Download Griffith`s Experiment (1928)

Chapter 16: DNA
Griffith’s Experiment (1928)
Streptococcus pneumonia bacteria
Found that harmless live bacteria mixed with heat-killed infectious
bacteria causes disease in mice (experiment {d} in the book)
Substance passed from dead bacteria to live bacteria =
“Transforming Factor”
Hershey and Chase (1952)
“Blender” experiment
worked with bacteriophage: viruses that infect bacteria
grew phage viruses in 2 media,radioactively labeled with either
o
35S
in their proteins
o
32P
in their DNA
infected bacteria with labeled phages
Radioactive phage & bacteria in blender
o
35S
phage
 radioactive proteins stayed in supernatant
 therefore, protein did NOT enter bacteria
o
32P
phage
 radioactive DNA stayed in pellet
 therefore, DNA did enter bacteria
o Confirmed DNA is “transforming factor”
Chargaff’s Rules (1947)
all 4 bases not in equal quantity
led to development of pairing rules:
o C–G
o A-T
nucleotide
PO
Watson and Crick (1962)
4
N
base
Structure of DNA: double helix
DNA backbone = sugar and phosphate group
5 CH
2

Rungs of the ladder = nucleotides
4

H bonds connect complementary base pairs
Antiparallel strands
O
1

ribose
3

O
H
2

o 3’ end (terminates with OH)
o 5’ end (terminates with phosphate group)
DNA Replication
Base pairing allows each strand to serve as a pattern for a new
strand
 Semi-conservative replication: parent DNA strands serve as a
template for replication
 daughter DNA is composed of one parent strand and one
new one
 Large team of enzymes coordinates replication
1. Unwind DNA: helicase enzyme; DNA stabilized by single-stranded
binding proteins
2. New nucleotides match up with template strands
a) bases can only be added to 3’ end of a growing DNA strand
b) grows 5’  3’
c) Leading strand: continuous formation
d) Lagging strand: discontinuous formation
 Okazaki fragments joined by ligase
DNA polymerase III can only extend an existing DNA molecule
 cannot start new one, cannot place first base
short RNA primer is built first by primase: starter sequences
DNA polymerase III can now add nucleotides to RNA primer
DNA polymerase I removes sections of RNA primer and replaces
with DNA nucleotides
Replication enzymes (review)
helicase
unwind double helix
primase
creates RNA primer
DNA polymerase III
adds nucleotides to primer
DNA polymerase I
ligase
removed RNA primer, replacing with
DNA nucleotides
joins Okazaki fragments
single-stranded binding
stabilize unwound DNA
proteins
Editing/ Proofreading DNA
 1000 bases/second = lots of typos!
 DNA polymerase I
proofreads & corrects typos
repairs mismatched bases
excises abnormal bases
repairs damage throughout life
reduces error rate from 1 in 10,000 to 1 in 100 million bases
What’s the big idea?
transcription
DNA
RNA
translation
protein