MCAT Biology : Ribosomes and Cytoskeleton

Study concepts, example questions & explanations for MCAT Biology

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Example Question #222 : Cell Biology, Molecular Biology, And Genetics

Human chromosomes are divided into two arms, a long q arm and a short p arm.  A karyotype is the organization of a human cell’s total genetic complement.  A typical karyotype is generated by ordering chromosome 1 to chromosome 23 in order of decreasing size. 

When viewing a karyotype, it can often become apparent that changes in chromosome number, arrangement, or structure are present.  Among the most common genetic changes are Robertsonian translocations, involving transposition of chromosomal material between long arms of certain chromosomes to form one derivative chromosome.  Chromosomes 14 and 21, for example, often undergo a Robertsonian translocation, as below.

1

A karyotype of this individual for chromosomes 14 and 21 would thus appear as follows:

Pic2

Though an individual with aberrations such as a Robertsonian translocation may be phenotypically normal, they can generate gametes through meiosis that have atypical organizations of chromosomes, resulting in recurrent fetal abnormalities or miscarriages.

During mitosis and meiosis chromosomes are separated from homologous pairs at the metaphase plate. A disorder in which of the following proteins could be expected to produce an aberration in cell division?

Possible Answers:

Myosin

Keratin

Tubulin

Collagen

Myosin AND tubulin

Correct answer:

Tubulin

Explanation:

Cell division and the corresponding separation of chromosomes occurs via the spindle apparatus, made of microtubules. Therefore, tubulin abnormalities can lead to abnormalities of division. Myosin is a tempting choice because myosin is involved in movement, but only in muscle tissue. Chromosomes do move during cell division, but without myosin involvement, and instead over tubulin pathways.

Example Question #223 : Cell Biology, Molecular Biology, And Genetics

Sexually transmitted diseases are a common problem among young people in the United States. One of the more common diseases is caused by the bacterium Neisseria gonorrhoeae, which leads to inflammation and purulent discharge in the male and female reproductive tracts.

The bacterium has a number of systems to evade host defenses. Upon infection, it uses pili to adhere to host epithelium. The bacterium also uses an enzyme, gonococcal sialyltransferase, to transfer a sialyic acid residue to a gonococcal surface lipooligosaccharide (LOS). A depiction of this can be seen in Figure 1. The sialyic acid residue mimics the protective capsule found on other bacterial species.

Once infection is established, Neisseria preferentially infects columnar epithelial cells in the female reproductive tract, and leads to a loss of cilia on these cells. Damage to the reproductive tract can result in pelvic inflammatory disease, which can complicate pregnancies later in the life of the woman.

Untitled

A scientist discovers that Neisseria uses flagella to move between host cells. Which of the following is true of the flagella?

Possible Answers:

They are different from cilia in terms of internal structure

They are also used in the female reproductive tract epithelium to move eggs toward the uterus

They have a 9+2 microtubule ultrastructure

They are more common in eukaryotes than in prokaryotes

They cannot be an antigen for host immune response

Correct answer:

They have a 9+2 microtubule ultrastructure

Explanation:

Flagella and cilia share a 9+2 ultrastructure, where there is a ring of 9 doublets of microtubules surrounding a central doublet, and connects via dynein arms.

Example Question #224 : Cell Biology, Molecular Biology, And Genetics

Most scientists subscribe to the theory of endosymbiosis to explain the presence of mitochondria in eukaryotic cells. According to the theory of endosymbiosis, early pre-eukaryotic cells phagocytosed free living prokaryotes, but failed to digest them. As a result, these prokaryotes remained in residence in the pre-eukaryotes, and continued to generate energy. The host cells were able to use this energy to gain a selective advantage over their competitors, and eventually the energy-producing prokaryotes became mitochondria.

In many ways, mitochondria are different from other cellular organelles, and these differences puzzled scientists for many years. The theory of endosymbiosis concisely explains a number of these observations about mitochondria. Perhaps most of all, the theory explains why aerobic metabolism is entirely limited to this one organelle, while other kinds of metabolism are more distributed in the cellular cytosol.

Phagocytosis of the sort that was involved in endosymbiosis makes use of microtubules to orient the cell appropriately. Which of the following processes also directly uses microtubules?

Possible Answers:

DNA replication

Transcription

Golgi modification of proteins

Mitosis

Translation

Correct answer:

Mitosis

Explanation:

Mitosis makes use of microtubules to organize and move DNA into appropriate daughter cells. These specific structures are known as mitotic spindles.

The other processes are closely linked to protein, enzyme, or ribosome function and do not involve microtubules.

