BIO SCI 98 Lecture Notes - Lecture 3: Missense Mutation, Nonsense Mutation, Acridine Orange
19 Jun 2018
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Types of mutations
-Single base substitutions: change of a single base in the DNA sequence of a gene
-Silent = change in codon does not change amino acid sequence
-Missense = results in a different amino acid encoded
-Nonsense = creates an early stop codon
-Frameshift mutants = insertion or deletion of nucleotides that alter the reading frame of
the coding sequence
-Deletion mutants = deletion of one or more nucleotides → sometimes large blocks of
genetic material are missing
Single base substitutions
Single base substitution - change of a single base in the DNA sequence of a gene
Silent - change in DNA sequence that does not change the amino acid sequence
Example: GAA (Glu) mutated to GAG (Glu) → both encode Glu so the mutation is silent
Missense - change in DNA sequence results in a change in the amino acid sequence
Example: GAA (Glu) mutated to GAC (Asp) → changes Glu to Asp
Some missense mutations are worse than others → Glu and Asp both neg charged so
their substitution won't have such a huge impact on protein
Nonsense - change in DNA sequence results in a stop codon
Example: GAA (Glu) mutated to TAA (UAA) → causes change from Glu to stop
Leads to truncated protein = non-functional
Transition mutation - purine is substituted for another purine
A to G or G to A
Most common type of mutation
Can lead to all 3 types of substitutions
Frameshift mutations
-Frameshift mutants: insertion or deletion of nucleotides that alter the reading frame of the
coding sequence
-Most frameshift mutations result in premature stop codon
Original Insertion mutation of G Deletion mutation of A
-Insertion and deletion mutations can negate each other
-If the number of nucleotides inserted or deleted is the same, then the reading frame is
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restored
-Will still have substitutions in between insertion and deletion points
-Insertion and deletion can be farther apart and reading frame will still be restored (but
there will be more substitutions in between them)
-Doesn't just apply to one nucleotide insertion/deletions
Reversion mutations
-Reversion mutation = a mutation that makes a protein dysfunctional gets a second
mutation that lets the protein function again
-Examples
-Missense mutation (Lys → Gly) in the active site causes protein to be non-
functional second protein returns amino acid to Lys⇒
-Same situation but Gly → Arg which is similar to Lys and protein is mostly
functional
-Nonsense mutation (Tyr → STOP) creates truncated protein second mutation ⇒
changes STOP → Try
-Same situation but STOP → Ser
-Frameshift mutation = insertion of a single nucleotide, second mutation deletes
single nucleotide near extra insertion and restores protein function
-Reversion mutations identified through functional assay
-Protein function must be restored to identify a reversion mutation
Biochemical basis of mutation
-Single base substitutions
-Chemical carcinogens
-Frameshift mutations
-Intercalating agents insert themselves
into DNA double helix
-Cause DNA stretching
-During DNA replication, as DNA
polymerase passes through strands
stretched by intercalating agents, they can
cause insertion or deletion mutations
-Acridine orange is an intercalating agent
used to make frameshift mutations in
DNA
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Ionizing radiation
-Type of radiation that causes
electrons to be released from
molecules, ionizing them
-Can directly damage
DNA
-Can indirectly damage DNA by creating free radicals from water molecules →
damage DNA
-Leads to double strand DNA breaks
-During repair, these breaks can lead to deletion mutations of several
nucleotides
-X-rays are common form of radiation
Things you should know about mutations
-Types of mutations (causes)
-Single base substitution: silent, missense, nonsense mutations (carcinogens)
-Frameshift mutations: insertion or deletion (intercalating agents)
-Deletion mutations: loss of large chunk of sequence (ionizing radiation)
-Transition mutation: A to G or G to A
-Reversion mutations: second mutation restores protein function
Be able to:
-Look at a nucleotide sequence and diagnose what kind of mutation occurred and what
effect it would have on the protein (none, mild, non-functional)
-Know what can cause each type of mutation (carcinogens, intercalating agents, ionizing
radiation, etc.) and generally how that occurs.
-Look at a mutant sequence and determine what kind of reversion mutation could correct
it in whole (perfect fix) or in part (restores some kind of function though not the original
sequence).
Is the code read continuously, or are there gaps in between each codon?
-How did these scientists figure it out?
Crick and Brenner: T4 bacteriophage mutants
-T4 bacteriophage: virus that infects bacteria
-Plaque assay - when mixed with bacteria and plated onto agar dishes, the plate will be
confluent with growing bacteria, but not where bacteriophage is growing
-The plaques are dark regions with dead bacteria
-B gene- required for bacteriophage to infect multiple strains of E. coli
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Document Summary
Single base substitutions: change of a single base in the dna sequence of a gene. Silent = change in codon does not change amino acid sequence. Missense = results in a different amino acid encoded. Nonsense = creates an early stop codon. Frameshift mutants = insertion or deletion of nucleotides that alter the reading frame of the coding sequence. Deletion mutants = deletion of one or more nucleotides sometimes large blocks of genetic material are missing. Single base substitution - change of a single base in the dna sequence of a gene. Silent - change in dna sequence that does not change the amino acid sequence. Example: gaa (glu) mutated to gag (glu) both encode glu so the mutation is silent. Missense - change in dna sequence results in a change in the amino acid sequence. Example: gaa (glu) mutated to gac (asp) changes glu to asp.
