Types of muscular dystrophy
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Types of muscular dystrophy
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[ID] => 553877
[post_author] => 12815
[post_date] => 2024-12-23 09:14:25
[post_date_gmt] => 2024-12-23 14:14:25
[post_content] => Practice Passage (Question 1-6)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Muscular dystrophy (MD) refers to a group of genetic disorders characterized by progressive muscle degeneration and weakness. It is caused by mutations in the genes responsible for the structure and function of muscles. MD can affect people of all ages, and its severity and progression vary widely.
Duchenne muscular dystrophy (DMD) is an X-linked recessive form of degenerative dystrophy usually diagnosed in early childhood (around ages 3 to 5). DMD is caused by the abnormal expression of dystrophin, a 500-kDa protein that helps to maintain the structural integrity of muscle fibers. In DMD patients, various deletions in the dystrophin gene lead to either a defective form of dystrophin or a complete lack of dystrophin. Scientists are able to identify sequences corresponding to the region of DNA that contains the deletion; they do this by using DNA probes made from the normal dystrophin sequence, which they hybridize with DNA from normal individuals and DMD patients. The deletions occur at different locations in each patient, suggesting that they occur de novo. The most severe and common deletion is located in the center of the locus. The sequence of the deletion in the coding strand has been determined to be 5’—GCCATAGAGCGA—3’.
Myotonic dystrophy, another common form of muscular dystrophy, usually presents in adulthood and results in progressive weakness and muscle wasting. Many people with this disorder suffer from myotonia, or prolonged muscle contractions, and are not able to relax their muscles after use. This dystrophy can affect voluntary skeletal muscle as well as involuntary cardiac and smooth muscle. There are two major types of myotonic dystrophy, which are referred to as type 1 (DM1) and type 2 (DM2). DM1 usually results from an increase in the number of CTG triplet repeats in the DMPK gene, which encodes for myotonic dystrophy protein kinase (DMPK), with a larger number of inserted repeats leading to more significant symptoms. DM2, on the other hand, results from an increase in the number of CCTG repeats in the intron of the CNBP gene, which codes for zinc finger protein 9 (ZNF9). Myotonic dystrophy can be difficult to diagnose as it presents similarly to other defects in the muscular system, and it is also challenging to treat due to its multi-systemic effects.
[post_title] => Types of muscular dystrophy
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[question] => What is a likely function of the ZNF9 protein?
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[answer] => 1
[description] => Reason for Correct Answer:
The passage states that ZNF9 is a “zinc finger protein.”
Zinc finger proteins are a family of proteins that contain zinc finger motifs, which are zinc-containing structural motifs involved in binding to nucleic acids.
The primary function of zinc finger proteins, including those containing CCHC-type zinc finger motifs like ZNF9, is to serve as transcription factors that bind to DNA or as RNA-binding proteins. They play a crucial role in regulating gene expression and RNA metabolism. When bound to DNA, they can either enhance or repress the transcription of specific genes. When bound to RNA, they can be involved in various aspects of RNA processing, such as splicing and stability.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Binding to RNA to mediate RNA metabolism and regulation
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[1] => Array
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[each_answer] => B. Catalysis of metabolic reactions in the mitochondria
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[2] => Array
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[each_answer] => C. Phosphorylation of downstream protein targets
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[3] => Array
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[each_answer] => D. Anchoring of dystrophin to the cell membrane
)
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[1] => Array
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[quiz_unique_key] => 1403770772
[question] => How many moles are present in one gram of purified dystrophin protein?
[value] => Array
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[answer] => 2
[description] => Reason for Correct Answer:
The passage states that dystrophin is a 500-kDa protein.
Daltons (Da) correspond to the molecular weight in grams per mole (g/mol).
To calculate the number of moles present in one gram of purified protein, use the following formula:
Plugging in the 1 gram of weight and the molecular weight of dystrophin (500 kDa = 500,000 Da = 500,000 g/mol):
You can also calculate this as 1 / (5×10⁵).
This would equal 0.2 x 10, or 2 x 10⁻⁶ moles.
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[answers] => Array
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[0] => Array
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[each_answer] => A. 2 x 10⁻³ moles
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[each_answer] => B. 2 x 10⁻⁶ moles
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[2] => Array
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[each_answer] => C. 5 x 10⁻³ moles
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[each_answer] => D. 5 x 10⁻⁶ moles
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[quiz_unique_key] => 1403770772
[question] => If a normal woman whose father had DMD married a normal man, what is the probability their two children will have DMD?
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[answer] => 1
[description] => Reason for Correct Answer:
The passage states that DMD is X-linked recessive.
If the woman’s father had DMD, then she inherited his abnormal X chromosome, as all daughters get their father’s X chromosome. The woman is therefore a carrier of the abnormal gene.
