DNA
The Blueprint of Life
Nucleic Acids
(Remember the macromolecules back in Chapter 2?)
Nucleic Acids - store the genetic instructions
for all life
Two types of nucleic acids: RNA and DNA
DNA
• Stands for:
Deoxyribonucleic Acid
• Type of sugar: Deoxyribose
• # of strands of nucleotides:
2 (Double Stranded) “double helix”
• Function: stores genetic material
RNA
• Stands for:
Ribonucleic Acid
• Type of sugar: Ribose
• # of strands of nucleotides:
1 (single stranded)
• Function:
Transfers genetic material
DNA is made up of small repeating units
(monomers) called nucleotides.
Nucleotide  has three parts:
• 1. A phosphate group
• 2. A sugar (called deoxyribose)
• 3. A Nitrogen Base – There are 4!
– Adenine “A”
– Thymine “T”
– Guanine “G”
– Cytosine “C”
Slide # 3
Components of DNA
DNA is made up small, repeating units called nucleotides.
Nucleotide has 3 parts
1 . Phosphate group
2. Deoxyribose sugar
3. Nitrogen base (4
different bases)
–
–
–
–
Adenine
Thymine
Cytosine
Guanine
Adenine
Guanine
Thymine
“A”
“T”
“C”
“G”
Phosphate
group
Go to
Section:
Cytosine
Deoxyribose
DNA  Structure:
Two strands of nucleotides bonded together
at the nitrogen bases (with hydrogen
bonds) and twisted.
flat
twisted
What is DNA Made Of? DNA is made of four building blocks called nucleotide
bases; adenine (A), thymine (T), guanine (G) and cytosine (C). Each strand of DNA is
made up of long strings of these nucleotides arranged in a specific order. Two strands
stick together and twist to form a helix.
Slide # 6
Rosalind Franklin: The Famous
Picture Taker
1.1952: Rosalind
Franklin: an expert in
x-ray crystallography.
2.She took the x-ray
picture of DNA -shows DNA is a
double helix.
3.1958: Franklin died
(37 years old)
Watson and Crick
Are responsible for
the 1st model of
DNA and described
its shape as a
double helix
(twisted ladder)
• The two sides of the DNA
ladder are made up of
sugar and phosphate
groups.
• The “steps” (center) of the DNA
ladder are made up of base
pairs :
adenine+thymine
Hydrogen
guanine+cytosine
bonds
Hydrogen bonds
Chargaff’s Rule =
A-T G-C
A (Adenine) bonds to T
(Thymine) and
C (Cytosine) bonds with G
(Guanine).
FYI !!!!
Base pairs are weakly
joined together by
hydrogen bonds to form
each step of the ladder.
Erwin Chargaff
The KEY!!!!
The sequence
(order) of the 4
base pairs
determines the
genetic
instructions or
‘code’ for life.
DNA Replication –
replica = copy
creating two identical DNA molecules by
copying the original DNA strand
DNA
Replication
Steps:
DNA
Polymerase
1. DNA unwinds
2. DNA unzips DNA polymerase (an
enzyme) break the weak hydrogen bonds
holding the base pairs together splitting
the DNA ladder down the middle.
DNA Replication
3. Enzymes hydrogen
bond new nitrogen
bases onto the
original DNA ½
ladders using
Chargaff’s basepairing rule (A-T and
G-C).
4. Finally, sugar and
phosphate groups
covalently bond to the
new nitrogen bases
and to each other to
finish off the two new
DNA strands.
Results:
two identical DNA
strands made up of
½ of the original
strand and ½ of the
new strand.
Parent strand: gray
Daughter strand: red
When does DNA copy itself?
The cell replicates its DNA right before
cell division. When two new cells are produced in cell
division, each new cell needs its own copy of the
DNA. (Identical Copy!)
What happens to the Cell after DNA Replication?
After DNA replication,
the cell will divide in half (cell division)
and give each new cell a copy of the DNA.
Chromosomes
As humans, we have 46 chromosomes in each of
our cells. Each chromosome has millions of
base pairs storing the codes that make up our
bodies.
***Your genetic code
depends on the
sequence “order”
of the bases in
your DNA.
Chromosome 
coiled DNA and protein
“spacers” called histones.
Histones keep the
chromosome from getting
tangled.
DUPLICATED CHROMOSOME
A chromosome that has gone through
DNA replication and has an identical
“sister” DNA copy attached to it.
We call each copy of this duplicated
chromosome a sister chromatid.