• Defined: Molecule that
stores genetic
information
• Monomer: Nucleotide
• Millions of nucleotides
bond to make nucleic
acids
• Gene: section of DNA
that holds instructions to
build a protein
Nucleotide
Structure
• Three parts
– Sugar molecule
– Phosphate group
– Nitrogen base
(A, T, C, or G)
• Nucleotides combine
to make nucleic
acids
• A pairs with T
• C pairs with G
T
C
C
G
A
• The nitrogen containing bases are the only
difference in the four nucleotides
• Nucleotides can be divided into 2 categories:
• Sugar (Deoxyribose)
connects with the phosphate
group
• 4 Different Bases:
– Adenine (A)
– Thymine (T)
– Guanine (G)
– Cytosine (C)
• Chargaff’s rules
– Rule: A always joins with T
– Rule: C always joins with G
• Human DNA: ~3 billion pairs
C
T
A
A
T
G
T
• James Watson and Francis Crick identified DNA’s
structure in 1953 (see video clip: Structure of DNA)
• Double Helix: spiral shape of DNA
• Nucleotide strands are connected by a hydrogen bond
• Defined: Process of copying DNA
– During interphase (S-stage)
• Step 1: DNA Helicase “unzips” the two DNA strands
(breaks the H bonds)
• Step 2: Free floating nucleotides (A, T, C, G) match up
• Step 3: DNA Polymerase reconnects the two strands
Step 1:
DNA Helicase
“unzips” the two
DNA strands
(breaks the H
bonds)
Step 2:
Free floating
nucleotides (A,
T, C, G) match
up
Step 3:
DNA
Polymerase
reconnects the
two strands
Both strands are
identical
Both strands are
composed of a
single strand of old
and a single strand
of new nucleotides.
This is called the
semi
conservative
model
DNA Replication Video Clip
enzyme
• Mutation: change in DNA
sequence
• Mutations can be corrected
– DNA Polymerase (enzyme)
“proofreads” DNA and
check/correct errors
• Pre-repair: 1 in 10,000
nucleotides has an error
• Post-repair: 1 in 1 billion
nucleotides has an error
ReView
1.
2.
3.
4.
Name the three parts of a DNA nucleotide.
Name the four nitrogen bases and how they pair with others.
Which nitrogen based are pyrimidines? Purines?
List what takes place during the three stages of DNA
replication.
5. Explain the semi-conservative model.
6. Fill in the missing DNA nucleotides:
A C ? T ? ? ? G C T ? ? A ? C ? T T
? ? C ? T AT ? ? ? C A ? G ? G ? ?
6. Where is the DNA mutation?
TAGGCCTGTCGAACAGT
ATCCGGACAACTTGT CA