Characteristic of Life!!
DNA is found in all living things and has the same structure!
• Nucleic Acids
• Examples - DNA and RNA
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2.35
DNA Location and Function
 Location – Nucleus (Eukaryotes)
 Function – contains the genetic instructions for
the development and function of living things.
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3.37
Monomer = Nucleotides
• DNA is made up of two long chains of nucleotides. Each
nucleotide has three parts.
– phosphate group
– deoxyribose sugar
– nitrogen-containing base (A,T,C,G)
• There are four different nucleotides
– Each contain a different nitrogen base.
phosphate group
deoxyribose (sugar)
nitrogen-containing
base
• Structure = Double helix
o Double = two strands
connected
o Helix = twisted
 Looks like a “twisted
ladder”.
• Main Parts
• Backbone
• Phosphate and 5 carbon
sugar
• Nitrogen bases
• Four Nitrogen Bases
• Adenine, Thymine, Cytosine, and Guanine
• Can be either a single or double carbon ring
• Pyrimidine = Single Carbon Ring
• Purine = Double Carbon Ring
DNA Discovery
• James Watson & Francis Crick
– Determined the threedimensional structure of
DNA.
– Identified the double helix
structure with a sugarphosphate backbone on the
outside and bases on the
inside.
• Other Contributors
– Rosalind Franklin’s x-ray images suggested that DNA was a
double helix of even width.
– Erwin Chargaff found that nucleotides can be found in even
ratios.
Chargaff’s Rules
– Adenine to Thymine
– Cytosine to Guanine
Nucleotide Paring Rules (Complementary Base Pairing)
• The base-pairing rules dictate
how nucleotides always pair up
in DNA.
– Adenine pairs with Thymine
– Cytosine pairs with Guanine
• Since a pyrimidine base (single
ring) pairs with a purine base
(double ring), the helix has an
even width.
G
C
A T
Nucleotide Bonding
• Backbone is connected by covalent bonds.
• Nitrogen bases are connected by hydrogen bonds.
hydrogen bond
covalent bond
DNA Replication – The process of copying a strand of DNA
Why do cells need to make a copy of their DNA?
• Occurs in the nucleus during interphase (Synthesis)
• Is a “semi-conservative” process
– The original (parent) DNA strand is used as a “Template” to
make the new DNA strand
– Results in 2 strands of DNA that are half old and half new (2
semi conservative DNA molecules)
DNA Replication: Semi-conservative
1. DNA helicase uncoils
and unzips DNA
2. New nucleotides line up
next to both old (parent)
chains of the DNA
DNA Replication – cont’d
3. DNA polymerase binds
the new nucleotides
together and
“proofreads” the new
strands for errors.
3. Results in 2 semiconservative strands
of DNA
DNA Replication
• Final Result – 2 Semi-Conservative Strands of DNA
DNA: the Genetic Code is carried in Triplets
• Genetic code = Order of nitrogenous bases
• Gene: a DNA segment that carries the
information for building a protein.
Reading DNA
• DNA is read in sets of three nucleotides called a triplet!
• 1 DNA triplet codes for 1 amino acid in a protein