LECTURE 5 : BPA // CHAPTER 11 DNA ANALYSIS

Question 1

Blood pattern analysis

Answer 1

• Through the examination of the size, shape and distribution of bloodstains and bloodstain patterns, in association with the underpinning sciences to determine the physical events giving rise to their origin
  Can answer the following questions
• What events occurred/ what did not occur
• When and in what sequence did they occur
• Who was there/ who was not
• Support or contradiction of statements given by POI/ alleged victim or witness

Question 2

Blood distribution interpretation

Answer 2

• Origin(s) of bloodstains
• Distances between impact areas of blood spatter and origin at time of bloodshed
• Type and direction of impact that produced bloodstains or spatter
• Objects(s) that produced particular bloodstain patterns
• Number of blows, shots, etc.that occurred
• Position of victim, assailant, or objects at the scene during bloodshed
• Movement and direction of victim, assailant or objects at scene after bloodshed

Question 3

Bloodstain classifications

Answer 3

gravity
contact
spatter
other

Question 4

gravity stain

Answer 4

• A bloodstain resulting from falling drop that formed due to gravity

Question 5

Contact

Answer 5

• A bloodstain resulting from contact between a blood-bearing surface and another surface

Question 6

Spatter (force)

Answer 6

• A bloodstain from n object striking liquid blood

Question 7

Directional blood stains and the determination of area of origin:

Answer 7

• A trigonometric relationship between the angle of impact and the width and length of the resultant bloodstain

AOI = sin-1 (W/L)

• The angle of impact of a blood droplet affects its shape
• They become longer and thinner at shallow angles
• The tail or spiny projections point in the direction of travel

Question 8

Forces affecting the movement and behaviour of blood

Answer 8

• Gravity
• Surface tension
• Initial force (KE) - increasing this means smaller droplets
• Air resistance
• Viscosity – aspirin

Question 9

Nature of DNA

Answer 9

• deoxyribonucleic acid (DNA) is a complex molecule found in nearly all cells in all animals and plants
• DNA is a special type of molecule known as a polymer, a molecule made up of repeating simpler units called monomers
• DNA located in two regions of a cell - nucleus in mitochondria

Question 10

Allele:

Answer 10

each form of a particular gene at a particular locus in the genome

Question 11

Phenotype:

Answer 11

observed characteristic by the gene

Question 12

Genotype:

Answer 12

alleles that make up the gene

Question 13

PCR

Answer 13

• Denaturation = DNA is added to the PCR tube that contains the reaction mixture & heated to 94 degrees. DNA denatures, hydrogen bonds break, resulting in single-stranded DNA
• Annealing = attach a short strand of synthetic DNA to each of the separated strands. These are called primers, they mark the starting points for addition of new bases to complete the reproduction of each strand. Temperature drops to 60 degrees.
• Extension = temperature raised to 72 degrees. Under the influence of Taq
polymerase, single bases (nucleotides) are added to the primer. New piece of
double-stranded DNA is produced. This process occurs at each of the complementary single strands created by the denaturation process, so the end result is that 2 identical pieces of double-standard DNA. This completes 1 PCR cycle. Temperature is raised once again to 94 degrees & the process repeats.

Question 14

Short tandem repeats

Answer 14

• During the 1980s another type of repeating unit containing 2-6 base pairs was developed. These are designated as microsatellites & are called short tandem repeats (STRs), which have the same basic arrangement as VNTRs containing repeating units of base pair sequences in tandem.
• STRs advantages over VNTRs:
   Exhibit high variability in a population – giving rise to high degrees of association of evidence with a suspect
   Small size of the repeats makes STRs much less sensitive to degradation of the DNA
   There are many microsatellites to choose from for forensic purposes.
• Primers are chosen so that they will anneal with DNA just outside each side of the STR
• DNA detected by laser induced fluorescene (capillary electro)
• Allelic ladders = strands of DNA made up of all common alleles present at each STR locus are used for calibration.
• The concept of individualization doesn’t apply to DNA typing because (1) all DNA
  associations are expressed as probabilities of a chance occurrence, (2) identical twins havethe same DNA, & (3) extreme care should be taken when evaluating such statistics of rare occurrences.

Question 15

Mitochondrial DNA

Answer 15

1. mtDNA is circular in shape
2. cells contain many mitochondria & each one has up to 10 copies of mtDNA. Thus, each cell contains hundreds or thousands of copies of mtDNA, whereas there are only 2 copies of nuclear (genomic) DNA.
3. mtDNA contains a noncoding region of 1100 base pairs that contains 2 hypervariable regions. These regions exhibit a high mutation rate so that over a period of many generations, sequence variations can occur at these sites. As a result, these regions can be quite useful in comparing known & questioned DNA samples.
4. All male & female mtDNA comes from the mother
5. mtDNA often shows a high degree of variation between unrelated people. However, because there are only 2 hypervariable regions in mtDNA, the population statistics are not nearly as discriminating as with nuclear DNA.
6. Because of the large number of copies per cell, mtDNA can be useful in typing samples that have low quantities of DNA or in exhibits that are degraded or very old.