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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


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

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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



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.



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!



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 16.8 Three alternative models of DNA replication

Figure 16.14 Priming DNA synthesis with RNA

Figure 16.16 A summary of DNA replication

Free-Write



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



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



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



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

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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 molecules 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



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



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



Collecting Reference Samples

o Known samples taken from suspects and victims

o Reference samples are compared to evidence collected from the crime scene



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



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



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?



chapter 7 dna - mahoneyspage.comdna segment length (base pairs) 400 500 1,200 1,300 1,500 x x x...

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