Chapter 13
• DNA : The Indispensable Forensic
Science Tool
Molecular Genetics’ Place in
the History of Genetics
• 1900s: Classical (Mendelian)
Genetics
• 1940-1960s: Biological Revolution
(Period of change in our
understanding of genetics at the
molecular level)
• 1990s: Advances in biotechnology
and cloning projects
Watson & Crick (1953)
• Discovered the
physical
structure and
chemical
composition of
DNA
Nitrogenous Bases
• Bases which contain nitrogen
• Two Types of Nitrogenous Bases:
• Pyrimidines
• Purines
• Will have to know structures of these bases
Pyrimidines
• Big Name…Small Structure
• Includes Cytosine, Thymine, and Uracil
• Uracil is found in RNA only!
Pyrimidine Structures
Purines
• Small Name… Large Structure
• Include Adenine and Guanine
Purine Structures
Deoxyribose
• The sugar component of DNA
• Deoxy– loses an OH- group on a Carbon
atom
What is the difference in sugars?
• Ribonucleic acids (RNA) and
deoxyribonucleic acids (DNA) contain
5-carbon sugars (ribose)
• RNA contains a ribose molecule
• DNA contains a deoxy-ribose molecule
(2'-deoxy-ribose)
Phosphoric Acid
Charged
oxygen
atoms make
the molecule
acidic
Sugar + Phosphate=
Phosphodiester Bond
Shorthand Notation
• Sugar + Phosphoric Acid
P
Deoxynucleotide
• Contains all 3 parts
• Phosphoric acid
• Deoxyribose
• Nitrogenous Base
(C,T,G,A)
Shorthand Notation
Phosphoric Acid
Purine or Pyrimidine
P
Pu/Py
Deoxyribose
Video Questions on Worksheet
Secrets of the Sequence
Just to Review
• Deoxyribonucleic Acid (DNA)
•
•
•
•
Chemical unit of heritable information
Capable of transformation
Unit structure is called a nucleotide
Base composition (C,T,G,A)
The Double Helix
Structure of DNA
Complimentary
Base Pairing
•C bonds with G
•A bonds with T
Edwin Chargaff (1949-53)
• Discovered the
base
composition
and chemistry
of DNA
Chargaff’s Observations
• Rule 1:
• The number of As = the number of Ts
• The number of Cs = the number of Gs
• **Illustrates the concept of base
pairing**
Chargaff’s Observations
• Rule 2:
• In double strand DNA,
• The sum of purines (A+G) = The sum of
pyrimidines (C+T)
Chargaff’s Observations
• Rule 3:
• % of (G+C) WILL NOT EQUAL % of
(A+T)
Chargaff’s Observations
• Rule 4:
• G-C Content Increases Stability of
Molecule : WHY??
•
•
•
•
A-T (2 Hydrogen Bonds)
C-G (3 Hydrogen Bonds)
More stable in higher temperatures
Info was used by Watson & Crick to model
the structure of DNA
Chargaff’s Rules
Example Problems
• If a dsDNA molecule has 10% G, how
much T does it have?
•Use Rule 1: 10% G then 10% C because they
must equal so 20% is C & G
•Assuming 100%, subtract C & G amounts
from 100. (100-20=80)
•80% is the amount of A & T together, so
to get T, divide 80/2=40%
The molecule has 40% Thymine!
Chargaff’s Rules
Example Problems
• Given the following: Is the molecule
ds or sstranded?
• A=18% T=26% C=26% G=30%
• Rule 2: A + G = C + T if dstranded
• Add percentages:
• 18 + 30 = 26 + 26
• 48 = 52 so the molecule is sstranded
Chargaff’s Rules
Example Problems
• Which molecule is more stable?
• ATACTGATGTCATAT
• CGATCGATCGATCGA
• Pair bases first, then look for C-G bonds
Molecule 1 has 4 C-G bonds
Molecule 2 has 8 C-G bonds
Molecule 2 is more stable!
More DNA Discoveries
Rosalind Franklin (1950-1953)
• Xray Diffraction of
dsDNA Molecule
• Physical evidence
of what DNA
looked like
• Regular structure,
specific width
• X-shape suggested
a helical structure
In 1953, Watson & Crick asked…
• How do chemical components come
together to make a molecular form?
