Lesson 1 of 9 / Mutation, cell division and cancer
A base change can alter the message
Which changes shift the reading frame?
I can explain substitutions, additions and deletions and their possible effects on amino-acid sequence.
I can explain gene mutations and numerical/structural chromosomal aberrations.Syllabus
Syllabus 8876, Core 2(d). Explain gene mutations and numerical/structural chromosomal aberrations.
About 6 min
Make a guess. You are not marked.
Three bases are deleted together from a gene. Does this shift the reading frame?
Show the answer
A multiple of three keeps the frame.
One codon is lost, so one amino acid is missing, but later codons are read normally.
The key idea
A gene mutation changes a DNA base sequence. A substitution can change a codon; an insertion or deletion not in a multiple of three can shift downstream codon grouping.
Explore the idea
Regroup the message after a change
Read from the same start. Bars mark codon boundaries, not bonds in the RNA.
RNA sequences display consequences of corresponding DNA changes. The incomplete final group is a fragment at the edge of this short example, not an extra complete codon.
Explanation
A gene mutation is a change in the nucleotide sequence of a gene. A substitution replaces one base with another. In a coding region, it may produce a synonymous codon, change one amino acid, or create a stop codon. The outcome depends on the original and new sequence.
Addition, also called insertion, places extra nucleotide material into the sequence; deletion removes it. If one or two bases are added or deleted within a coding sequence, the triplet reading frame downstream changes. Many later codons can then differ, often leading to a different or prematurely shortened polypeptide.
An insertion or deletion of a multiple of three bases does not shift the subsequent frame, although it can add or remove amino acids and still alter function. Position also matters: a change in a critical region can have a larger effect than a similar change elsewhere.
Mutation supplies new alleles. It need not be directed towards what an organism needs, and not every mutation produces an observable phenotype. Follow the chain from sequence to RNA codons, amino-acid sequence, folding and function before judging the effect.
Step by step
- 1
Identify the sequence change
Replacement, addition and loss are different operations.
- 2
Regroup into codons
Keep the original start and check whether downstream triplets shift.
- 3
Assess the product
Use a codon table or supplied meanings; do not assume every altered codon changes the amino acid.
Worked example
A one-base insertion
An mRNA coding segment is AUG|CCU|GAA|GGC. A U is inserted immediately after AUG. Regroup the sequence from the same start.
One way to explain it
AUG|UCC|UGA|AGG|C... . All following groups shift. UGA is a stop codon in the standard code, so this example can terminate prematurely.
Why this answer works
- Insert the base before regrouping.
- Read triplets from the unchanged start.
- The effect is not restricted to one added amino acid.
Watch out for this
A student says: "Every three-base deletion causes a frameshift." What is wrong with this?
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Deleting a multiple of three preserves the downstream triplet frame, although amino acids can be lost and protein function can change.
Check your understanding
Which coding-region change most directly produces a downstream frameshift?
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Insertion of one nucleotide.
Codons are triplets, so one extra base changes subsequent grouping.
Exam-style questions
Check your understanding
Which mutation would NOT cause a frameshift?
Show the answer
Deleting three adjacent bases
Removing three bases removes one codon; the downstream reading frame is unchanged.
Quick recall
Which changes shift the reading frame?