Spontaneous mutation (short answer)

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Spontaneous Mutation

Spontaneous mutations are mutations that arise naturally in cells without the influence of any external mutagen (like radiation or chemicals).
They occur due to errors in normal cellular processes, mainly during DNA replication, repair, or recombination.

These mutations are random and occur at a low but constant rate.

Causes of Spontaneous Mutations

  1. Errors in DNA replication (most common)

  2. Tautomeric shifts in nitrogen bases

  3. Spontaneous deamination (e.g., cytosine → uracil)

  4. Depurination (loss of purine base: A or G)

  5. Oxidative damage due to reactive oxygen species (ROS)

  6. Slippage during replication (in repetitive sequences)

Types of Spontaneous Mutations

1. Base Substitution (Point Mutation)

A single base is changed, inserted, or deleted.

(a) Transition

One purine ↔ another purine (A ↔ G)
or
One pyrimidine ↔ another pyrimidine (C ↔ T).

(b) Transversion

Purine ↔ pyrimidine
(A ↔ C / A ↔ T / G ↔ C / G ↔ T)

2. Frameshift Mutation

Caused by insertion or deletion of one or two bases during replication.
This shifts the reading frame and produces a completely altered protein.

Occurs commonly due to slippage in repetitive DNA sequences.

3. Spontaneous Depurination and Depyrimidination

Loss of a base from DNA:

  • Depurination: loss of A or G (more common)

  • Depyrimidination: loss of C or T

This creates an AP site (apurinic/apyrimidinic).
DNA polymerase may insert the wrong base → mutation.

4. Spontaneous Deamination

Removal of an amino group from bases:

  • Cytosine → Uracil

  • Adenine → Hypoxanthine

  • Guanine → Xanthine

This leads to incorrect base pairing.

5. Tautomeric Shifts

Nitrogen bases temporarily shift to rare forms (tautomers).
Example:

  • Keto ↔ Enol

  • Amino ↔ Imino

This causes mispairing, e.g.,
T (enol form) pairs with G instead of A.

6. Errors During Recombination

During meiosis, unequal crossing over can generate:

  • Deletions

  • Duplications

  • Inversions

Summary Table

Type of Spontaneous MutationDescription
TransitionPurine ↔ purine or pyrimidine ↔ pyrimidine
TransversionPurine ↔ pyrimidine
FrameshiftInsertion/deletion causing reading-frame shift
DepurinationLoss of A or G base
DeaminationRemoval of amino group; C→U
Tautomeric shiftRare base forms causing mispairing
Recombination errorsUnequal crossing over → deletions/duplications

If you want, I can also provide short notes, long answer format, or diagram-based explanation.

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