Question 1

For each protein,there is a pH at which negative and positive charges are equal.This pH is referred to as the ___________.

Accepted Answer

The term used to describe the pH at which a protein has a net charge of zero is the isoelectric point (pI). It is also known as the isoelectric pH or the isoelectric focusing point. At the isoelectric point, the protein has an equal number of positively charged Lysine and Arginine residues and negatively charged Aspartic acid and Glutamic acid residues.

Question 2

How are fragments already in a mass spectrometer further fragmented if this becomes necessary?

Accepted Answer

Fragments in a mass spectrometer can be further fragmented through collision with an inert gas, such as argon or helium. This process, known as collision-induced dissociation (CID) or collision-induced fragmentation (CIF), can break apart larger peptides into smaller fragments for analysis. Alternatively, pepsin and trypsin are proteolytic enzymes used for protein digestion and are not typically used to further fragment peptides in a mass spectrometer. Nitrogen and oxygen gases are not commonly used for fragmentation in mass spectrometry.

Question 3

Why does ion exchange chromatography depend on the pH of the medium?

Accepted Answer

Ion exchange chromatography separates molecules based on their net surface charge. The charged groups on the molecules interact with oppositely charged ions immobilized on the column matrix. The net charge on each molecule is determined by the pH of the surrounding medium because the charge of ionizable groups such as amino (NH3+) and carboxylate (COO-) groups on amino acids, or the phosphate groups on nucleic acids, is pH-dependent. Therefore, the pH of the medium will control the charge of the molecules and thus their interactions with the column matrix.

Question 4

Which of the following is most likely to elute first from a gel filtration column if each of them is essentially globular?

Accepted Answer

The answer of Which of the following is most likely...

Question 5

Why do phenol and buffered saline separate into two phases when one centrifuges the mixture during the DNA extraction procedure?

Accepted Answer

Phenol and buffered saline are immiscible, which means they cannot mix completely to form a homogeneous solution. This leads to the formation of two separate phases when centrifuged, with phenol forming the upper phase and buffered saline forming the lower phase.

Question 6

When making a measurement in a spectrophotometer if you are illuminating a specimen with ultraviolet light,into what kind of container should it be placed?

Accepted Answer

Ultraviolet light has a shorter wavelength and a higher energy compared to visible light. For this reason, it can be absorbed or scattered by most materials, including glass and plastic. Therefore, a special container made of quartz, which is transparent to UV light, should be used to hold the specimen. This container should also have flat sides to allow for accurate measurement of light absorption or transmission.

Question 7

What is the name of the procedure in which proteins separated on a polyacrylamide gel are transferred with the application of a current to a nitrocellulose filter placed against the gel and subsequently identified by their interaction with specific antibodies?

Accepted Answer

This procedure is known as a Western blot. The other options (A, B, and C) refer to similar procedures for detecting DNA or RNA fragments on a gel, but do not involve the transfer and identification of proteins. Option E is not a valid term for any scientific procedure.

Question 8

The amount of light passing through a solution unabsorbed or _________ is measured by ________ on the other side of the cuvette.

Accepted Answer

The amount of light passing through a solution unabsorbed, i.e. transmitted light, is measured by photocells on the other side of the cuvette. Therefore, option C is the correct choice. Thermocouplers and charged-coupling devices are not commonly used to measure the amount of transmitted light, and absorbed light is not directly measured in this context.

Question 9

Electron cryomicroscopy can be used to cause membrane proteins to become closely packed at very low temperatures into two-dimensional crystalline arrays within the plane of a membrane.The structures of such proteins are then determined from combined,high-resolution electron microscope images of many different protein molecules taken at various angles.This process is called __________.

Accepted Answer

The answer of Electron cryomicroscopy can be used to cause...

Question 10

Which technique could be used for molecular weight (mass)determinations in proteins?

