Cell Bio HW Questions

While small, uncharged molecules can diffuse through the hydrophobic core of a phospholipid bilayer, larger polar molecules such as glucose must enter cells by binding to

carrier proteins that facilitate the passage of specific molecules across membranes.

Which of the following lipids are distinctly enriched in lipid rafts?

cholesterol or sphingloplipds

A disulfide bond is formed between _______ residues.

cysteine

Enzymes act by

decreasing the activation energy

In the fluid mosaic model of biological membrane structure, transmembrane proteins are

embedded nearly randomly in the lipid bilayer

Cholesterol, a membrane lipid in animals, has a chemical structure similar to

estradiol

Which statement about the glucose transporter is false?

flow through it is unidirectional

Phospholipids consist of a 3-carbon core to which two fatty acids and a phosphate group are linked. The most common 3-carbon core is

glycerol

Two examples of membrane lipids that are present in small amounts are

glycolipids and phosphatidylinositol

The two strands of DNA in the double helix are held together by

hydrogen bonds between the bases of each strand.

Lipids with unsaturated fatty acids

increase the fluidity of membranes

Introducing a double bond into a fatty acid puts a(n) _______ into the conformation of the molecule.

kink

Membrane proteins are able to move

laterally within the plane of a membrane.

Which of the following classes of amino acids is buried within the folded structure of the protein?

non polar

The ingestion of large particles by cells is a form of endocytosis known as

phagocytosis.

Proteins must have more than one _______ to have a quaternary structure.

polypeptide chain

Which class of molecule accelerates transport across biological membranes?

proteins

Which of the following is not involved in receptor-mediated endocytosis?

pseudopodia

LDL uptake by cells is one of the functions of

receptor-mediated endocytosis

The enzyme that synthesizes DNA using RNA templates is called

reverse transcriptase

Although Na+ is smaller than K+, its passage through the K+ channel is blocked by the _______ filter.

selectivity

A PCR reaction does not require

sodium dodecyl sulfate.

Which molecule serves as an adaptor between amino acids and mRNA during translation?

tRNA

The α (alpha) helix is an example of which level of protein structure?

tertiary

Which statement about the relative concentrations of Na+ and K+ inside and outside of a typical mammalian cell is true?

the concentration of Na+ outside the cell is 30 times higher then inside cell, and the concentration K+ inside the cells is 10 to 30 times higher then outside the cell

About 50% of the mass of most biological membranes consists of lipids and about 50% consists of proteins. Therefore,

the membranes contain more molecules of lipid than of protein.

The process by which proteins are made from RNA templates is called

translation.

Passive transport across a membrane refers to

transport in the energetically favorable direction

Which of the bases is not found in DNA?

uracil

The most abundant molecule in cells is

water

A feature common to most transmembrane proteins is a(n)

α-helical region of about 20 to 25 hydrophobic amino acids.

Enzymes affect the transition state of a chemical reaction by

- providing a surface on which reactions converting substrate to product can occur more rapidly. - binding to substrate(s). - altering the conformation of substrate(s) to approach that of the transition state.

How many amino acids are commonly incorporated into proteins?

20

A restriction enzyme with a four-base recognition site would cleave DNA with a statistical frequency of once every

256 base pairs

How many molecules would be produced from two molecules of DNA following five rounds of PCR amplification?

64

The genetic code is said to be degenerate. What does "degenerate" mean in this context?

A single amino acid may be coded for by more than one codon. (degenerate = redundancy)

Why can't a digest of human genomic DNA be resolved by gel electrophoresis?

A typical digest of the human genome produces too many fragments to resolve

Imagine you are a student trying to decode the genetic code. An artificial mRNA molecule consisting of poly-CA (5ʹ... CACACACACACACAC..3ʹ) yields a polypeptide consisting solely of histidines and threonine, and another artificial mRNA consisting of poly-CAA (5ʹ...CAACAACAACAACAA...3ʹ) yields three different polypeptides: polythreonine, polyglutamine, and polyasparagine. On the basis of this information, which codons can you assign to which amino acids?

ACA - threonine CAC - histidine The others you cannot know for sure with this information.

The characterization of restriction endonucleases was a key step in the development of recombinant DNA technology. What is the function of these enzymes?

Cleavage of DNA at specific sequences

A cDNA molecule is

DNA synthesized from an RNA template using the enzyme reverse transcriptase.

Which statement about ABC transporters is false?

Each member of the family is a Cl- channel.

EcoRI and NotI are two DNA restriction enzymes that recognize the specific sequences GAATTC and GCGGCCGC, respectively. Which of these restriction enzymes would cleave DNA most frequently, and why?

EcoRI. There are only six base pairs in the specific sequences recognized by EcoRI, whereas NotI would occur less frequently—only once in 48 base pairs, based on four possible nucleotides in a specific sequence of eight.

