Chapter 16
Molecular Basis of Inheritance
DNA genetic material
• Chromosomes
composed of DNA
+ protein
DNA base composition
Nucleotide base
Guanine
Cytosine
Thymine
Adenine
Guanine, C5H5N5O
DNA is a polymer of nucleotides
Chargaff’s rules (1950)
[T] = [A]
[G] = [C]
A certain chromosome is 19% A.
What is the % of C?
DNA structural model
Watson, Crick, Franklin 1953
X-ray crystallography
DNA is helical
Spacing of bases
Width of helix suggested 2 strands
DNA double helix
Sugar-phosphate
“backbone”
Anti parallel
strands
• Bases face inward
• Hydrogen bonds
connect bases
A - T (2 bonds)
G - C ( 3 bonds)
Original DNA
copied to new DNA
helix
Original DNA
broken up and
combined in
new DNA
1 strand original
DNA maintained
in new DNA
Meselson and Stahl 1950s
1. Label DNA (E. coli) with
15N in growth media
2. Transfer E. coli to 14N
media for 1 generation
(20 min)
Results:
The density of the DNA is
intermediate
Cells grown longer 14N,
make lighter DNA
What would the DNA density be after 20 more
minutes of cell group?
14N
15N
DNA 1.710 gm/cm3
DNA1.724 gm/cm3
DNA replication is semi-conservative
DNA replication: mechanism (E. coli)
E. coli genome = 4 X 10 6 bp DNA
1 circular chromosome
1 origin of replication (ori)
• Ori nucleotides
– Replication proteins attach to ori
– Forms a replication bubble
• Two strands of DNA open
Replication fork in both directions
Proteins in DNA replication Table 16.1
1. DNA polymerase (enzyme)
Adds nucleotides 5’  3’ direction only
2. Helicase (enzyme) – unwinds double helix
3. Single stranded binding protein (SSB) binds to
DNA strands to stabilize them
4. Topoisomerase (enzyme) – breaks, rejoins
DNA to relieve physical stress
5. Primase – synthesizes a primer
Leading strand is
Lagging strand is
Each strand is a template for new DNA
DNA replication leading strand: steps
1. Primase (enzyme)
– synthesizes primer complementary to leading strand
– primer is ~10 bases
2. DNA polymerase (pol III) synthesizes new
strand 5’  3’
G, A, T, C nucleotides complementary to template strand
500 nuc/sec
Continuous elongation until end of chromosome
DNA Synthesis steps: lagging strand
1. Primase makes RNA primer
2. DNA adds nucleotides to primer in 5’  3’
direction only
3. DNA pol III detaches
Okazaki fragment
• ~ 1, 000 nucleotides
long
4. Another primer
added, another
Okazaki fragment
formed
Many primers needed
5. Gaps between
primers filled in
6. Ligase enzyme
bonds fragments
DNA replication Fig. 16.17
Telomeres, the protective ends
Linear DNA has telomeres
• No genes
• Repetitive DNA TTAGGG
up to 1000 times
5'...TTAGGG
TTAGGG TTAGGG TTAGGG TTAGGG TTAGGG..3‘
3'...AATCCC AATCCC AATCCC AATCCC AATCCC AATCCC..5'
Human chromosomes capped by telomeres
• Chromosomes shorten with each cell division
• When telomeres are too short
 cell senescence
(irreversible)
~ 125 cell divisions (humans)……life span?
Telomeres shorten ~100 bp each time cell divides
Mouse fibroblasts in culture
• Cells that do not divide often
– Example: heart muscle
Telomeres do not shorten with age
• Lagging strand problem
• Animation garland
• Embryonic cells, some wbc, stem cells, cancer
cells express telomerase
White blood cell
cervical cancer cell
embryo