Describe frameshift mutations — Biology notes

What Is a Frameshift Mutation?

Why the Reading Frame Matters

DNA is read in triplet codons (groups of 3 nucleotides). The ribosome starts at a start codon (AUG in mRNA) and reads continuously in3-base increments until a stop codon (UAA, UAG, UGA). There are three possible reading frames:

Original DNA: ATG|CAT|GC|TA

Frame 1: ATG | CAT | GGC | TA → Met-His-Gly-Stop
Frame 2: A | TGC | ATG | GCT | AA → (different codons)
Frame 3: AT | GCA | TGG | CTA | A → (different codons)

The cell uses Frame 1 (defined by the start codon position). A frameshift changes which frame is read.

How Frameshift Mutations Occur

Mechanism: Insertion

Before: ATG|CAT|GGC|TAA → Met-His-Gly-Stop

Insert 1 base (A): ATG|**CA|TGG|CTA|A

Reading frame shifts right by 1
New codons: Met-Thr-Trp-Leu-... (completely different protein)

Why this happens at the molecular level:

DNA polymerase slipage during replication (especially in repetitive sequences)
Errors in DNA repair (non-homologous end joining can add/lose bases)
Transposable elements inserting themselves into genes

Mechanism: Deletion

Before: ATG|CAT|GGC|TAA → Met-His-Gly-Stop

Delete 1 base (A in CAT): ATG|C**_**T|GGC|TAA → ATG|CTG|GCT|AA

Reading frame shifts left by 1
New codons: Met-Leu-Ala-... (completely different)

Molecular causes:

DNA polymerase skipping bases (slippage in repetitive regions)
Exposure to mutagens (radiation, certain chemicals)
Spontaneous depurination creating abasic sites

Why Frameshifts Are So Destructive

Three catastrophic outcomes:

Nonsense protein: Every amino acid after the shift is wrong
- Protein cannot fold correctly
- Non-functional or misfolded (degraded by proteasome)
Premature stop codon: Shifted frame often hits stop codons early
- Truncated protein (shortened)
- Usually non-functional
- Subject to nonsense-mediated decay (mRNA destroyed)
Lost stop codon: Original stop codon read through
- Translation continues into3' UTR
- Abnormally long protein with garbage sequence
- Usually unstable and degraded

Worked example Worked Example 1: Single Base Insertion

Original gene:

DNA  5'-ATG CC TAT GCA TGA-3'
mRNA: 5'-AUG CCG UAU GCA UGA-3'
Protein: Met - Pro - Tyr - Ala - Stop

Mutation: Insert T after position 7 (after CCG)

DNA:  5'-ATG CCG T|T|AT GCA TGA-3'
               ↑ inserted T
mRNA: 5'-AUG CCG U|U|AU GCA UGA-3'

New reading frame:

mRNA: AUG CCG UA UGC AUG A...
     Met - Pro - Leu - Cys - Met - ...

Why each step?

Step 1: Insertion creates CCG U → but codons are 3 bases, so we get CCG then UUA
Step 2: UA codes for Leucine (not Tyrosine like original UAU)
Step 3: Every subsequent codon is shifted: UGC (Cys), AUG (Met), etc.
Result: After position 2(Pro), the entire protein sequence is altered

Worked example Worked Example 2: Two-Base Deletion

Original:

DNA:  5'-ATG GCA TTC GA TGA-3'
Protein: Met - Ala - Phe - Glu - Stop

Mutation: Delete GC (bases 4-5)

DNA:  5'-ATG __ A TC GAA TGA-3'
       → 5'-ATG AT CGA ATG A-3'

New reading:

mRNA: AUG AU CGA AUG A...
     Met - Ile - Arg - Met - ...

