A protein was purified to homogeneity. The determination of the mass by gel‑filtration chromatography yields a value of 60 kDa. Chromatography in the presence of urea results in a 30 kDa species. Repeating the chromatography in the presence of both urea and β‑mercaptoethanol, however, results in a single molecular species of 15 kDa. What do the data suggest about the structure of the protein?

In the given case, the homogeneity of the protein is done. By performing, the technique of gel-filtration chromatography, the mass of the protein obtained is 60 kilodalton. When the chromatography is performed in the existence of urea, a protein of mass 30 kilodalton is obtained. The ratio between the two obtained mass is 60/30 = 2, thus, it shows that the protein comprises two subunits and each of them weighs 30 kilodaltons.  

When the same technique of chromatography is performed in the existence of beta-mercaptoethanol and urea, a single protein of mass 15 kilodalton is produced. Now, the ratio is 30/15 = 2. Therefore, each subunit of the protein comprises two units of mass 15 kilodalton.  

Hence, it shows that the protein is a tetramer of 15 kilodalton subunits and each subunit produces dimers of mass 30 kilodalton and is attached by disulfide bonds.  

an individual’s sex (i.e., whether they are a male or female) is determined by the sex chromosomes. most people have two sex chromosomes, one that is inherited from their mother and one that is inherited from their father. typically, females have two x chromosomes (xx) and males have one x chromosome and one y chromosome (xy). conditions caused by changes (“mutations”) in genes located on the x chromosome are considered x-linked.

x-linked recessive inheritance

most x-linked conditions are recessive. this means that in a person with two x chromosomes (most females), both copies of a gene (i.e., one on each x chromosome) must have a change or mutation whereas in a person with one x chromosome (most males), only one copy of a gene must have a mutation. a female with a mutation in one copy of a gene on the x chromosome is said to be a “carrier” for an x-linked condition. a male with a mutation in a gene on the x chromosome is typically affected with the condition. because females have two copies of the x chromosome and males have only one x chromosome, x-linked recessive diseases are more common among males than females. however, x-linked recessive diseases can occur in both males and females.

for x-linked recessive disorders, an unaffected carrier mother who has a mutation in a gene on the x chromosome can transmit either the x chromosome with this mutation or a “normal” x chromosome to her children. if the father is unaffected, none of her daughters will be affected and all of her daughters will be unaffected—since they will inherit at least one normal x chromosome from their father. however, each daughter will have a 50% chance of being an unaffected carrier like her mother and a 50% chance of both x chromosomes being normal.