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Chapter 7
DNA
© 2012 Cengage Learning. All Rights Reserved
Objective: You will be able to identify the components of DNA structure.
Do Now: • Read the introduction on p. 160
• List the ways that DNA can be used in forensics
2
3 3
A 200-Year Old Mystery
o August 13, 1792, the French royal family is arrested and imprisoned.
o The king and queen were executed.
o The heir , Luis, Was imprisoned along with sister.
o After 2 years they said Luis died
o Conspiracy that loyalists helped him escape • People throughout Europe started to claim they were Luis
o Did he die in prison or escape?
4
Deoxyribonucleic acid (DNA)—the genetic material that makes each individual unique.
• Only identical twins have the same DNA
• DNA copies itself when cells divide
• DNA contains the blueprint for making proteins essential for life processes
• DNA is found in biological evidence
• DNA is individual evidence
5
Structure and Function
o Nucleotides—subunits of DNA • Deoxyribose
• Phosphate group
• Nitrogenous base • Adenine
• Thymine
• Cytosine
• Guanine
6
Structure and Function
o Each nitrogenous base can form hydrogen bonds with a specific base on a complementary strand of DNA
o Adenine always pairs with thymine
o Guanine always pairs with cytosine
Activity
o You and a partner will build a piece of DNA that is 6 base pairs long on each side
o One bag will contain all of the materials that you need
o Black is sugar, red is phosphate group and yellow represents the bond between them
o Create a key so that I know what each color represents
o Use p. 161 in your textbook to help
3.4 nm
1 nm
0.34 nm
Hydrogen bond
(a) Key features of DNA structure
(b) Partial chemical structure
3 end
5 end
3 end
5 end
T
T
A
A
G
G
C
C
C
C
C
C
C
C
C
C
C
G
G
G
G
G
G
G
G
G
T
T
T
T
T
T
A
A
A
A
A
A
Figure 16.7a
Objective: You will be able to discuss the
function of DNA.
Do Now: • Without looking at your notes from yesterday, list as many
concepts as you can about the structure of DNA
9
Figure 17.UN01
DNA RNA Protein
Figure 5.25-3
Synthesis of mRNA
mRNA
DNA
NUCLEUS
CYTOPLASM
mRNA
Ribosome
Amino acids Protein
Movement of mRNA into cytoplasm
Synthesis of protein
1
2
3
Figure 17.4
DNA template strand
TRANSCRIPTION
mRNA
TRANSLATION
Protein
Amino acid
Codon
Trp Phe Gly
5
5
Ser
U U U U U
3
3
5 3
G
G
G G C C
T
C
A
A
A A A A A
T T T T
T
G
G G G
C C C G G
DNA molecule
Gene 1
Gene 2
Gene 3
C C
Figure 17.5 Second mRNA base
Fir
st
mR
NA
base (
5 en
d o
f co
do
n)
Th
ird
mR
NA
base (
3 en
d o
f co
do
n)
UUU
UUC
UUA
CUU
CUC
CUA
CUG
Phe
Leu
Leu
Ile
UCU
UCC
UCA
UCG
Ser
CCU
CCC
CCA
CCG
UAU
UAC Tyr
Pro
Thr
UAA Stop
UAG Stop
UGA Stop
UGU
UGC Cys
UGG Trp
G C
U
U
C
A
U
U
C
C
C A
U
A
A
A
G
G
His
Gln
Asn
Lys
Asp
CAU CGU
CAC
CAA
CAG
CGC
CGA
CGG
G
AUU
AUC
AUA
ACU
ACC
ACA
AAU
AAC
AAA
AGU
AGC
AGA
Arg
Ser
Arg
Gly
ACG AUG AAG AGG
GUU
GUC
GUA
GUG
GCU
GCC
GCA
GCG
GAU
GAC
GAA
GAG
Val Ala
GGU
GGC
GGA
GGG Glu
Gly
G
U
C
A
Met or
start
UUG
G
14
Structure and Function
o Nitrogenous base pairs information
o Genes—sections of DNA that code for a protein
o Proteins catalyze, regulate, and control chemical reactions in the cell.
DNA RNA Proteins
15 Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Structure and Function
o Alleles—an alternative form of a gene for a certain trait.
o An individual inherits two alleles for each gene, one from each parent.
Free Write
o Write a paragraph that discusses the function of DNA.
16 Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Objective: You will be able to compare and contrast DNA and RNA
Do Now:
• Turn to page 170 in your textbook
• Begin reading the first case study (Colin Pitchfork)
Figure 5.25-3
Synthesis of mRNA
mRNA
DNA
NUCLEUS
CYTOPLASM
mRNA
Ribosome
Amino acids Polypeptide
Movement of mRNA into cytoplasm
Synthesis of protein
1
2
3
DNA and RNA Similarities
o Both are polynucleotides made from three components:
• Sugar
• Phosphate
• Nitrogenous base – Adenine
– Guanine
– Cytosine
o These nucleotides are joined by covalent bonds to form a linear molecule
• This molecule has a 5’ and a 3’ end
Structural Differences in DNA and RNA
o RNA is single stranded
o RNA uses Uracil instead of Thymine
o RNA has the sugar ribose instead of deoxyribose
20
Figure 5.26c
Nitrogenous bases
Cytosine (C)
Thymine (T, in DNA)
Uracil (U, in RNA)
Adenine (A) Guanine (G)
Sugars
Deoxyribose (in DNA)
Ribose (in RNA)
(c) Nucleoside components
Pyrimidines
Purines
Functional Differences
o DNA is the hereditary material…uhm HELLO!
o RNA is created by the DNA to help build a protein that needs to be made
DNA RNA
o Draw the following Venn diagram in your notebook
Bad Blood!