Why this is catastrophic:

Original protein: Met-Ala-Phe-Glu (4 amino acids)
Mutant: Met-Ile-Arg-Met-... (completely different chemistry)
Original stop codon (UGA) is now read as AUG| → shifted out of frame
Translation likely continues until hitting a new stop codon in the shifted frame

Frameshift Mutations in Real Diseases

Common Mistakes (and Why They Feel Right)

Common mistake Mistake 1: "Any insertion/deletion is a frameshift"

Why this feels right: You hear "indel = frameshift" as shorthand. Why it's wrong: Insertions/deletions of 3, 6, 9, 12... bases (multiples of 3) do NOT shift the frame. They add/remove whole codons.

Example that breaks the rule:

Original: ATG|CAT|GC|TAA
Delete CAT (3 bases): ATG|GC|TAA → Met-Gly-Stop
Reading frame preserved! Just missing one amino acid.

The fix: "Frameshift" requires non-multiple-of-3 indel. Use the term in-frame insertion/deletion for multiples of 3.

Common mistake Mistake 2: "Frameshifts always produce non-functional proteins"

Why this feels right: You learned frameshifts scramble everything downstream.

Why it's wrong:

Late frameshifts (near the end of the gene) may preserve most of the protein
Suppressor mutations: A second frameshift downstream can restore the frame Example:

Original: ATG|CAT|GGC|TAA (Met-His-Gly-Stop)
Insert A at position 4: ATG|ACA|TGG|CTA|A...
          ↓ (frameshift 1)
Delete T at position 10: ATG|ACA|TGG|C_A|A... → ATG|ACA|TG|CA|...
          ↓ (frameshift 2, opposite direction)
Result: Met-Thr-Trp-Gln-... (frame restored after position 10)

Only the region between the two frameshifts is garbled. If that region isn't critical, the protein might retain partial function.

The fix: Frameshifts are usually lethal to protein function, but context matters (location, compensatory mutations).

Frameshift vs. Other Mutations

Type	Reading Frame	Severity	Example
Point (substitution)	Preserved	Low-Medium	GA→GUA (Glu→Val, sickle cell)
Insertion (3n bases)	Preserved	Low-Medium	Insert CAT (add His)
Deletion (3n bases)	Preserved	Low-Medium	ΔF508 in CF (lose Phe)
Frameshift (+1, +2)	Shifted	High	All downstream scrambled
Frameshift (−1, −2)	Shifted	High	Premature stop common

Why frameshifts are uniquely severe: They have a multiplicative effect—each wrong codon compounds, and one mistake cascades through the entire protein.

Molecular Consequences Timeline

DNA level: Insertion/deletion occurs (replication error, mutagen)
Transcription: Mutant DNA → mutant mRNA (frame is shifted in the transcript)
Translation: Ribosome reads wrong frame → produces nonsense amino acid sequence
Protein folding: Misfolded protein (hydrophobic residues in wrong places, etc.)
Cellular response:
- If premature stop: Nonsense-mediated decay (mRNA destroyed)
- If full translation: Unfolded protein response (protein degraded by proteasome)
Phenotype: Loss of protein function → disease (if gene is essential)

Detection and Repair

How cells try to prevent frameshifts:

Proofreading by DNA polymerase δ/ε (3'→5' exonuclease activity)
Mismatch repair (MMR) system catches slippage errors
Failure of these systems → higher frameshift rate (e.g., in microsatellite instability cancers)

Why repair often fails:

Frameshifts in repetitive sequences (like AAAAA) are hard to detect (template slippage is common)
If the mutation happens in a germ cell, it's passed to offspring

Clinical detection:

DNA sequencing (looking for indels in patient genes)
Protein truncation test (checks for premature stops)

Connections

Point mutations — single base changes that preserve the reading frame
Nonsense mutations — point mutations creating premature stops (frameshifts often cause these)
Genetic code — why triplet reading frame matters
DNA replication errors — slippage causes most frameshifts
Nonsense-mediated decay — mRNA quality control triggered by frameshift-induced early stops
Tay-Sachs disease — real-world example of frameshift lethality
Sickle cell anemia — contrast with point mutation (to see why frameshifts are worse)

Recall Explain to a 12-Year-Old

Imagine you're reading a coded message where every 3 letters make one word: "THE CAT ATE THE BIG RAT"

Now someone adds an extra letter X after THE:

"THE XCA TAT ETH EBI GRA T"

Suddenly, every word is nonsense! That's a frameshift. DNA works the same way—genes are read 3 letters at a time (like THE, CAT, ATE). If you add or remove a letter that's not a group of 3, every "word" after that point becomes gibberish. Your cells can't make the right protein, which can cause diseases.

