Repeated sequence (DNA)

Tandem and interspersed repeat schematic

Huntington's disease (5880985560)

Fragile X Chromosomal Differences

Repeated sequences (or repetitive elements, repetitive DNA) are patterns of nucleic acid sequences that occur in multiple copies throughout the genome. Repeated DNA sequences can be classified into several types based on their structure and the mechanism by which they are replicated. These sequences play a significant role in genomics, evolution, and the structure and function of the genome.

Types of Repeated Sequences

Repeated sequences in DNA can be broadly categorized into two main types: tandem repeats and interspersed repeats.

Tandem Repeats

Tandem repeats are sequences that are repeated one after another in a head-to-tail fashion. They can be further divided into:

• Satellite DNA: Comprises large arrays of tandemly repeated sequences. Satellite DNA can be found in heterochromatin, particularly in centromeres and telomeres, and is involved in chromosome structure and segregation.
• Minisatellites: Also known as variable number tandem repeats (VNTRs), these sequences are 10 to 60 base pairs long and are used in DNA fingerprinting.
• Microsatellites: Also known as short tandem repeats (STRs), these sequences are 1 to 6 base pairs in length and are highly polymorphic. They are used in genetic studies, forensic science, and population genetics.

Interspersed Repeats

Interspersed repeats are scattered throughout the genome and can move to new locations. They include:

• Transposable elements (TEs): Sequences that can move within the genome through a process called transposition. TEs are divided into two classes: retrotransposons, which move by a copy-and-paste mechanism, and DNA transposons, which move by a cut-and-paste mechanism.
• SINEs (Short Interspersed Nuclear Elements): Short DNA sequences that can be copied and inserted into new locations in the genome. They do not encode proteins but can influence gene expression and genome structure.
• LINEs (Long Interspersed Nuclear Elements): Longer sequences that can also move around the genome. Some LINEs are capable of encoding proteins necessary for their own transposition.

Functions of Repeated Sequences

Repeated sequences have various functions in the genome, including:

• Structural roles in chromosomes, such as in the formation of centromeres and telomeres.
• Regulation of gene expression through the presence of regulatory elements within or near repeated sequences.
• Facilitating recombination and genome rearrangements, which can drive evolution and speciation.
• Serving as markers for genetic studies and forensic analysis due to their high polymorphism.

Evolutionary Significance

Repeated sequences are a major source of genetic variation and evolution. They can lead to genetic diversity through the generation of new alleles by unequal crossing-over and by serving as sites for recombination. Their movement and amplification within the genome can cause mutations, which may lead to genetic diseases or provide material for evolutionary adaptation.

Challenges in Genomic Studies

The presence of repeated sequences poses challenges in genome sequencing and assembly due to their repetitive nature, making it difficult to accurately assemble the genome. Advanced sequencing technologies and computational methods are continually being developed to address these challenges.