Chapter 7Biologically Unique

Copyright © 2012 by W. H. Freeman and Company

Introduction

• How DNA helped free an innocent man?• Roy Brown– Found guilty of homicide and sentenced to 25

years to life in prison– Expert stated bite marks on the victim’s body

matched Brown’s teeth– Brown maintained his innocence

Innocence Project

• Brown contacted the innocence project– Nonprofit organization founded in 1992 by

Neufeld and Scheck– Mission to use DNA evidence to free people

wrongly convicted of crimes– Not available when Brown was convicted

What is DNA?

• DNA stands for deoxyribonucleic acid.– DNA is a hereditary molecule that is passed from

parents to offspring.– DNA is common to all living organisms.– From plants to bacteria to fungi to humans– DNA serves as the instruction manual for how to

build an individual. – Do you look like your parents, aunts,

grandparents?

Where is DNA found?

• DNA is found in the nucleus of eukaryotic cells. It is in the form of chromosomes.

What is a chromosome?

• A chromosome consists of a single DNA molecule wrapped around proteins.

Humans have 23 pairs of chromosomes• One chromosome from each pair is inherited from the biological mother.• One chromosome from each pair is inherited from the biological father.• A chromosome 1 from each parent, etc.Karotype

Chromosomes

• DNA in most of cells (except RBCs)• Scientists can collect evidence as blood,

semen, saliva, hair, skin, etc. from the crime scene and extract DNA

• Used to identify a perpetrator

Humans have 23 pairs of chromosomes• The 23rd chromosome pair determines a

person’s sex.– XX = female– XY = male

DNA testing

• Innocence Project has freed more than 240 people from prison since 1992 (death row)

• Exposed flawed justice system• Dishonest witnesses, crooked police officers,

unprepared lawyers, mistaking identity• DNA has helped improve the system• Junk science convicted Roy Brown

The structure of DNA: Nucleotides

• Nucleotides have one of four bases:– Adenine (A)– Thymine (T)– Guanine (G)– Cytosine (C)

The structure of DNA: Complementary base pairing

• The complementary base pairing rule can be used to determine a sequence of a

complementary strand of DNA.

The structure of DNA: Nucleotide sequences

• The specific sequence of nucleotides along a strand of DNA is unique to each individual.

• With the exception of identical twins• Order is different for each individual

The structure of DNA: A double helix

• The two strands of nucleotides pair up and twist around each other to form a spiral-shaped double helix.

The structure of DNA: A double helix

• The sugars and phosphates form the outside “backbone” of the helix.

• The bases form the internal “rungs,” like steps on a twisting ladder. (spiral staircase)

Brown

• Filed a Freedom of Information Act request for information about his case.

• Affidavits of four people suspicious of another person the day of the crime

• What happened next to create a DNA profile of the perpetrator?

DNA Replication

• Cells make an identical copy of a DNA molecule through DNA replication.

DNA Replication• During DNA replication, the hydrogen bonds

that hold base pairs together are broken and the helix is unwound.

DNA Replication

• After the helix is unwound, DNA polymerase adds complementary nucleotides using the rules of base pairing.

DNA replication is semi-conservative

• At the end of DNA replication, two copies of the original DNA molecule exist.– Each molecule consists of an original and a new

strand.

DNA Replication exercise

• A T C G C A T C A T C G A A T C• T A G C G T A G T A G C T T A G• What is the complementary sequence?

The Polymerase Chain Reaction• PCR is a laboratory technique

used to replicate a specific DNA segment.

• Scientists add nucleotides, DNA polymerase, and primers (short segments of DNA that show the section to which the DNA polymerase should bind).

The Polymerase Chain Reaction

• During each round of PCR, the two nucleotide strands separate. Each strand is used as a template for complementary base pairing.

The Polymerase Chain Reaction• PCR can make

billions of copies from a starting sample of just a few molecules of DNA.

DNA Profiling

• Genome – one complete set of genetic instructions encoded in the DNA of an organism

• DNA profiling takes advantage of the fact that no two people have the exact same DNA sequence.

Short Tandem Repeats

• STRs are sections of a chromosome in which DNA sequences are repeated.– For example, the sequence AGCT may be repeated

over and over again.

Short Tandem Repeats

• We all have STRs in the same places along our chromosomes, but the exact length of our STR varies from person to person because no two people have the same exact DNA sequence.

Noncoding regions of DNA

• Not all sequences of DNA serve as instructions for making proteins.

• DNA sequences that do not hold instructions to make cellular proteins are called noncoding regions.– STRs are found in noncoding regions of DNA.

Noncoding regions of DNA

• Coding sequences of DNA (make proteins) are extremely similar from person to person, but noncoding sequences vary much more between individuals.

• Variation in noncoding sequences provides a genetic fingerprint.

STRs and DNA Profiling

• Forensic scientists use the differences in STR lengths to distinguish between individuals.

Making a DNA profile: Step 1

• Collect cells and extract DNA.

Making a DNA profile: Step 2

• Use PCR to amplify multiple STR regions.

Making a DNA profile: Step 3

• Separate STRs using gel electrophoresis.

Gel electrophoresis• Gel electrophoresis is a laboratory technique that separates

fragments of DNA by size.– It separates replicated STRs according to their length.

Gel electrophoresis

• Gel electrophoresis works by applying an electrical current to a gel loaded with DNA.

• The electrical current causes polar DNA to migrate through the gel.

• Shorter fragments travel further, whereas longer fragments remain near the top.

Making a DNA profile: Step 4• The separated segments of DNA create a specific patter of bands that is

visible under fluorescence and unique to each person.

Reading a DNA profile

• Different individuals have different DNA banding patterns because they have different DNA sequences.

Several STR regions are used to create a DNA profile

• 520% of people share the same number of repeats at any one STR site.

• The combined pattern of STR repeats at multiple sites is unique to a person.

DNA evidence is more reliable than other forms of evidence

• Some evidence commonly presented in criminal cases can be unreliable.– Error rates for bite mark identification can be as

high as 91%.– Hair analysis can only exclude a suspect, not

positively identify one.

• Except for identical twins, no two people share exactly the same DNA.