For questions 29-32, use the following statement. The MC1R gene encodes a hormon

Question

For questions 29-32, use the following statement.

The MC1R gene encodes a hormone receptor that binds the hormone alpha-melanotropin. The function of the receptor hormone complex is to convert the red pigment, pheomelanin, to eumelanin, which is a black or brown pigment in skin and hair. A mutation in the MC1R gene limits the ability of some red heads to convert pheomelanin to eumelanin so these individuals are unable to tan and have red hair.

29. Normally, the MC1R receptor binds alpha melanotropin, but cortisol can also activate this receptor. The Kd for cortisol at this receptor is 10-3 M (millimolar). These results indicate that the MC1R receptor has a ________ affinity for cortisol. So, a __________ cortisol concentration would be expected to activate the receptor.

a. high, high
b. high, low
c. low, high
d. low, low
e. Cannot be determined

30. Changing an arginine to a histadine in the MCR1 receptor protein causes the red head mutation. The mRNA codons for arginine are CGU and CGC, and the mRNA codons for histadine are CAU and CAC. What is the base change in the DNA that causes the mutation?

a. adenine to guanine
b. guanine to cytosine
c. guanine to adenine
d. cytosine to thymine
e. There is no way to know given the available information.

31. The alpha melanotropin that binds to the MC1R receptor is initially synthesized as a 285 amino acid protein called pre-pro-opiomelanocortin. After the signal peptide is removed during translation, the remaining protein is further cleaved in synaptic vesicles to yield alpha-melanotropin. Based on this description you would predict that the resulting molecule would:

a. enter the cytoplasm by passing through the phospholipid bilayer. b. be hydrophobic.
c. bind to a cytoplasmic receptor.
d. affect the activity of adenylyl cyclase

e. all except d are correct.

32. Red heads have light skin, freckles and red hair because ___________________ are unable to convert pheomelanin to eumalanin.

a. keratinocytes
b. fibrocytes
c. melanocytes
d. connective tissue cells e. b and d are correct

Explanation / Answer

The MC1R gene provides instructions for making a protein called the melanocortin 1 receptor. This receptor plays an important role in normal pigmentation. The receptor is primarily located on the surface of melanocytes, which are specialized cells that produce a pigment called melanin. Melanin is the substance that gives skin, hair, and eyes their color. Melanin is also found in the light-sensitive tissue at the back of the eye (the retina), where it plays a role in normal vision.

Melanocytes make two forms of melanin, eumelanin and pheomelanin. The relative amounts of these two pigments help determine the color of a person's hair and skin. People who produce mostly eumelanin tend to have brown or black hair and dark skin that tans easily. Eumelanin also protects skin from damage caused by ultraviolet (UV) radiation in sunlight. People who produce mostly pheomelanin tend to have red or blond hair, freckles, and light-colored skin that tans poorly. Because pheomelanin does not protect skin from UV radiation, people with more pheomelanin have an increased risk of skin damage caused by sun exposure.

29. A ) C is the answer generally cortisol is the low affinity to the receptor ( MC1R receptor ) when higher concentrations it can activte the MC1R receptor

30.A ) C is the answer

31.A ) D is the answer because alpha melano tropin is working through the G-prptein receptor mechanism on the cell membrane by affect the activity of adenyl cyclase

32.A ) C is the answer

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