An inverted repeat is a segment of DNA followed by the reverse complement of its sequence. This means the complementary base pairs to the sequence (G to C, A to T and vice versa) follow the sequence, but in the opposite direction. Inverted repeats can be read in opposite directions on the two different strands of DNA.
Inverted repeats don't need to follow immediately after the first segment of the repeated DNA. In some situations, the inverted repeats occur many base pairs downstream of the first copy. This means that DNA can sometimes form structures different from the standard double helix when the strands become separated. Each strand contains the sequence as well as its complement, so it's possible for the DNA to form single stranded loops and bind to itself forming a cruciform.
Inverted repeats serve biological functions that are both beneficial and detrimental to life forms. Inverted-repeat structures are frequently found near the binding sites of transcription factors, which makes it easier for the cell to initiate transcription of the DNA into RNA. However, inverted-repeat structures can also cause the DNA to bind to itself and cause portions of the DNA to be missed during transcription. Inverted repeats are also found near transposons, which are individual DNA sequences that can jump from one section of the genome to another. Although transposon activity is frequently harmless, it can disrupt essential genes, leading to cell death or illness.