• How does structure relate to the
biological properties of DNA?
Things they Knew…
• In the structure lies the mechanism
for…
• 1. Replication
• 2. Transfer of info to future
generations
• 3. Information Storage
4. Molecular basis of mutation
What they did…
• Watson & Crick used the chemical info
from Chargaff and physical info from
Franklin to find DNA was a helix.
• Chemistry + Physical = Helix
How It Works…
Phosphodiester
Bonds
Link the DNA
Backbone
How the strands make a helix…
The Story of Sam Sheppard
DNA at Work
• DNA is the fundamental unit of heredity.
• DNA codes for proteins which control all
aspects of physical appearance, from eye
color to height.
• Changes in or absences of these proteins
result in genetic disorders such as
hemophilia, muscular dystrophy, and
Huntington’s disease.
DNA Double Helix and Replication
• The nature of the Helix makes it easy to
replicate the DNA
• When the Helix unwinds, both strands can
be copied at the same time to make 2
brand new, identical DNA Helices with the
help of certain enzymes and proteins
• Very similar to making a carbon copy
DNA Typing
• Process of distinguishing
one individual from another
• Using DNA, which is specific
to every individual, Forensic
Scientists are able to match
suspects based on portions
of DNA found at a crime
scene
RFLPs - Restriction fragment length
polymorphisms
• Segments of DNA that are used in DNA typing
• A large portion of DNA in the human genome
seems to act as filler DNA and does not code for
any proteins.
• These gaps in coding DNA are called tandem
repeats - sequences of letters that are repeated
several times.
• The letters involved and the lengths of the
tandem repeats are specific to every individual
Polymerase Chain Reaction (PCR)
• The main enzyme involved in DNA
replication is called DNA Polymerase.
• Scientists can use DNA Polymerase to
replicate and amplify small pieces of DNA
found at a crime scene into workable
samples.
• The DNA is then used in several different
lab tests to link a suspect to a crime or
crime scene.
PCR used for DNA Typing
• PCR typing replaced RFLP DNA typing as the
dominant method in the mid-1990s.
• PCR uses a much smaller sample size
which means that it can characterize DNA
extracted from small amounts of blood,
semen, and saliva.
• Ex: Envelope seals, cigarette butts, soda
cans, stains on clothes and bedding, etc.
Mitochondrial DNA
• Found inside the Mitochondria of the cell
and is inherited solely from the mother.
• Can be used when nuclear DNA is not
available due to charred remains, small
quantities like hair shafts, etc.
• However, mtDNA analysis is more rigorous,
time consuming, and costly when
compared to nuclear DNA profiling.
Mitochondrial DNA cont’d
• There are hundreds of thousands of
copies of mtDNA in each cell
• mtDNA is in a loop instead of a strand
or double helix
• Reference samples for lab testing can
be obtained from any maternal
relative
DNA and the FBI Database
CODIS - Combined DNA Index System
• A computer software program that stores
local, state, and national databases of DNA
from convicted offenders, unsolved crime
scene evidence, and profiles of missing
persons.
• Allows investigators to compare new
evidence to preexisting cases and/or
convicted offenders and possibly solve the
crime.
Collection and Preservation
• In order to properly collect and preserve
DNA evidence, investigators must be sent to
a crime scene immediately.
• Care must be taken so that the investigator
does not make personal contact with the
evidence; latex gloves, shoe covers, and
face masks must be used.
Collection and Preservation cont’d
• All clothing from the victim and suspect(s)
needs to be sent to a lab to test for saliva,
semen, and blood samples, along with
other fabrics in and around the crime
scene.
• Each stained article should be packaged
separately in a paper bag or in a well
ventilated box.
Interesting cases with DNA evidence
• Bill Clinton impeachment trial
• DNA taken from Monica Lewinsky’s dress.
• Anna Nichole Smith’s child
• DNA tests to determine paternity
• Phillip Spector – movie producer
• DNA found on breast of breast of victim
• Yale Student disappearance
• Bloody clothes found in tiles of the wall
matched the suspect