Transcript Chapter 16 notes

Chapter 16
The Molecular Basis of
Inheritance
DNA
• Deoxyribonucleic Acid – now known to be
the genetic material
• function studied in microbes
DNA
1928 Griffith studied Streptococcus pneumoniae
-found 2 strains: smooth (encapsulated - s) &
rough – r
Experiment – when mice were injected with s,
pathogenic
- s heat killed - harmless to mice
- heat killed s mixed with r - mice died
- live r - harmless to mice
DNA
• Blood samples taken from dead mice contained
live s
• Somehow r cells acquired capsule ability from
dead s & passed it to offspring
- this phenomenon is called transformation –
assimilation of external genetic material by a
cell
DNA
1944 Avery – determined transforming agent to
be DNA
1952 Hershey & Chase – using bacteriophages
(T4) which infect E. coli (to verify Avery’s work)
- they knew viruses were made of DNA & protein
& knew viruses cause host to make new viruses
Hershey-Chase experiment
• Radioactively labeled T2 with 35S mixed with
bacteria, agitated in blender to separate outer
phage from cells, centrifuged & measured
radioactivity in supernatant
Results: radioactivity in supernatant, therefore,
protein did not enter the bacteria
Further evidence
• DNA replicates prior to cell division
• Chargaff using paper chromatography analyzed
various DNA samples & found DNA to be very
species-specific & also very diverse
DNA Structure
• Polymer of nucleotides:
1) sugar (deoxyribose)
backbone 2) phosphate
3) nitrogenous base:
purines (adenine & guanine)
pyrimidines (cytosine & thymine)
Chargaff’s rules:
A-T, G-C
DNA Structure
• 2 nucleotide strands held together by weak
hydrogen bonds
• X-ray crystallography showed 2 antiparallel
strands of S-P wound around N-bases
• Watson & Crick described structure as
double helix
DNA Replication
• Process where DNA makes an exact copy of
itself as a result of complementary base pairing:
1) molecule unwinds, then unzips (2 strands
separate) due to helicase
2) new DNA nucleotides line up on both strands
3) strands recoil; continues through molecule
Messelson-Stahl experiment
1) bacteria grown in 15N, then to 14N
2) DNA isolated & mixed with CsCl, centrifuged
for days
3) DNA moves to areas where density equals
CsCl
Results:
• 3 different densities indicating 3 different
combinations
DNA Replication
• Begins at origin sites forming Y – shaped forks
• helicases – unwind & single-strand binding
proteins stabilize
• topoisomerases – break & reseal DNA strands
to allow them to unlink
• primer – short segment of RNA whose base
sequence is complementary to parent DNA
portion needed to start new DNA segment
DNA Replication
• DNA polymerases – catalyze DNA synthesis
5’ 3’
• DNA ligase – links Okazaki fragments together
• errors are usually removed by enzymes
(natural incidence 1/10,000, completed is
1/1billion)
DNA Replication
• Repairs:
mismatch repair – polymerase & several
proteins in eukaryotes
excision repair – errors cut out & removed, then
replaced with correct
nucleotides by DNA ligase &
DNA polymerase
http://www.nobel.se/medicine/educational/dna/a/replication/lagging_ani.html
http://www.nobel.se/medicine/educationl/dna/a/replication/lagging ani.html
:// http highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animat