Example Question #225 : Cell Biology, Molecular Biology, And Genetics

Which of the following is true of microtubules?

Possible Answers:

Microtubules are responsible for cytoplasmic streaming

Microtubules are involved in phagocytosis

Actin forms a major component of microtubules

Microtubules are larger than microfilaments

Correct answer:

Microtubules are larger than microfilaments

Explanation:

Microtubules are a component of the cell cytoskeleton formed by polymers of tubulin protein. They are larger than microfilaments and make up the internal structures of cilia and flagella. Microtubules have a positive and negative end. The negative end of a microtubule attaches to a microtubule-organizing center (MTOC) within the cell, which then allows the microtubule to grow away from the MTOC at its positive end. 

Example Question #226 : Cell Biology, Molecular Biology, And Genetics

Which of the following is not a function of microtubules?

Possible Answers:

Component of sarcomere scaffold structure

Produce mitotic spindle

Component of cilia structure

Component of the cytoskeleton

Component of flagella structure

Correct answer:

Component of sarcomere scaffold structure

Explanation:

Microtubules are made of the tubulin protein and play integral roles in cell structure. They are prominent in the cytoskeleton and form the fundamental structures for cilia and flagella. The mitotic spindles are also comprised of microtubules, and are used to draw apart the sister chromatids of each chromosome during cell division.

Microtubules do not play a significant role in the structure or function of sarcomeres. Actin and myosin compose the main functional basis of the sarcomere, while titin and the Z disc proteins provide the sarcomere structure.

Example Question #227 : Cell Biology, Molecular Biology, And Genetics

What happens at the minus-end of actin filaments when the concentration of G-actin is above its critical concentration?

Possible Answers:

Monomers are lost from it.

Monomers bind GTP.

Monomers add on to it.

Monomers undergo dynamic instability.

Monomers are not lost from it or added on to it.

Correct answer:

Monomers add on to it.

Explanation:

Monomers are lost when concentration of G-actin is below its critical concentration. Monomers are gained when concentration of G-actin is above its critical concentration. If it is in between the critical concentrations, the actin filaments will undergo treadmilling, which is the addition of monomers on the (+) end and loss of monomers on the (–) end.

Example Question #228 : Cell Biology, Molecular Biology, And Genetics

Actin is the major protein that composes which part of the cytoskeleton?

Possible Answers:

Microtubules

Microfilaments

Intermediate filaments

Cilia

Correct answer:

Microfilaments

Explanation:

The cytoskeleton is important for cell support and movement. It is composed of microfilaments made of actin, microtubules made of tubulin, intermediate filaments that bear tension, and cilia/flagella which are made of microtubules (tubulin).

Example Question #229 : Cell Biology, Molecular Biology, And Genetics

Which of the following structures that promote cell motility generates motion by sliding actin microfilaments?

Possible Answers:

Microvilli

Cilia

All of these answers

Flagella

Correct answer:

Microvilli

Explanation:

The only choice that consists of actin microfilaments is microvilli. These motile structures are composed of cross-linked actin microfilaments. Cilia and flagella are composed of a "9+2" organization of microtubule doublets and singlets (nine doublets surrounding two singlets).

Example Question #230 : Cell Biology, Molecular Biology, And Genetics

Arp2/3 is a protein complex that helps nucleate branch points on __________ chains.

Possible Answers:

microtubule

actin microfilament

intermediate filament

glycogen

Correct answer:

actin microfilament

Explanation:

Arp2/3 is a large protein complex that is specifically responsible for aiding in the organization of the actin microfilament cytoskeleton. In particular, it helps nucleate branch points from already formed actin microfilaments. Arp2/3 is not involved in the branching of microtubules, intermediate filaments, or glycogen.

Example Question #1 : Ribosomes And Cytoskeleton

In which of the following structures do actin microfilaments play a crucial role?

I. Contractile ring formed during cytokinesis

II. Sarcomeres

III. Adherens junctions

IV. Eukaryotic flagella 

Possible Answers:

III and IV

I and II

I, II, and III

II, III, and IV

I, II, III, and IV

Correct answer:

I, II, and III

Explanation:

Eukaryotic flagella are primarily made up of microtubule doublets and singlets organized in a "9+2" manner (two singlets surrounded by nine doublets). Actin microfilaments are not present in flagella.

The contractile ring formed during cytokinesis consists of actin and myosin, and helps separate the two daughter cells to conclude mitosis. Sarcomeres consist of actin and myosin overlaps that are crucial to muscle contraction. Adherens junctions are specialized cell junctions that use the actin cytoskeleton to anchor adjacent cells.

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