What are tandem repeats, and how are they used in DNA profiling?

Sourav PanNovember 9, 2024

Answered

Tandem repeats are sequences of DNA in which two or more nucleotides are repeated directly adjacent to each other. These repeats can vary in length and number, and they play significant roles in genetics, including applications in DNA profiling. Here’s a detailed overview of tandem repeats and their use in DNA profiling:

What Are Tandem Repeats?

1. Definition:
   • Tandem repeats consist of repeating units of nucleotides that are arranged in a head-to-tail fashion on a chromosome. They can be classified into various types based on the length of the repeat units:
     • Microsatellites (Short Tandem Repeats, STRs): Typically consist of 1-6 base pairs repeated multiple times (e.g., (CA)n).
     • Minisatellites: Comprise longer repeat units, usually 10-60 base pairs, repeated a variable number of times.
     • Satellite DNA: Longer tandem repeats that can be found in specific regions such as centromeres.
2. Genetic Variation:
   • The number of repeats can vary among individuals, making tandem repeats highly polymorphic. This variation is useful for distinguishing between different individuals, as unrelated people are likely to have different numbers of repeat units at specific loci.

Use of Tandem Repeats in DNA Profiling

1. DNA Profiling Techniques:
   • Short Tandem Repeat (STR) Analysis: STRs are the most commonly used markers in forensic DNA profiling due to their high variability and ease of analysis. STR analysis involves amplifying specific loci that contain tandem repeats using polymerase chain reaction (PCR).
   • The PCR process allows for the amplification of these regions even from small or degraded DNA samples, making STR analysis particularly valuable in forensic contexts.
2. Applications in Forensics:
   • Identification: STR profiling is used to create a DNA profile for individuals, which can then be compared against samples from crime scenes or databases. The unique patterns of STRs allow forensic scientists to identify or exclude potential suspects with a high degree of certainty.
   • Database Comparisons: Many countries maintain national DNA databases where profiles based on STRs are stored. When new samples are collected from crime scenes, they can be compared against these databases to find matches or links to known offenders.
   • Exoneration and Conviction: STR profiling has been instrumental in solving cold cases, exonerating wrongfully convicted individuals, and linking suspects to crimes through genetic evidence.
3. Advantages of Using Tandem Repeats:
   • High Polymorphism: The variability in repeat numbers makes STRs highly discriminatory for individual identification.
   • Efficiency: STR analysis requires only small amounts of DNA and can be performed quickly using automated systems.
   • Robustness: This method is effective even with degraded samples, which is often the case in forensic investigations.