24 Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Objective: You will be able to discuss the replication of DNA
Do Now: • Turn to page 170 in your textbook
• Begin reading the first case study (Tommie Lee Andrews)
Figure 16.7 A model for DNA replication: the basic concept (Layer 1)
Figure 16.7 A model for DNA replication: the basic concept (Layer 2)
Figure 16.7 A model for DNA replication: the basic concept (Layer 3)
Figure 16.7 A model for DNA replication: the basic concept (Layer 4)
Figure 16.8 Three alternative models of DNA replication
Figure 16.14 Priming DNA synthesis with RNA
Figure 16.16 A summary of DNA replication
Free-Write
33 Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Objective: You will be able to explain the techniques used in forensics to compare DNA
Do Now: • Read “DNA Identification” on p. 162
• What are polymorphisms
35 Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
What Causes Variation in DNA?
o 99.5% of DNA is the same for everyone
o Marker—a sequence of DNA base pairs associated with a specific trait
o Matching markers from two samples determines the likelihood of an individual being the same as the reference sample • Called DNA fingerprinting
Sequence of DNA Fingerprinting
1. Extract DNA
2. Cut DNA into small pieces with restriction enzymes
3. Replicate DNA using PCR
4. Gel electrophoresis
36
37 Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Step 1: Extraction
o Organic Extraction—removing biological evidence from materials
• Example: Blood from bed sheets
o Differential Extraction—separates vaginal epithelial cells from sperm cells
38 Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Chelex Extraction
o Used when a small amount of biological evidence is available
o Sample placed in boiling water with Chelex beads
o The cells in the sample burst open to expose and preserve DNA
39
Step #2: Cut DNA into fragments
DNA Segment
Length (base pairs) 400 500 1,200 1,300 1,500
X X X
Enzyme Enzyme Enzyme Enzyme Y
Describe the results you would expect from the cutting of the DNA
segment above. Assume that the digestion occurred under
appropriate conditions and went to completion.
I. DNA digested with only enzyme X
II. DNA digested with only enzyme Y
III. DNA digested with enzyme X and enzyme Y combined
IV. Undigested DNA
Objective: You will be able to discuss how gel electrophoresis separates DNA.
Do Now: • Read “Electrophoresis” on p. 166
• Explain how/why the DNA moves in gel electrophoresis
Sequence of DNA Fingerprinting
1. Extract DNA
2. Cut DNA into small pieces with restriction enzymes
3. Replicate DNA using PCR
4. Gel electrophoresis
44
Step 3: The Polymerase Chain Reaction (PCR)
• Break hydrogen bonds by
heating
• Add primers
• Add nucleotides (A, G, T, C)
• Cool down
• Repeat steps
Figure 13.24a
Mixture of DNA mol- ecules of different sizes
Cathode Anode
Wells
Gel
Power source
(a) Negatively charged DNA molecules will move toward the positive electrode.
Figure 20.8 Gel electrophoresis of macromolecules
Figure 13.24b
Restriction fragments
(b) Shorter molecules are impeded less than longer ones, so they move faster through the gel.
This is electrophoresis for ALL DNA. Don’t use anymore
Figure 20.9 Using restriction fragment patterns to distinguish DNA from different alleles
Switched to RFLP Analysis because one locus (location)
o Enzymes break DNA into different sized restriction fragments
o The different sized fragments are called length
polymorphism…hence the name RFLP o Can produce extremely low random match probabilities
Figure 20.17 DNA fingerprints from a murder case
Begin this activity by reading p.191
53
Which Suspect, A
or B, cannot be
excluded from the
class of potential
perpetrators of
this assault?
Objective: You will be able to compare and contrast DNA and RNA
Do Now:
• Begin reading “Probes” on p. 168
• Explain what a probe does in DNA fingerprinting
Short Tandem Repeats (STRs)
o STRs are short sequences of DNA that are repeated numerous times
• Normal length is 2-5 base pairs
• Example is “gata”
o Different people typically have different #’s of repeats
o By far the most common method of establishing a DNA fingerprint
59 Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
CODIS
o Combined DNA Index System • An FBI nationwide database
• Stores DNA fingerprints of individuals who have been convicted of certain crimes
• Forensic scientists can compare DNA fingerprints of suspects based on 13 core STR locations
60 Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Collecting Biological Evidence
o Guidelines • Never use bare hands
• Avoid sneezing and coughing
• Use clean protective gloves for each item collected
• Package each sample separately
• Air dry evidence located on objects; store in an envelope or paper bag, not plastic bags
• Establish a chain of custody
61 Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Collecting Reference Samples
o Known samples taken from suspects and victims
o Reference samples are compared to evidence collected from the crime scene
62 Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Collecting DNA Evidence
o Biological samples • Avoid contamination
• Retain original surface when possible
• Reference samples for comparative analysis
o Stop degradation • Protect sample from environmental exposure
• Remove moisture by drying out and freezing the sample
63 Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Collecting Biological Evidence
o Examples • Skin cells
• Blood
• Saliva
• Semen
• Hair
Problems with Sexual Assault Cases and DNA
o Sperm is very durable and long lasting
o Sperm on clothing can be washed and transferred to other clothing in wash
o So may be used to show someone had sex with another when they may not have
65 Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Mitochondrial DNA (mtDNA) Analysis
o mtDNA is inherited from the mother
o Although it does not work as an individualistic evidence, it can trace maternal lineage
Hmmmmmm……..
o Would you be ok if Mahopac decided to place cameras and radars along the road?
o Should it be legal to take a sample of DNA from everyone convicted of a crime?
o Should the government have access to the DNA of everybody?
66 Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
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