Molecular Genetics - Auburn City Schools

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23 Οκτ 2013 (πριν από 3 χρόνια και 7 μήνες)

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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?


Ribo
nucleic acids (RNA) and
deoxy
ribo
nucleic 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

P

Pu/Py

Phosphoric Acid

Deoxyribose

Purine or Pyrimidine

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?



A T A C T G A T G T C A T A T


C G A T C G A T C G A T C G A



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