Question 6

What morphological and histochemical differences are there between L and M cones, and what percentage identity do they have?

Answer 6

No known morphological or histochemical differences.

They are 98% identical.

Question 7

Describe the cones in order of density.

Answer 7

L>M>S

Question 8

Which cone type is infrequent at the fovea?

Answer 8

S cones.

Question 9

What family of protein are opsins categorised as?

Answer 9

G-protein coupled receptors.

Question 10

True or false

The gene for all three cone types are found on the same chromosome.

Answer 10

False, S cones are found on #7, L and M cones are found on #X

Question 11

Describe the three major events for trichromacy.

Answer 11

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

Question 12

Where do photoreceptor genes arise from?

Answer 12

From a single ancestor.

Duplication and sequence diversification leads to more pigments.

Question 13

Rhodopsin shares high percentage identity with which pigment? What does this indicate?

Answer 13

Rhodopsin and blue pigments show common amino acid sequences.
Indicates blue pigments evolved first.

Question 14

How did a third opsin locus arise, and in what species?

Answer 14

Old world primates.

Believed to be due to an unbalanced crossover event.

Question 15

When a third opsin locus arose, was the locus control unit copied over?

Answer 15

No.

Question 16

During recombination, what drives alignment?

Answer 16

DNA sequence similarity.

Question 17

How do incorrect offset alignments occur during recombination?

Answer 17

Up/Downstream repetitive sequence similarities

Question 18

What kind of mutations can an incorrect offset alignment during recombination result in?

Answer 18

Insertional and deletional mutations.

Question 19

Define intragenic recombination.

Answer 19

When half of one gene is recombined into half of another gene.

Question 20

By how many amino acids do L and M cones differ?

Answer 20

15

Question 21

What was responsible for the conversion of the second M pigment gene copy to L?

Answer 21

Point mutations

Question 22

True or false
Everyone has one copy of the L and M pigment genes.
Consider the answer, does this have any effect?

Answer 22

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,

Question 23

What is the ratio of L:M cones.

Answer 23

Varies between 1.5:1 to 9:1.

Question 24

Consider the genes for the pigments. To which pigment is there a bias towards and why?

Answer 24

Heavy bias to L cones, possibly due to its proximity to the LCR region. It is closer than the M copies.

Question 25

Is the neural wiring able to accommodate to unfamiliar additional pigment genes?

Answer 25

Yes, due to neural plasticity.

Question 26

Define protanopia, deuteranopia, and tritanopia.

Answer 26

Protanopia - red deficit
Deuteranopia - green deficit
Tritanopia - blue deficit

Question 27

Distinguish between di/monochromacy and anomalous trichromacy.

Answer 27

Di/monochromacy - complete lack of a given cone(s)

Anomalous trichromacy - mild deficiency in a given cone(s)

Question 28

What is anomalous trichromacy/dichromacy commonly caused by?

Answer 28

Intragenic recombination

Question 29

What is typically the main cause intragenic recombination?

Name 3 less common causes.

Answer 29

Chromosomal misalignment during recombination
Less common causes:
-Point mutation
-Deletion
-Single L/M hybrid gene in a protanope