Do You Know USMLE Step
For those of you who love all things biology and living organisms especially anatomy and how stuff works in the human body then this is the quiz for you. If you are prepared to test your knowledge try it out.
- 1.A sample of human DNA is subjected to increasing temperature until the major fraction exhibits optical density changes due to disruption of its helix (melting or denaturation). A smaller fraction is atypical in that it requires a much higher temperature for melting. This smaller, atypical fraction of DNA must contain a higher content of
- A.
Adenine plus cytosine
- B.
Cytosine plus guanine
- C.
Adenine plus thymine
- D.
Cytosine plus thymine
- E.
Adenine plus guanine
- 2.A newborn baby has a sibling with sickle cell anemia (141900) and is at risk for the disease. Which of the following is the appropriate diagnostic test for sickle cell anemia in this baby?
- A.
DNA amplification
- B.
Hemoglobin antibodies
- C.
DNA restriction
- D.
Red cell counting
- E.
DNA fingerprinting
- 3.. A farming couple in Northern Michigan consult their physician about severe skin rashes and ulcers noted over the past year. They also have lost many cattle over the past year, and claim that their cattle feed changed in consistency and smell about 1 year ago. Chemical analysis of the feed shows high concentrations of polychlorinated biphenyls, a fertilizer related to known carcinogens. The physician sends the chemical to a laboratory for carcinogen testing, which is performed initially and rapidly by
- A.
Inoculation of the chemical into nude mice
- B.
Incubation of mutant bacteria with the chemical to measure the rate of reverse or "back" mutations
- C.
Incubation with stimulated white blood cells to measure the impact on DNA replication
- D.
Computer modeling based on the structures of related carcinogens
- E.
Incubation with mammalian cell cultures to measure the rates of malignant transformation
- 4.Patients with hereditary nonpolyposis colon cancer [HNPCC (114500)] have genes with microsatellite instability, that is, many regions containing abnormal, small loops of unpaired DNA. This is a result of a mutation affecting
- A.
Mismatch repair
- B.
Chain break repair
- C.
Base excision repair
- D.
Depurination repair
- E.
Nucleotide excision repair
- 5.A culture of bacteria not resistant to tetracycline develops an infection from a virus that is derived from the lysis of tetracycline-resistant bacteria. Most of the bacterial progeny of the original culture is found to have become resistant to tetracycline. What phenomenon has occurred?
- A.
Conjugation
- B.
Colinearity
- C.
Recombination
- D.
Transformation
- E.
Transduction
- 6.Which of the following molecules is found in a nucleoside?
- A.
A pyrophosphate group
- B.
A 1' base linked to a pentose sugar
- C.
A 5'-phosphate group linked to a pentose sugar
- D.
A 3'-phosphate group linked to a pentose sugar
- E.
A terminal triphosphate
- 7.Which of the following enzymes can be described as a DNA-dependent RNA polymerase?
- A.
DNA ligase
- B.
Primase
- C.
DNA polymerase III
- D.
DNA polymerase I
- E.
Reverse transcriptase
- 8.. The DNA sequence M, shown below, is the sense strand from a coding region known to be a mutational "hot spot" for a gene. It encodes amino acids 21 to 25. Given the genetic and amino acid codes CCC = proline (P), GCC = alanine (A), TTC = phenylalanine (F), and TAG = stop codon, which of the following sequences is a frame-shift mutation that causes termination of the encoded protein? M 5'-CCC-CCT-AGG-TTC-AGG-3'
- A.
-CCA-CCT-AGG-TTC-AGG-
- B.
-GCC-CCT-AGG-TTC-AGG-
- C.
-CCA-CCC-TAG-GTT-CAG-
- D.
-CCC-CTA-GGT-TCA-GG—
- E.
-CCC-CCT-AGG-AGG——
- 9.The hypothetical "stimulin" gene contains two exons that encode a protein of 100 amino acids. They are separated by an intron of 100 bp beginning after the codon for amino acid 10. Stimulin messenger RNA (mRNA) has 5' and 3' untranslated regions of 70 and 30 nucleotides, respectively. A complementary DNA (cDNA) made from mature stimulin RNA would have which of the following sizes?
- A.
500 bp
- B.
400 bp
- C.
300 bp
- D.
100 bp
- E.
70 bp
- 10.The hypothetical "stimulin" gene with two exons encoding a protein of 100 amino acids is found to have abnormal expression in cell culture. Which of the following mutations would produce a 500-bp stimulin mRNA and a 133–amino acid peptide that reacts with antibodies to stimulin protein?
- A.
Splice junction mutation preventing RNA splicing
- B.
Frame-shift mutation in codon #2
- C.
Silent point mutation in the third nucleotide of codon #50
- D.
Nonsense mutation at codon #2
- E.
Deletion of exon 1
- 11.Template-directed RNA synthesis occurs in which of the following?
- A.
Point mutation
- B.
Triplet repeat expansion
- C.
Initiation of the polymerase chain reaction
- D.
Expression of oncogenes
- E.
Repair of thymine dimmers
- 12.Which of the following most correctly describes mammalian messenger RNAs?
- A.
They are usually transcribed from both DNA strands
- B.
They are normally double-stranded
- C.
Their content of uridine equals their content of adenine
- D.
They have an overall negative charge at neutral pH
- E.
Their ratio of ribose to purine bases equals 1
- 13.The western blot uses what type of probe?
- A.
Antibody
- B.
MRNA
- C.
Products of polymerase chain reaction (PCR)
- D.
TRNA
- E.
