Archival Notice
This is an archive page that is no longer being updated. It may contain outdated information and links may no longer function as originally intended.
Home | Glossary | Resources | Help | Contact Us | Course Map
Uniquely designed primers will lead to generation of specific amplicons. The length of the primers is usually 18-30 bases, because the random combination of this size of a primer will hit less than once per total genomic sequence (3 billion base pairs).04
For example,
- There is a 1/4 chance (4-1) of finding an A, G, C or T in any given DNA sequence.
- There is a 1/16 chance (4-2) of finding any di-nucleotide sequence (e.g. AG).
- There is a 1/256 chance (4-4) of finding a given 4-base sequence (e.g. AGCT).
So, a seventeen base sequence will statistically be present only once in every 417 bases or approximately 17 billion.
A 17-mer or longer primer should be complex enough so that the likelihood of annealing to sequences other than the chosen target is very low. Primers of this length generally are unique sequences in the human genome; however, it is important to ensure that portions of the primer do not have sequence or cross-homology with the target. Computer programs such as BLAST can be used to find regions of local similarity between sequences. The program compares nucleotide sequences to sequence databases and calculates the statistical significance of matches.
Primers longer than 30 bases do not demonstrate higher specificity. Additionally, long amplicons are more likely to cross-hybridize with other primers and sequences in the reaction mixture, and this can terminate the DNA polymerization.09
Additional Online Courses
- What Every First Responding Officer Should Know About DNA Evidence
- Collecting DNA Evidence at Property Crime Scenes
- DNA – A Prosecutor’s Practice Notebook
- Crime Scene and DNA Basics
- Laboratory Safety Programs
- DNA Amplification
- Population Genetics and Statistics
- Non-STR DNA Markers: SNPs, Y-STRs, LCN and mtDNA
- Firearms Examiner Training
- Forensic DNA Education for Law Enforcement Decisionmakers
- What Every Investigator and Evidence Technician Should Know About DNA Evidence
- Principles of Forensic DNA for Officers of the Court
- Law 101: Legal Guide for the Forensic Expert
- Laboratory Orientation and Testing of Body Fluids and Tissues
- DNA Extraction and Quantitation
- STR Data Analysis and Interpretation
- Communication Skills, Report Writing, and Courtroom Testimony
- Español for Law Enforcement
- Amplified DNA Product Separation for Forensic Analysts