Cystic fibrosis (CF) is the most common inherited disease of Caucasians. It is due to mutations in an ABC transporter, the cystic fibrosis transmembrane conductance regulator (CFTR), which is a Cl- and bicarbonate channel. Why have efforts at gene therapy to correct CF concentrated on the lungs when CF affects other tissues as well?

Even though CF affects other tissues, the immediate cause of most CF-caused deaths is bacterial infections in the lung due to mucus accumulation. CF is caused by a mutation in the CFTR gene which produces the ABC transporter protein CFTR. As a chloride channel, CFTR maintains a balance of water and salt inside of the cell. When the transporter is mutated, the change in salt and water balance causes dehydrations of mucus and a loss of salt via sweat. In terms of gene therapy, introduction of a wild-type, properly function CFTR to the lung is the most biologically relevant approach to treatment.

The drug ouabain inhibits the plasma membrane Na+-K+ pump. Why does it also inhibit glucose transport across the intestinal epithelial layer?

Glucose transport across the intestinal epithelial layer requires two Na+ ions to enter the cell and transport the glucose with it, in order to carry the glucose against its concentration gradient and maintain energetic favorability. So when the Na+-K+ pump is inhibited, glucose cannot be carried because it will not diffuse in an energetically favorable process.

In experiments using polymerase chain reactions (PCR), it is often more difficult to amplify through regions of DNA that are high in GC content versus those regions that are either lower in GC content or are AT-rich. Based on your knowledge of DNA structure, explain why.

In base pairing between strands of DNA, there are three hydrogen bonds between each G-C base pair and only two hydrogen bonds between A-T base pairs. In PCR, heat is used to melt the hydrogen bonds between strands of DNA. The energy required to melt regions of DNA high in GC are significantly higher than those that are either less so or are AT-rich.

The resting plasma membrane potential as described by the Nernst equation is the sum of the movement of many ions. However, in practice, the movement of _______, a single ion, is the major component determining membrane potential.

K+

Which of the following will not solubilize a typical peripheral membrane protein?

Mildly hypertonic saline

Transport of glucose into the intestinal epithelium is driven by ion gradients established by the

Na+-K+ pump.

Which character is not necessary in a plasmid vector to facilitate successful cloning?

Restriction site

Why are E. coli DNA polymerases not used in the polymerase chain reaction (PCR)?

Taq DNA polymerase must be used because it is thermally stable. To use E. coli DNA polymerase at such high temperatures would make it non functional.

Which statement correctly represents the distinction between the terms "genotype" and "phenotype"?

The genotype is the genetic composition of an organism, and the phenotype is its physical appearance.

Detergents are used to solubilize membranes because they are dual molecules possessing both hydrophobic and hydrophilic portions. Why are they also used as cleaning reagents (to clean, for example, clothes and dishes)?

The hydrophobic part of the detergent molecule binds to the hydrophobic substances that you are trying to clean (ex. oil, grease, dirt) and the hydrophilic part of the detergent molecule binds to the water that you are using to clean. In this way, we can use detergent to take off the hydrophobic substance and it will then be washed away with water.

According to the "central dogma" of molecular biology, information flows from DNA to RNA to protein. In its original version, the dogma stated that information does not flow in the reverse direction. To what extent is this true?

The reproductive cycle of retroviruses includes a step, catalyzed by reverse transcriptase, in which a DNA copy of the viral RNA genome is synthesized. DNA made from an RNA template contradicts the original version of the dogma. However, it has not yet been found that any nucleic acid can be made using a protein template, so it appears that information does not flow backward from proteins.

If adenine makes up 22% of the bases in a given double-stranded DNA molecule, what percentage is made up of guanines?

Then Thymine is also 22%. And the other 56% is Guanine and Cystine, so 28% are guanine.

Which statement about ion channels is true?

They are opened either by the binding of ligands or by changes in electric potential across the membrane.

How does water enter a cell from the extracellular space?

Water enters a cell from extracellular space and diffuses across the membrane via the process of osmosis. In the direction of their concentration gradient, water molecules diffuse into the more hypertonic solution (whether that be in or out of the cell) the molecules diffuse across the membrane through simple diffusion and lessens osmotic pressure in the more hypertonic region.

Why is dissociation of ligands from their receptors in endosomes an important feature of receptor-mediated endocytosis?

When ligands dissociate from their receptors they allow them to be reused as the receptors return to the plasma membrane.

The pH of endosomes and lysosomes is

acidic

Active transport differs from facilitated diffusion in that

active transport involves the transport of molecules up their concentration gradient.

Mutations in the internalization signal of endocytic receptors prevent their interaction with

adaptor proteins