chapter 16
DESCRIPTION
Chapter 16. Molecular Basis of Inheritance. DNA genetic material. Chromosomes composed of DNA + protein. DNA base composition. Nucleotide base Guanine Cytosine Thymine Adenine. Guanine, C 5 H 5 N 5 O. DNA is a polymer of nucleotides. Chargaff’s rules (1950). - PowerPoint PPT PresentationTRANSCRIPT
Chapter 16
Molecular Basis of Inheritance
DNA genetic material
• Chromosomes composed of DNA + protein
DNA base composition
Nucleotide base
GuanineCytosineThymineAdenine
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 crystallographyDNA is helicalSpacing of basesWidth 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 DNAcopied to new DNAhelix
Original DNA broken up and combined in new DNA
1 strand original DNA maintained in new DNA
Meselson and Stahl 1950s1. 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?
15N DNA1.724 gm/cm3
14N DNA 1.710 gm/cm3
DNA replication is semi-conservative
DNA replication: mechanism (E. coli)
E. coli genome = 4 X 10 6 bp DNA1 circular chromosome1 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 helix3. 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
Each strand is a template for new DNA
Leading strand is
Lagging strand is
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 strand500 nuc/secContinuous elongation until end of chromosome
DNA Synthesis steps: lagging strand1. Primase makes RNA primer2. 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
Human chromosomes capped by telomeres
5'...TTAGGG TTAGGG TTAGGG TTAGGG TTAGGG TTAGGG..3‘ 3'...AATCCC AATCCC AATCCC AATCCC AATCCC AATCCC..5'
• 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
• Embryonic cells, some wbc, stem cells, cancer cells express telomerase
White blood cell cervical cancer cell embryo