Accepted Answer

SDS-PAGE (polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate) is a commonly used technique for determining the molecular weight of proteins. The technique separates proteins based on their size, with smaller proteins migrating more quickly through the gel than larger ones. When combined with a protein ladder of known sizes, the molecular weight of a protein can be estimated based on its migration distance relative to the ladder.

Question 11

What is usually the first step in the purification of DNA?

Accepted Answer

The first step in DNA purification is usually the homogenization of cells and isolation of nuclei, as DNA is located within the nucleus of cells.

Question 12

What happens to the solution viscosity after DNA has been released into it by treating nuclei with a buffered saline solution containing SDS?

Accepted Answer

The release of DNA into the solution increases the viscosity markedly because DNA is a large, highly charged molecule that can entangle and increase the resistance to flow.

Question 13

Non-denaturing gel electrophoresis (PAGE)has the disadvantage that it ____________.

Accepted Answer

Non-denaturing PAGE separates proteins based on multiple properties, including charge, size, and shape, which can complicate the interpretation of results.

Question 14

To what are the reflection positions and intensities on the photographic plate of an X-ray crystallography experiment related?

Accepted Answer

The reflection positions and intensities on the photographic plate of an X-ray crystallography experiment are related to the electron densities of proteins. X-rays in crystallography scatter off of the electrons in the protein molecules, and the resulting diffraction pattern provides information about the electron density distribution in the crystal. This electron density data is then used to determine the protein's atomic structure through a process known as phase determination.

Question 15

In which chromatographic method are the charges of the proteins in question the basis of their purification?

Accepted Answer

Ion exchange chromatography separates proteins based on their net charge. Proteins with a net positive charge will bind to negatively charged resins, while proteins with a net negative charge will bind to positively charged resins. This allows for the selective purification of proteins based on their charge characteristics.

Question 16

What is the significance of the varying porosity of gel filtration media?

Accepted Answer

The varying porosity of gel filtration media allows proteins or nucleic acids to diffuse in and out of the beads differentially, based on their size and shape. This allows for separation and purification of molecules based on their molecular weight.

Question 17

Most proteins have an isoelectric point ________ 7.

Accepted Answer

The isoelectric point (pI) of a protein is the pH at which the protein has no net charge, i.e., the protein is neutral. For most proteins, this point is below pH 7, so the best choice is C, "below." However, there are exceptions to this, including some basic proteins that have a pI above 7, and some acidic proteins that have a pI far below 7.

Question 18

As solvent passes through a column,it is collected as it drips out of the bottom of the column into a series of tubes; each of these tubes is called a(n)____________.

Accepted Answer

The process described in the question is that of chromatography, where a mixture of substances is separated using a solvent and a column filled with a stationary phase. As the solvent passes through the column, different substances interact differently with the stationary phase and are separated accordingly. The solvent that has passed through the column and contains a purified substance is known as a fraction. Therefore, the tubes collecting the fractions are also known as fractions or fraction collectors. Choice D is the only option that accurately describes a collection of fractions.

Question 19

In the yeast two-hybrid assay,the ____domain and _____ domain of a transcription factor are fused to two different proteins.If the proteins interact with each other,the transcription factor can allow reporter gene expression.

Accepted Answer

In the yeast two-hybrid assay, the DNA binding domain and activation domain of a transcription factor are fused to two different proteins. If the proteins interact with each other, the transcription factor can allow reporter gene expression. Therefore, the correct choice is A, which includes DNA binding and activation.

Question 20

What is the term for a variety of techniques in which a mixture of dissolved components is fractionated as it moves through a porous matrix?

Accepted Answer

Chromatography is the general term for a variety of techniques that use a matrix (such as chromatography paper or a column packed with silica gel or other porous material) to separate the components of a mixture based on their interactions with the matrix and with a mobile solvent phase. Different types of chromatography, including size-exclusion chromatography, ion-exchange chromatography, and affinity chromatography, use various mechanisms to separate different components.

Quiz 18: Techniques in Cell and Molecular Biology