How did they know that DNA was the molecule that was inherited?
I. DNA = Deoxyribonucleic Acid
A. Basic unit is a nucleotide
Phosphate group
Nitrogen Base
5 carbon sugar: deoxyribose
B. Stores information to make proteins
C. Structure:
1. Phosphate-sugar backbone
2. Double stranded
a. The two strands run in opposite directions, they are
said to be “anti-parallel”
Double Helix
Antiparallel Double Strands
Showing the
anti-parallel double
strands
3. A twisted Helix shape (like a phone cord)
4. Discovered by James Watson and Francis Crick in 1953
a. received Nobel prize along with Maurice Wilkins
b. Rosalind Franklin also contributed, but died before
Nobel Prize was given
5. Nitrogen bases are held together by weak Hydrogen
bonds
a. Purines: Adenine (A)
Guanine (G)
b. Pyrimidines: Cytosine (C)
Thymine (T)
c. Purines always pair with pyrimidines
A – T and G – C
6. Erwin Chargaff
a. knew components of DNA, not structure
b. analyzed base components of different organisms
c. amounts of bases vary, but are present in a
characteristic ratio.
Adenines = Thymines
Guanines = Cytosines
in humans
A = 30.9%
T = 29.4%
G = 19.9%
C = 19.8%
6. In all cells of the human
body, the DNA strand is
approximately 3m (3
billion bases) long and is
broken into 46 pieces.
These pieces are called
“chromosomes”
7. The strands are coiled
tightly around proteins
called “histones”
II. DNA Replication
“ It has not escaped our notice that the specific pairing we have postulated
immediately suggests a possible coupling mechanism for the genetic
material” –Watson and Crick
A. Meselson-Stahl experiment:
1. Tested three hypotheses for DNA replication.
What data would look like with
different densities.
If conservative, then:
Parent:
1st rep:
Centrifuge
results:
2nd rep:
Centrifuge
results:
If semiconservative, then
If dispersive, then
Conclusion: Semiconservative
B. Why is DNA replication so important?
1. So new cells have an exact copy of the old cell’s DNA
C. When does replication occur?
2. Before the cell divides
D. Process:
1. An enzyme called “DNA Helicase” attaches to DNA
strand and breaks the hydrogen bonds between A/T and
C/G
2. An enzyme “DNA polymerase” attaches free floating
bases to the single DNA strand
3. The result is two identical strands of DNA
4. This occurs at the same time at
different parts of the DNA strand
because it is so long
a. you see “Replication Bubbles”
all along the strand.
5. DNA polymerase can only add
bases to a free 3’ end, therefore the
daughter strand is always built in the
5’ to 3’ direction.
6. At every replication fork, both DNA strands are being
replicated
a. Leading strand – DNA polymerase moves along the
strand in the direction of replication
b. Lagging strand – DNA polymerase moves against the
direction of replication and has to build fragments that
will be joined later (“Okazaki fragments”)
DNA ligase joins
the fragments
together
7. Errors do occur: 1 error per billion nucleotides
a. DNA polymerase goes back to check mistakes
8. How long does this take? Approximately 50 nucleotides
per second