What is DNA?
- stands for deoxyribonucleic acid
- is the deciphering of its structure and decoding of its genetic information were turning points in our understanding of the underlying concepts of inheritance
What did DNA finally allow forensic scientists to do?
- a way to link biological evidence with individuals (blood, semen, hair, saliva etc.)
How many types of DNA does a human cell contain?
two
What are the two types of DNA that a human cell contains?
- nuclear
- mitochondrial
What is the structure of a nuclear DNA strand?
a very large molecule made by linking a series of repeating units called nucleotides
What is a nucleotide?
composed of sugar, a phosphorus-containing group and a nitrogen-containing molecule called a base
What are the four types of bases associated with DNA structure?
- Adenine (A)
- Thymine (T)
- Guanine (G)
- Cytosine (C)
What is a DNA molecule comprised of?
two strands each wrapped around the other in the form of a double helix
How are the bases of each DNA strand aligned?
in a manner known as complementary base pairing
What is the result of complementary base pairing?
adenine pairs with
thymine and guanine pairs with cytosine
What is PCR testing?
- polymerase chain reaction testing
- a technique for replicating small quantities of DNA or broken pieces of DNA found at a crime scene
How is PCR testing done?
- DNA is heated to separate it
- primers (short strands of DNA used to target specific regions of DNA replication) are added which hybridize with the strands
- DNA polymerase and free nucleotides (A,T,G, and C) are added to rebuild each of the separated strands
- the cycle is then repeated 25-30 times
What do tandem repeats in DNA offer a forensic scientist?
offer a means of distinguishing one individual from another - there is a tremendous variation in the number of repeats each of us have for that specific tandem
What is the latest method of DNA typing?
short tandem repeat (STR)
What is short tandem repeat?
STRs are locations on the chromosome that contain short tandem repeats that repeat themselves within the DNA molecule
*STRs normally consist of repeating sequences of 3 to 7 bases in length,
and the entire strand of an STR is also very short, less than 450 bases in length.
*This means that STRs are much less susceptible to degradation and may
often be recovered from bodies or stains that have been subjected to
extreme decomposition
What are the advantages of STR?
- because of the shortness ideal candidates for multiplication by PCR
- usually produce a 2 band pattern; thus the interpretation of mixtures is simplified. An STR pattern arising from two individuals will have four peaks
What is RFLP?
- tandem repeats known as restriction fragment length polymorphisms use up until the mid-1990s
RFLPs are length differences associated with relatively long repeating
DNA strands and form the basis for one of the first DNA typing
procedures
*Typically, a core sequence consists of 15 to 35 bases in length and
repeats itself up to a thousand times
*The key to understanding DNA typing lies in the knowledge that
numerous possibilities exist for the number of times a particular
sequence of base letters can repeat itself on a DNA strand
PCR vs RFLP
- PCR technology cannot be applied to RFLP DNA typing
- the RFLP strands are too long often out numbering in the thousands of bases
- PCR is best used with DNA strands that are no longer than a couple hundred bases
How do you type STRs?
- by examining the distance the STR has migrated on the electrophoretic plate one can determine the number of AATG repeats in the STR
How is sex determined by STRs?
- the amelogenin gene (which is the gene for tooth pulp) is shorter by 6 bases in the X chromosome than the Y chromosome
- when the amelogenin gene is amplified by the PCR and seperated by electrophoresis, males who have X and Y chromosome, show two bands; females, who have two X chromosomes have just one band
What is mitochondrial DNA?
- another type of DNA used for individual characterization
- located outside the cells nucleus and is inherited from the mother
What is mitochondrial DNA testing?
Mitochondrial DNA typing does not approach STR analysis in its discrimination power and thus is best
reserved for samples, such as hair, for which STR analysis may not be
possible
*Forensic analysis of mtDNA is more rigorous, time consuming, and costly
when compared to nuclear DNA analysis
*Also, all individuals of the same maternal lineage will be indistinguishable by mtDNA analysis
*Two regions of mtDNA HV 1 and HV 2, have been found to be highly variable and a procedure known as
sequencing is used to determine the order of base pairs
What is CODIS?
combined DNA index system - a computer software program developed with the FBI that maintains local, state and national databases of DNA profiles of convicted offenders, unsolved crime scene evidence and profiles of missing persons
