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Tutor profile: Sumanya K.

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Sumanya K.
Experienced Science and Math Tutor of 3 years
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Questions

Subject: Basic Math

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Question:

The length of two sides of a rectangle are 2 cm and 4 cm. What is the perimeter of the rectangle?

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Sumanya K.
Answer:

Though only two side lengths of the rectangle are given, all four are known. In a rectangle, opposite sides have the same lengths. Therefore, the side opposite to the 4 cm side is also 4 cm, and the side opposite to the 2 cm side is also 2 cm. The side lengths are: 4 cm, 4 cm, 2 cm, and 2 cm. To find the perimeter of the rectangle, we need to add all of the sides together. 4 cm + 4 cm + 2 cm + 2 cm = 12 cm. The perimeter of the rectangle is 12 cm.

Subject: Biochemistry

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Question:

What is the difference between competitive, noncompetitive, and uncompetitive inhibition for enzymes?

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Sumanya K.
Answer:

Enzymes have an active site, in which the substrate binds and is converted to product, and other binding sites. The difference between the three types of inhibition is based on where and how different inhibitors bind to the enzyme. In competitive inhibition, an inhibitor binds to the active site of the enzyme, competing with the substrate. Competitive inhibition can be overcome by increase substrate concentration, so the substrate eventually crowds out the inhibitor and binds to the enzyme instead of the inhibitor. However, competitive inhibition increases the Km of the enzyme. Km is defined by the substrate concentration that allows the enzyme to reach half of its maximum velocity (the maximum rate at which the enzyme can convert substrate to product). It is a measure of substrate-enzyme affinity; a lower Km value indicates a higher affinity between the enzyme and substrate. Competitive inhibition increases Km because more substrate than normal is necessary to overcome the competitive inhibitor and bind to the enzyme. However, the maximum velocity (Vmax) of the enzyme is not affected. In noncompetitive inhibition, an inhibitor binds to another site on the enzyme that is not the active site. This changes the structure of the enzyme, affecting its ability to convert substrate to product effectively. Since the substrate is not competing with the inhibitor to bind to the active site, the Km is not affected. However, since the enzyme's ability to convert substrate to product is changed, the Vmax of the enzyme is decreased in noncompetitive inhibition. In uncompetitive inhibition, the inhibitor binds to the enzyme-substrate complex (enzyme and substrate are bound). The inhibitor prevents the enzyme from completing the substrate-to-product transition, and does not allow the complex to disband. Since the substrate is bound to the enzyme for long periods of time, it can be said that the substrate and enzyme have a higher affinity for each other, decreasing the Km. Since the substrate cannot be converted to product when the inhibitor is bound, the Vmax of the enzyme is decreased. To summarize: Competitive Inhibition: Inhibitor binds to the active site, Km increases, Vmax is unaffected Noncompetitive Inhibition: Inhibitor binds to a different site, Km is unaffected, Vmax decreases Uncompetitive Inhibition: Inhibitor binds to the enzyme-substrate complex, Km decreases, Vmax decreases

Subject: Biology

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Question:

You discover a single gene with two alleles that controls a mouse's color. The brown phenotype (allele B) is dominant, and the white phenotype (b) is recessive. You have two brown male mice and one white female mouse, but you don't know which mice have which genotype. How can you figure out the genotype of each mouse using a genetic cross?

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Sumanya K.
Answer:

Since white phenotype is recessive, there must be two copies of its allele in the genotype for it to show up phenotypically. Therefore, the genotype of the white female mouse must be: bb. However, since the brown phenotype is dominant, the genotypes of the two male mice could be: BB or Bb. A genetic cross must be done with the female mouse to determine their genotypes. Let's start by crossing the white female mice with one of the brown male mice. Their offspring will inherit one allele from each parent. We know the female's genotype is bb. If the male's genotype is BB, the offspring's genotypes could be: Bb, Bb, Bb, or Bb (this can be determined from a Punnett square). All of the potential offspring's genotypes contain the dominant B gene, indicating that they will have the brown phenotype, since only one allele is required for a dominant phenotype to be expressed. Therefore, if all offspring from the cross are brown, the brown male's genotype must be BB. If the male's genotype os Bb, the offspring's genotypes could be: Bb, Bb, bb, or bb (this can be determined from a Punnett square). In this case, half of the potential genotypes contain the dominant B allele, while the other half contain two copies of the recessive b allele. This means that half of the potential phenotypes are brown because they contain at least one copy of the dominant allele, while the other half of the potential phenotypes are white because they contain two copies of the recessive allele. Therefore, if any of the offspring from the cross are white, the brown male's genotype must be Bb. This is because if the genotype was BB, all of the offspring from the cross would be brown and none would be white, as mentioned earlier. This approach can be repeated when crossing the white female with the other brown male to determine its genotype.

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