Question

why is PCR so useful for forensic scientist that have an old blood sample

Answer 1

PCR = Polymerase chain reaction

It is an in-vitro DNA amplification method.
Steps involved in PCR:
a. Denaturation
b. Annealing
c. Extension
d. Repeat

Components of PCR:
a. Template DNA
b. Primers
c. Taq polymerase
d. dNTPs
e. Buffer

PCR plays an essential role in forensic studies. Often, minute quantities of blood/tissue samples are obtained from the crime scene. These samples are used to isolate genomic DNA. The genomic DNA from suspects is also isolated. PCR is performed on these samples to amplify specific genomic segments that exhibit high polymorphism. The PCR product can be directly used as a marker if it is an SSLP. It can also be used for restriction digestion if it is an RFLP. The patterns of DNA profiling from the crime scene samples and suspects is compared at several polymorphic loci and the culprit can be confirmed.