Download Chapter 16 DNA

Chapter 16
Molecular Basis of Inheritance
DNA genetic material
• Chromosomes
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?
Purines are larger than pyrimidines
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
DNA replication: history
Can read quotes from W + C’s paper in textbook
3 models of DNA replication, 1 correct
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)
Density centrifugation
6M CsCL gradient cf 2 days 50,000 rpm
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)
1 circular chromosome
1 origin of replication (ori)
• Ori
– Specific sequence of 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 pol 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 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)
Mouse fibroblasts in culture
• Cells that do not divide often
– Example: heart muscle
Telomeres do not shorten with age
• Telomeres shorten ~100 bp each time cell
divides
• Lagging strand problem
• Embryonic cells, some wbc, stem cells, cancer
cells express telomerase
White blood cell
cervical cancer cell
embryo
Telomerase enzyme
synthesizes telomeres
• Animation garland