3. Week 4 STR summary

Short Tandem Repeats (STRs)

• Type of size polymorphism; also known as microsatellites.

• Small DNA fragments (2-6 bp) repeated in tandem; total length 100-400 bp.

STRs vs VNTRs

• STRs: repeat units of 2-6 base pairs, cover less distance (100-400 bp).

• VNTRs: repeat units of hundreds of base pairs, typically repeated 4 to 20 times.

Location of STRs

• Non-clustered; unevenly distributed along chromosomes.

• Found in non-coding DNA; variable density due to replication errors.

Types of Repeats

• Mononucleotide, dinucleotide, trinucleotide, tetranucleotide, pentanucleotide, hexanucleotide.

STR Typing Techniques

• PCR: Amplification of STR regions.

• Capillary Electrophoresis: Enhanced accuracy in detecting DNA fragments using lasers and fluorescence.

Capillary Electrophoresis Details

• Operates via electrical current in a capillary tube.

• Uses fluorescent signals for detection; utilizes dyes or labeled primers.

Allelic Ladders and STR Typing

• Allelic ladders serve as references for STR allele comparison.

• Multiplex PCR allows simultaneous amplification of multiple STRs in one reaction.

Characteristics of Good STR Markers

• Narrow allele size range to avoid overlap; small product size for better amplification.

• Larger repeat units (4 bp preferred).

CODIS STRs

• Core set of 13 STR markers used in the Combined DNA Index System by the FBI.

Random Match Probability

• Calculated using allele frequencies in population data; predicts match likelihood.

• Uses Hardy-Weinberg Equilibrium; calculations vary by ethnic group.

Key Takeaways

• Understanding STRs and their significance in forensic DNA analysis.

• Familiarity with methodology of PCR and capillary electrophoresis for STR typing.