Chapter 9:
DNA: The Molecule of Heredity
Chapter 9: DNA
What is DNA?
Answer: Molecule that carries the blueprint of life
General Features:
• DNA is packages in chromosomes (DNA + Proteins)
• Gene = Functional segment of DNA located at a
particular place on a chromosome
Chapter 9: DNA
(Figure 9.1)
Chapter 9: DNA
Structure of DNA:
• Nucleic acid composed of individual nucleotides
Chapter 9: DNA
Structure of DNA:
• Nucleic acid composed of individual nucleotides
• 4 types of nucleotides (nitrogen base differences):
Purines (double-ring bases)
Pyrimidines (single-ring bases)
Chapter 9: DNA
How are the Nucleic Acids arranged to form DNA?
1) Chargaff (1940s):
• DNA contains equal amounts of adenine and thymine
• DNA contains equal amounts of cytosine and guanine
2) Franklin and Wilkens (early 1950s):
• Used X-ray diffraction
• DNA is a helical molecule (corkscrew)
• DNA has a uniform diameter (2 nm)
• DNA consists of repeating sub-units
Chapter 9: DNA
How are the Nucleic Acids arranged to form DNA?
1) Chargaff (1940s):
2) Franklin and Wilkens (early 1950s):
3) Watson and Crick (1953)
• DNA is two separate strands of linked nucleotides
• Phosphate groups bonded to sugar groups (backbone)
• Antiparallel (strands opposite one another)
• DNA strands held together by hydrogen bonding
between individual base pairs
• Complementary base pairing:
• Adenine → Thymine
• Guanine → Cytosine
• Strands twisted to form double helix
Chapter 9: DNA
(Figure 9.3)
Chapter 9: DNA
The order of nucleotides in DNA can encode vast amounts
of information:
• 10 nucleotides = > 1,000,000 possible sequences
• Organisms have millions (bacteria) to billions
(plants / animals) of nucleotides
Chapter 9: DNA
DNA Replication
• The copying of DNA is a critical event in a cell’s life
• Cell reproduce by mitosis to produce two daughter
cells from a single parental cell
• Each daughter cell must receive an exact copy of DNA
• DNA replication produces two DNA double helices through
process termed semiconservative replication
• Parental strands unwind and separate
• Daughter strands are formed using parental strands
as templates
• Parental strand and new daughter strand wrap together
to form new double helix
Semiconservative
Replication:
Chapter 9: DNA
Chapter 9: DNA
DNA Replication - A Closer Look...
Step 1: DNA Strand Separation
• DNA Helicase (enzyme) breaks hydrogen bonds
holding nitrogen bases together (unzips)
• Requires energy (ATP)
• Multiple DNA helicases work on single chromosome
• form replication bubbles; time-saving mechanism
DNA Helicase
Chapter 9: DNA
DNA Replication - A Closer Look...
Step 2: DNA Synthesis
• DNA Polymerase (enzyme) joins nucleotide sub-units
to form new strands of DNA
• Forms complementary strands to parent strands
(Recognizes nucleotide and adds appropriate base)
C
A
T
G
C
C
G
T
A
T
G
A
G
G
Polymerase
A
A
T
T
A
T
A
T
G
A
C
T
C
C
Chapter 9: DNA
DNA Replication - A Closer Look...
Step 2: DNA Synthesis
• DNA Polymerase (enzyme) joins nucleotide sub-units
to form new strands of DNA
• Forms complementary strands to parent strands
(Recognizes nucleotide and adds appropriate base)
• DNA Polymerase can only move in one direction
• One strand synthesized as continuous molecule
• Second strand synthesized in small pieces
DNA
Polymerase
DNA Ligase: Attaches pieces together
Chapter 9: DNA
DNA Replication - Mistakes Happen...
• DNA replication is a rapid process
• 700 nucleotides / second
• DNA polymerase mismatches 1 bp / 10,000 bps
• Proof-reading enzymes correct mistakes
• Overall mistake rate = 1 bp / 1,000,000,000 bps
• Other hazards damage DNA
• Body temperature = 98.6 °F = 10,000 bp lost / day
• Ultraviolet light
Sun Burns:
Chapter 9: DNA
1st Degree
2nd Degree
UV light from sun damages DNA
If damage is great enough,
cell will commit cellular suicide
If damage in region of DNA
controlling replication, cancer may result
A symmetry
B order (irregular)
Color (irregular)
Diameter (large)
3rd Degree
Chapter 9: DNA
DNA Replication - Mistakes Happen...
• DNA replication is a rapid process
• 700 nucleotides / second
• DNA polymerase mismatches 1 bp / 10,000 bps
• Proof-reading enzymes correct mistakes
• Overall mistake rate = 1 bp / 1,000,000,000 bps
• Other hazards damage DNA
• Body temperature = 98.6 °F = 10,000 bp lost / day
• Ultraviolet light
• X-rays
¾ Repair enzymes fix damaged DNA
Chapter 9: DNA
DNA Helicase
DNA
Polymerase
DNA Ligase: Attaches pieces together