Download DNA Structure and Replication

Structure of DNA
Watson and Crick used two sets of observations to infer the structure of DNA
X-ray crystallography: DNA is a helix
Subunit analysis
Subunits of nucleic acids
(DNA)
(RNA)
Polymerization of DNA subunits by condensation
T
Deoxyribose
The double helix
“Complementary” base pairs
stick together through H-bonds
Note: antiparallel
The double helix
The helical twist pulls the hydrophobic
bases close together
DNA chains can be very long
! E. coli chromosome: 4.6x106 base pairs:
4.6x 106 x .34 nm = 1.5x106 nm = 1.5 mm
! Human DNA: 6x10-12 g/cell x 1/660 mol bp/g x
6.023x1023 bp/mol bp x 0.34x10-9 m/bp = 1.9
m
! Bacterial, viral DNA “chromosomes” are circles
! DNA in human chromosomes (and DNA of all
eukaryotes) are linear strands
! “Z-DNA”, left-handed double helix, is also
possible
Melting and re-association of DNA
! H-bonds are weak bonds, broken by high
temperature
! DNA “melts”--strands separate--at 85-95oC
! When cool, the strands can re-associate, if they
have complementary base sequences
! Use this technique to recognize specific
sequences
! make radioactive DNA of particular
sequence
! fix unknowns to membrane
! add radioactive “probe”
! slowly cool to allow re-association
! wash off unassociated DNA
! measure radioactivity
Question: Is the gene for Starlink Bt toxin
present in my taco shells?
Sample 1
Sample 2
Sample 3
32P:
radioactive
phosphorus
In eukaryotes, DNA chains coil to become
shorter and thicker
Telomere s
Centromere
Replication of chromosomes
! Eukaryotes: chromosomes are linear (two to
several hundred per cell; humans = 46)
! Distinguished visually by length, position of
“centromere”
! Replication during S phase gives two pieces
(“chromatids”), connected by centromere
! Progeny and parent DNA have identical base
sequences
G1
G2
Why do parent and progeny DNAs have identical base sequences?
•Parent DNA serves as a template for the synthesis of progeny DNA
DNA synthesis: the molecular mechanism
•Separate the template strands
•Add subunits: deoxyribonucleotide triphosphates
•Connect (form new sugar-phosphate bonds)
Complications:
! Splitting H-bonds
! Priming (DNA synthesis
needs a primer)
! Anti-parallel templates
Complications:
! Splitting H-bonds: unwinding enzyme, DNA
helicase, binding proteins
! Priming (DNA synthesis needs a primer):
RNA "primase" makes RNA; DNA added
! Antiparallel templates: Okazaki fragments of
new DNA on one strand (for a short time)
Summary
! The information in DNA is in the base sequence.
! The complementary strands of DNA represent
two copies of all the information in the cell.
! Through hybridization, one strand can be used
to identify a complementary strand.
! Duplication of DNA information is needed before
a cell reproduces.
! DNA polymerase (together with other proteins)
uses the two existing DNA strands as templates
to synthesize two new strands.