It's like reading a book where every word after page 50 is in the wrong language—you can't understand anything that comes after the mistake!

#flashcards/biology

What is a frameshift mutation? :: A mutation caused by insertion or deletion of nucleotides (not a multiple of 3) that shifts the reading frame, changing all downstream codons and amino acids.

Why are frameshifts worse than point mutations?

Point mutations change1 amino acid; frameshifts change EVERY amino acid after the mutation, usually destroying protein function.

Does deleting 6 bases cause a frameshift?

No. Deleting 6 bases (multiple of 3) removes2 amino acids but preserves the reading frame. Only non-multiples of 3 cause frameshifts.

What are the three possible outcomes of a frameshift?

(1) Nonsense protein (all downstream amino acids wrong), (2) Premature stop codon (truncated protein), (3) Lost stop codon (abnormally long protein).

Why do frameshifts often create premature stop codons?

The shifted reading frame exposes different triplets, which statistically hit one of the3 stop codons (UAA, UAG, UGA) quickly.

What is nonsense-mediated decay (NMD)?

A cellular quality control system that detects and degrades mRNAs containing premature stop codons, often triggered by frameshift mutations.

Give a real disease caused by frameshift mutation :: Tay-Sachs disease (4-base insertion in HEXA gene → truncated enzyme → lipid accumulation → neurodegeneration).

Can two frameshift mutations cancel each other out?

Yes. If a second frameshift (opposite direction) occurs downstream, it can restore the reading frame. The region between them is still garbled, but the rest may be functional.

What causes frameshifts at the molecular level?

DNA polymerase slippage (especially in repetitive sequences), replication errors, faulty DNA repair, or transposable elements inserting into genes.

Why are repetitive sequences (like AAAAA) hotspots for frameshifts?

The DNA polymerase can slip on identical bases, losing track of position and adding/skipping a base during replication.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Frameshift mutation ek bahut serious genetic problem hai jo DNA mein hota hai. Sochiye ap ek sentence padh rahe ho jisme har3 letters ka ek word hai: "THE CAT ATE RAT." Agar aap bech mein ek letter add kar do ya delete kar do—jaise "THEXCAT ATE RAT"—toh bad ke sare words badal jate hain: "THX ECA TAT ER." DNA bhi aise hi kaam karta hai. Genes ko 3- bases (nucleotides) ke groups mein padha jata hai jinhe "codons" kehte hain, aur har codon ek amino acid banata hai. Jab 1 ya 2 bases insert ya delete ho jaate hain (3 ke multiple mein nahi), toh pura reading frame shift ho jaata hai. Iska matlab mutation point ke bad har codon galat ho gaya—ur isse pura protein barbad ho jata hai.

Yeh point mutation sezyada dangerous hota hai kyunki point mutation sirf ek amino acid badalta hai, lekin frameshift sare downstream amino acids ko galat bana deta hai. Aksar isse premature stop codon aa jata hai (protein bech mein hi ruk jata hai) ya phir bilkul nonsense protein banta hai jo fold nahi hota aur cell mein kaam nahi kar paata. Diseases jaise Tay-Sachs (brain disorder) aur kuch types ke cystic fibrosis frameshift mutations ki wajah se hote hain. Isliye jab bhi DNA replication mein polymerase slip karta hai ya repair system fail ho jaata hai, frameshift ka risk badh jata hai—aur yeh genetic diseases ka major cause ban sakta hai. Yeh concept samajhna zaroori hai kyunki yeh dikhata hai ki DNA mein ek chhoti si galti kitna bada impact daal sakti hai puri protein machinery par.

Test yourself — Mutations & Gene Regulation

Connections