The woman will have a 50% chance of passing the abnormal X-linked gene to each child. However, only boys will have DMD, as the girls will get a normal X chromosome from their father.
Therefore, the chance of DMD for each child =
(chance of getting abnormal gene) x (change of being boy) = 0.5 x 0.5 = 0.25
or, 25%.
For two children, you must multiply their probabilities; therefore, the chance of both =
(chance of DMD in child 1) x (chance of DMD in child 2) = 0.25 x 0.25 = 0.0625
or, 6.25%.
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[answers] => Array
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[0] => Array
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[each_answer] => A. 6.25%
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[each_answer] => B. 12.5%
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[each_answer] => C. 25%
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[each_answer] => D. 50%
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[question] => Which type of mutation characterizes the most common form of DMD deletion?
I. Frameshift mutation
II. Loss-of-function mutation
III. Gain-of-function mutation
[value] => Array
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[answer] => 2
[description] => Reason for Correct Answer:
In gain-of-function mutations, the mutant form of the protein interferes with the function of the normal protein, usually resulting in a dominant phenotype.
Loss-of-function mutations render the mutated allele non-functional; since the wild-type allele can still function normally, this usually results in a recessive phenotype.
The passage describes deletions in the dystrophin gene as leading to “either a defective form of dystrophin or a lack of dystrophin completely”; in addition, it states that DMD is autosomal recessive. This is consistent with loss of function mutations (II).
The “most common mutation”, according to this passage, is the deletion of 5’—GCCATAGAGCGA—3’. The number of nucleotides in this segment of DNA is a multiple of three, suggesting that there is not a frameshift mutation (not I) – i.e. not a shift in the reading frame of the coding sequence.
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[0] => Array
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[each_answer] => A. I only
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[each_answer] => B. II only
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[each_answer] => C. I and II only
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[each_answer] => D. I and III only
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[question] => Based on the information in the passage, how are scientists able to identify deletion sequences using DNA probes?
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[answer] => 3
[description] => Reason for Correct Answer:
DNA probes are designed based on a known sequence of DNA. The DNA sequence from normal individuals is known, and the DNA from DMD patients is unknown.
The passage states that scientists are “able to identify sequences corresponding to the region of DNA containing the deletion” and that they do this by “using DNA probes made from the normal dystrophin sequence, which they hybridize with DNA from normal individuals and DMD patients.”
The probes for the experiment are therefore designed to hybridize with sequences from normal individuals, and would therefore not recognize DNA from DMD patients that was different.
Therefore, by analyzing which probes do not hybridize to DMD DNA, the scientists can determine the missing sequences.
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[each_answer] => A. The DNA probes hybridize to the sequences in DMD DNA that are different from normal DNA, indicating the deleted sequences.
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[each_answer] => B. All of the DNA probes hybridize to DMD DNA, while some do not hybridize to normal DNA, indicating the deleted sequences.
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[each_answer] => C. All of the DNA probes hybridize to normal DNA, while some do not hybridize to DMD DNA, indicating the deleted sequences.
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[each_answer] => D. The DNA probes hybridize to normal DNA only when complementary sequences are not present in DMD DNA.
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[quiz_unique_key] => 1403770772
[question] => Which is NOT true regarding the mutations present in DM1 and DM2?
[value] => Array
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[answer] => 3
[description] => Reason for Correct Answer:
The passage states that DM1 usually results from an increase in the number of CTG triplet repeats in the DMPK gene.
This means that subsequent generations would be more likely to experience worse symptoms of DM because of the phenomenon known as genetic anticipation; repeats in CTG would tend to expand in successive generations, leading to earlier onset and increased severity of symptoms.
The abnormal DMPK protein in DM1 is generally larger due to the expanded CTG repeats, and this altered protein function contributes to the symptoms of the disease.
Unlike the DM1 mutation, the DM2 mutation occurs in an intron. While mutations in coding regions (exons) can directly affect the amino acid sequence of a protein, mutations in introns can have indirect effects by influencing the processing, stability, and regulation of RNA. (Not all intronic mutations will necessarily disrupt RNA processing but they are more likely to, and the DM2 mutation does.)
DM1 is NOT likely to be caused by a missense mutation, as it is typically caused by an expanded CTG trinucleotide repeat in the DMPK gene. A missense mutation occurs when a codon is altered such that it produces a different protein, not when additional codons or trinucleotide repeats are added.
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[each_answer] => A. Subsequent generations are likely to experience worse symptoms of DM1.
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[each_answer] => B. The mutant DMPK protein is larger than the normal protein.
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[each_answer] => C. DM1 is more likely to result from a missense mutation.
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[each_answer] => D. DM2 is more likely to result from defects in RNA processing.
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