Slipped strand mispairing (SSM) is a mutation process which occurs during DNA replication

DNA replication

DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...

. It involves denaturation

Denaturation (biochemistry)

Denaturation is a process in which proteins or nucleic acids lose their tertiary structure and secondary structure by application of some external stress or compound, such as a strong acid or base, a concentrated inorganic salt, an organic solvent , or heat...

 and displacement of the DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 strands, resulting in mispairing of the complementary bases. Slipped strand mispairing is one explanation for the origin and evolution of repetitive DNA sequences

Repeated sequence (DNA)

In the study of DNA sequences, one can distinguish two main types of repeated sequence:*Tandem repeats:**Satellite DNA**Minisatellite**Microsatellite*Interspersed repeats:**SINEs...

. Slipped Strand Mispairing has also been shown to function as a Phase variation

Phase variation

Phase variation is a method for dealing with rapidly varying environments without requiring random mutation employed by various types of bacteria, including Salmonella species. It involves the variation of protein expression, frequently in an on-off fashion, within different parts of a bacterial...

 mechanism in certain bacteria.

Self-acceleration

SSM events can result in either insertions or deletions. Insertions are thought to be self-accelerating: as repeats grow longer, the probability of subsequent mispairing events increases. Insertions can expand simple tandem repeats by one or more units. In long repeats, expansions may involve two or more units. For example, insertion of a single repeat unit in GAGAGA expands the sequence to GAGAGAGA, while insertion of two repeat units in [GA]₆ would produce [GA]₈ .

Evolution of diverse adjacent repeats

The combination of SSM events with point mutation

Point mutation

A point mutation, or single base substitution, is a type of mutation that causes the replacement of a single base nucleotide with another nucleotide of the genetic material, DNA or RNA. Often the term point mutation also includes insertions or deletions of a single base pair...

 is thought to account for the evolution of more complex repeat units. Mutations followed by expansion would result in the formation of new types of adjacent short tandem repeat

Short tandem repeat

A short tandem repeat in DNA occurs when a pattern of two or more nucleotides are repeated and the repeated sequences are directly adjacent to each other. The pattern can range in length from 2 to 5 base pairs and is typically in the non-coding intron region...

 units. For example, a transversion

Transversion

In molecular biology, transversion refers to the substitution of a purine for a pyrimidine or vice versa. It can only be reverted by a spontaneous reversion. Because this type of mutation changes the chemical structure dramatically, the consequences of this change tend to be more drastic than those...

 could change the simple two- base repeat [GA]₁₀ to [GA]₄GATA[GA]₂. This could then be expanded to[GA]₄[GATA]₃[GA]₂ by two subsequent SSM events. Simple repetitive DNA sequences

Repeated sequence (DNA)

In the study of DNA sequences, one can distinguish two main types of repeated sequence:*Tandem repeats:**Satellite DNA**Minisatellite**Microsatellite*Interspersed repeats:**SINEs...

 containing a variety of adjacent short tandem repeats are commonly observed in non-protein coding regions of eukaryotic genomes.