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IB DP Biology Mock Exam HL Paper 2 Set 3 - 2025 Syllabus

IB DP Biology Mock Exam HL Paper 2 Set 3

Prepare for the IB DP Biology Exam with our comprehensive IB DP Biology Exam Mock Exam HL Paper 2 Set 3. Test your knowledge and understanding of key concepts with challenging questions covering all essential topics. Identify areas for improvement and boost your confidence for the real exam

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Question 

Duchenne muscular dystrophy (DMD) is a sex-linked inherited disorder caused by a recessive mutation in the dystrophin gene on the X-chromosome. Mutations may involve base substitutions, deletions, or insertions, all of which disrupt normal muscle function.

(a) Using a Punnett grid, determine the likelihood that the children of an unaffected male and a female carrier will inherit DMD.

(b) Explain the difference between deletion mutations and insertion mutations.

CRISPR-Cas9 gene editing offers a potential treatment by correcting the defective dystrophin gene. The diagram below illustrates how dystrophin expression can be restored:

(c) Describe how CRISPR-Cas9 technology could be used to alter the mutant dystrophin gene so that a functional protein is produced.

(d) Give one reason why pluripotent stem cells, rather than multipotent stem cells, may be preferred for this gene-editing approach.

▶️ Answer/Explanation
Detailed solution

(a)

Unaffected male: \(X^D Y\)
Carrier female: \(X^D X^d\)
(D = normal allele, d = DMD allele)

 \(X^D\)\(Y\)
\(X^D\)\(X^D X^D\)\(X^D Y\)
\(X^d\)\(X^D X^d\)\(X^d Y\)

Only the son with genotype \(X^d Y\) will develop DMD.
Therefore, the chance of a child inheriting DMD is 25%.

(b)

• A deletion mutation removes one or more nucleotides from the DNA sequence.
• An insertion mutation adds extra nucleotide bases into the sequence.
Both can cause frameshifts, altering downstream amino acids.

(c)

CRISPR-Cas9 can modify the dystrophin gene in multiple ways:

  • Correcting single-base errors: The guide RNA can direct Cas9 to the mutated base, allowing repair to restore the correct codon.
  • Removing inserted DNA: If excess bases cause a frameshift, CRISPR can excise the unwanted segment.
  • Adding missing DNA: Cas9 can introduce the absent nucleotides when deletions disrupt the reading frame.

In each case, the goal is to restore a functional dystrophin protein.

(d)

Pluripotent stem cells can produce almost any cell type in the body, including both cardiac muscle and skeletal muscle, both of which are affected in DMD. Multipotent stem cells have a much narrower differentiation potential, making them less suitable for whole-body treatment.

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