Virtual Lab 3: DNA fingerprinting - Introduction

To identify specific DNA pattern in one individual, DNA fingerprinting DNA 指紋分析 can be used.
Within the genome of an organism, there are coding DNA responsible for coding proteins and non-coding DNA which lie between the genes with unknown function.  Among the non-coding DNA (>95% in human genome), there are two kinds of DNA segments that can be used in fingerprinting.  They show variations among individual (polymorphism) by the number of repetitive base sequence.  They are

  • Variable number tandem repeats (VNTRs) 變量數目的串聯重複 – consists of repetitive base sequence of 20-100 base pairs
  • Short Tandem repeats (STRs) 短串聯重複 – consists of repetitive base sequence of 2-6 base pairs

With different number of repeats, different lengths of the VNTRs can be obtained after cutting by
  restriction enzyme  .  Gel electrophoresis 凝膠電泳 can be used to separate these DNA segments.  A DNA fingerprint can then be obtained after DNA denaturation, adding DNA probes and staining.  This method is known as Restriction Fragment Length Polymorphism (RFLP) 限制性片段長度多態性 analysis.

Comparing VNTRs, STRs require a much smaller amount of DNA sample.  Being amplified by   Polymerase Chain Reaction (PCR)  , fragments of STRs can be separated by gel electrophoresis.  Since the STRs are located in highly variable region, no DNA denaturation and DNA probes are needed.   

This method of using STRs, called Polymerase Chain reaction – Short Tandem Repeat (PCR-STR) analysis is mostly used nowadays.

In embryo screening, coding DNA will be analyzed instead of VNTRs and STRs.  Using similar principle of RFLP analysis, specific disease causing mutations in genes will be detected by using DNA probes.