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Continuity and change - Mutations and gene editing

Grade 12IBBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Mutations are permanent changes in the nucleotide sequence of DNADNA, which can be caused by replication errors or mutagens such as UVUV radiation and chemical agents.

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Point mutations include substitutions (replacing one base), insertions (adding a base), and deletions (removing a base).

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Substitution mutations are classified as Silent (no change in amino acid), Missense (change to a different amino acid), or Nonsense (creation of a premature stop codon).

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Frameshift mutations occur when insertions or deletions (not in multiples of 33) shift the triplet reading frame, typically resulting in a non-functional protein.

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The CRISPR-Cas9 system is a gene-editing tool consisting of a guide RNARNA (gRNAgRNA) that directs the Cas9Cas9 endonuclease to a specific DNADNA target sequence.

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For Cas9Cas9 to bind, the target DNADNA must be immediately followed by a Protospacer Adjacent Motif (PAMPAM), commonly 5′−NGG−3′5'-NGG-3'.

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Somatic mutations occur in body cells and affect only the individual, whereas germline mutations occur in gametes and can be passed to offspring (F1F_{1} generation).

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Gene editing can be used for Gene Knockout (disrupting a gene) or Gene Knock-in (inserting a functional sequence).

📐Formulae

Mutation Frequency=Number of MutantsTotal Number of IndividualsMutation \ Frequency = \frac{\text{Number of Mutants}}{\text{Total Number of Individuals}}

DNA Sequence Change: 5′−GAG−3′→5′−GTG−3′\text{DNA Sequence Change: } 5'-GAG-3' \rightarrow 5'-GTG-3'

Amino Acid Change: Glu→Val\text{Amino Acid Change: } Glu \rightarrow Val

💡Examples

Problem 1:

In the β\beta-globin gene, the 6th6^{th} codon changes from GAGGAG to GUGGUG in the mRNAmRNA sequence. Determine the type of mutation and its effect on the resulting polypeptide.

Solution:

This is a base substitution mutation (specifically a transversion). The codon GAGGAG codes for Glutamic acid, while GUGGUG codes for Valine. This is a missense mutation.

Explanation:

Because Glutamic acid is hydrophilic and Valine is hydrophobic, the hemoglobin molecules polymerize under low oxygen conditions, causing the red blood cell to take a 'sickle' shape. This is the molecular basis of Sickle Cell Anemia.

Problem 2:

A DNADNA sequence 5′−ATG GCA CCG−3′5'-ATG \ GCA \ CCG-3' undergoes a deletion of the second 'G'. Predict the new sequence and the impact on the reading frame.

Solution:

Original: 5′−ATG GCA CCG−3′5'-ATG \ GCA \ CCG-3' New: 5′−ATG CAC CG...−3′5'-ATG \ CAC \ CG...-3'

Explanation:

The deletion of the GG at position 44 causes a frameshift. All subsequent codons (GCA→CACGCA \to CAC, etc.) are altered, which usually results in a completely different amino acid sequence and often a premature stop codon downstream.

Problem 3:

Explain the role of the PAMPAM sequence in CRISPR-Cas9 gene editing.

Solution:

The PAMPAM sequence (typically 5′−NGG−3′5'-NGG-3') is a short DNADNA sequence found on the target DNADNA, but not on the gRNAgRNA.

Explanation:

The Cas9Cas9 enzyme first recognizes and binds to the PAMPAM sequence. Once bound, it unwinds the DNADNA to see if the gRNAgRNA matches the adjacent target sequence. If there is no PAMPAM, Cas9Cas9 will not cut, preventing the system from targeting the bacterial cell's own DNADNA (in its natural prokaryotic state).