Mutant and normal oligonucleotides
- 14.How many high-energy phosphate-bond equivalents are utilized in the process of activation of amino acids for protein synthesis?
- A.
Zero
- B.
One
- C.
Two
- D.
Three
- E.
Four
- 15.An immigrant family from rural Mexico brings their 3-month-old child to the emergency room because of whistling inspiration (stridor) and high fever. The child's physician is perplexed because the throat examination shows a gray membrane almost occluding the larynx. A senior physician recognizes diphtheria, now rare in immunized populations. The child is intubated, antitoxin is administered, and antibiotic therapy is initiated. Diphtheria toxin is often lethal in unimmunized persons because it
- A.
Inhibits initiation of protein synthesis by preventing the binding of GTP to the 40S ribosomal subunit
- B.
Binds to the signal recognition particle receptor on the cytoplasmic face of the endoplasmic reticulum receptor
- C.
Shuts off signal peptidase
- D.
Blocks elongation of proteins by inactivating elongation factor 2 (EF-2, or translocase)
- E.
Causes deletions of amino acid by speeding up the movement of peptidyl-tRNA from the A site to the P site
- 16.Methionyl–transfer (t) RNA is used for initiation of protein synthesis by which of the following?
- A.
Chloroplast ribosomes
- B.
Eukaryotic mitochondrial ribosomes
- C.
Eukaryotic cytoplasmic ribosomes
- D.
Bacterial ribosomes
- E.
Bacterial cytoplasm
- 17.Which of the following is required for certain types of eukaryotic protein synthesis but not for prokaryotic protein synthesis?
- A.
Ribosomal RNA
- B.
Messenger RNA
- C.
Signal recognition particle
- D.
Peptidyl transferase
- E.
GTP
- 18.A major obstacle to gene therapy involves the difficulty of homologous gene replacement. Which of the following strategies addresses this issue?
- A.
A recombinant vector contains complementary DNA sequences that will facilitate site-specific recombination
- B.
A recombinant vector expresses antisense nucleotides that will hybridize with the targeted mRNA
- C.
A recombinant vector replaces inessential viral genes with a functional human gene
- D.
A recombinant vector transfects patient cells, which are returned to the patient
- E.
A recombinant vector contains DNA sequences that target its expressed protein to lysosomes
- 19.A family in which several individuals have arthritis and detached retina is diagnosed with Stickler syndrome. The locus for Stickler syndrome has been mapped near that for type II collagen on chromosome 12, and mutations in the COL2A1 gene have been described in Stickler syndrome. The family became interested in molecular diagnosis to distinguish normal from mildly affected individuals. Which of the following results would be expected in an individual with a promoter mutation at one COL2A1 gene locus?
- A.
Western blotting detects no type II collagen chains
- B.
Southern blotting using intronic restriction sites yields normal restriction fragment sizes
- C.
Reverse transcriptase–polymerase chain reaction (RT-PCR) detects one-half normal amounts of COL2A1 mRNA in affected individuals
- D.
Fluorescent in situ hybridization (FISH) analysis using a COL2A1 probe detects signals on only one chromosome 12
- E.
DNA sequencing reveals a single nucleotide difference between homologous COL2A1 exons
- 20.Gyrate atrophy (258870) is a rare autosomal recessive genetic disorder caused by a deficiency of ornithine aminotransferase. Affected individuals experience progressive chorioretinal degeneration. The gene for ornithine aminotransferase has been cloned, its structure has been determined, and mutations in affected individuals have been extensively studied. Which of the mutations listed below best fits with test results showing normal Southern blots with probes from all ornithine aminotransferase exons but absent enzymatic activity?
- A.
Duplication of entire gene
- B.
Two-kb deletion in coding region of gene
- C.
Two-kb insertion in coding region of gene
- D.
Deletion of entire gene
- E.
Missense mutation
- 21.. Hurler's syndrome (252800) is caused by a deficiency of L-iduronidase, an enzyme normally expressed in most human cell types. It was demonstrated by Neufeld that exogenous L-iduronidase could be taken up by deficient cells via a targeting signal that directed the enzyme to its normal lysosomal location. Which of the following therapeutic strategies would be the most realistic and efficient mode of therapy?
- A.
Germ-line gene therapy
- B.
Heterologous bone marrow transplant
- C.
Infection with a disabled adenovirus vector that carries the L-iduronidase gene
- D.
Injection with L-iduronidase purified from human liver
- E.
Autologous bone marrow transplant after transfection with a virus carrying the L-iduronidase gene
- 22.Which of the following mutations is most likely to be lethal?
- A.
Substitution of adenine for cytosine
- B.
Substitution of cytosine for guanine
- C.
Substitution of methylcytosine for cytosine
- D.
Deletion of three nucleotides
- E.
Insertion of one nucleotide
- 23.. In the following partial sequence of mRNA, a mutation of the template DNA results in a change in codon 91 to UAA. What type of mutation is it? 88 89 90 91 92 93 94 GUC GAC CAG UAG GGC UAA CCG
- A.
Missense
- B.
Silent
- C.
Nonsense
- D.
Suppressor
- E.
Frame shift
- 24.Which one of the following causes a frame-shift mutation?
- A.
Transition
- B.
Transversion
- C.
Deletion
- D.
Substitution of purine for pyrimidine
- E.
Substitution of pyrimidine for purine
- 25.Which of the following regulators are said to act in "cis"?
- A.
The lac repressor and mammalian transcription factors
- B.
The lac repressor and the lac operator
- C.
The lac operator and mammalian enhancers
- D.
The lac operator and mammalian transcription factors
- E.
Mammalian transcription factors and enhancers
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