Types of Genetic Mutations and their Consequences

Essential Genetic Nomenclature

• Mutation: A change that occurs in the DNA sequence, which can be caused by mistakes when the DNA is copied or as the result of environmental factors.

• Point Mutation: A genetic mutation where a single nucleotide base is changed, inserted, or deleted from a sequence of DNA or RNA.

• Frameshift Mutation: A genetic mutation caused by indels (insertions or deletions) of a number of nucleotides in a DNA sequence that is not divisible by three. This shifts the "reading frame" of the genetic message.

• Mutagen: A physical or chemical agent that changes the genetic material, usually DNA, of an organism and thus increases the frequency of mutations above the natural background level.

• DNA (Deoxyribonucleic Acid): The hereditary material in humans and almost all other organisms, carrying genetic instructions for development, functioning, and reproduction.

• mRNA (Messenger RNA): A single-stranded molecule of RNA that corresponds to the genetic sequence of a gene and is read by a ribosome in the process of synthesizing a protein.

• Translation: The process in which ribosomes in the cytoplasm or endoplasmic reticulum create proteins after the process of transcription of DNA to RNA.

• Protein: Large biomolecules, or macromolecules, consisting of one or more long chains of amino acid residues.

mRNA and Protein Translation Baseline

To understand the effects of mutations, an original (wild-type) sequence must be established for comparison.

• Original DNA Sequence: $TAC - ACA - CGT - AAT - ATT$

• Original mRNA Sequence: $AUG - UGU - GCA - UUA - UAA$

• Original Protein Product: $\text{Met} - \text{Cys} - \text{Ala} - \text{Leu} - \text{Stop}$

Substitution Mutations and Their Effects

Substitution occurs when one base is exchanged for another.

Substitution Mutation #1

• Error: An $A$ is present instead of a $U$ at the 5th nucleotide position.

• Mutated mRNA Sequence: $AUG - UGA - GCA - UUA - UAA$

• Protein Product: $\text{Met} - \text{Stop}$

• Effect on Protein: This is a nonsense mutation. Because the second codon was changed to a "STOP" codon ($UGA$), the translation process ended prematurely. The resulting protein is significantly shorter (truncated) and likely non-functional compared to the original.

Substitution Mutation #2

• Error: A $C$ is present instead of an $A$ at the 9th nucleotide position.

• Mutated mRNA Sequence: $AUG - UGU - GCC - UUA - UAA$

• Protein Product: $\text{Met} - \text{Cys} - \text{Ala} - \text{Leu} - \text{Stop}$

• Effect on Protein: This is a silent mutation. Although the DNA/mRNA sequence changed, both the original codon ($GCA$) and the mutated codon ($GCC$) code for the same amino acid, Alanine ($Ala$). Therefore, the resulting protein product is identical to the original.

Frameshift Mutations: Insertion and Deletion

Frameshift mutations alter the reading frame of the codons, often changing every amino acid in the sequence after the point of mutation.

Insertion Mutation (Frameshift)

• Error: An extra $C$ is inserted into the mRNA at the 8th position.

• Mutated mRNA Sequence: $AUG - UGU - GCC - AUU - AUA - A...$

• Protein Product: $\text{Met} - \text{Cys} - \text{Ala} - \text{Ileu} - \text{Ileu}$

• Effect on Protein: The insertion shifted the reading frame. This changed the amino acids following the insertion (the 4th and 5th amino acids became $Ileu$ instead of $Leu$) and eliminated the original stop codon. The protein will continue to be built until a new stop codon is reached elsewhere on the strand, making it entirely different in structure and length.

Deletion Mutation (Frameshift)

• Error: A $G$ was deleted from the mRNA (originally at the 7th position).

• Mutated mRNA Sequence: $AUG - UGU - CAU - UAU - AA$

• Protein Product: $\text{Met} - \text{Cys} - \text{His} - \text{Tyr}$

• Effect on Protein: By deleting a base, the entire reading frame shifted to the left. This transformed the third codon from $GCA$ to $CAU$ ($His$) and the fourth from $UUA$ to $UAU$ ($Tyr$). Much like the insertion mutation, this results in a protein with an incorrect amino acid sequence that will likely fail to perform its biological function.

Standard Genetic Code Reference Table

This table describes the relationship between mRNA codons and their corresponding amino acids.

• First Letter U:

  • $UUU / UUC$: Phe

  • $UUA / UUG$: Leu

  • $UCU / UCC / UCA / UCG$: Ser

  • $UAU / UAC$: Tyr

  • $UAA / UAG$: Stop

  • $UGU / UGC$: Cys

  • $UGA$: Stop

  • $UGG$: Trp

• First Letter C:

  • $CUU / CUC / CUA / CUG$: Leu

  • $CCU / CCC / CCA / CCG$: Pro

  • $CAU / CAC$: His

  • $CAA / CAG$: Gln

  • $CGU / CGC / CGA / CGG$: Arg

• First Letter A:

  • $AUU / AUC / AUA$: Ileu

  • $AUG$: Met (Start)

  • $ACU / ACC / ACA / ACG$: Thr

  • $AAU / AAC$: Asn

  • $AAA / AAG$: Lys

  • $AGU / AGC$: Ser

  • $AGA / AGG$: Arg

• First Letter G:

  • $GUU / GUC / GUA / GUG$: Val

  • $GCU / GCC / GCA / GCG$: Ala

  • $GAU / GAC$: Asp

  • $GAA / GAG$: Glu

  • $GGU / GGC / GGA / GGG$: Gly

Questions & Discussion

Q: What happened to the protein product in Substitution Mutation #1 compared to the original?A: The translation stops when the "stop" codon is reached. Because the mutation created a premature stop codon ($UGA$), the protein synthesis was terminated after only one amino acid ($Met$), resulting in a truncated, non-functional protein.

Q: What effect did the insertion mutation have on the protein product?A: The insertion caused a frameshift. This resulted in the third amino acid remaining Ala (due to the specific placement of the extra C), but it changed the subsequent amino acids to $Ileu$ and destroyed the stop codon ($UAA$ was broken apart). Every codon after the insertion is potentially changed, fundamentally altering the protein.

Q: What occurred in the Deletion Mutation relative to the original sequence?A: The deletion of a single base ($G$) shifted the reading frame, causing the ribosome to read a new set of triplets. The protein changed from $\text{Met} - \text{Cys} - \text{Ala} - \text{Leu}$ to $\text{Met} - \text{Cys} - \text{His} - \text{Tyr}$. Frameshift mutations like this usually have catastrophic effects on protein function.