Solving the DNA mystery 1949- Erwin Chargaff discovered two
rules that helped lead to the discovery of the double helix
structure of DNA. This strongly hinted towards the base pair makeup
of the DNA, although Chargaff was not able to make this connection
himself. The first and best known achievement was to show that in
natural DNA the number of guanine units equals the number of
cytosine units and the number of adenine units equals the number of
thymine units. He also discovered that the total amount of purines
equals the total amount of pyrimidines in an organisms genome. G =
C A = T Purines = Pyrimidines
Slide 5
Nucleotide Structure in detail Sugar molecule containing 5
carbons that has lost the OH (hydroxyl group) on its 2 carbon
Deoxyribose sugar Group of four oxygen atoms surrounding a central
phosphorous atom found in the backbone of DNA Phosphate group An
alkaline, cyclic molecule containing nitrogen Held together by
hydrogen bonds The source of variation is found in the nitrogenous
bases. Nitrogeneous base
Slide 6
Nucleotides making up DNA DNA is composed of many nucleotides
Nucleotides are held together by phosphodiester bonds Therefore,
DNA is a polymer
Slide 7
Deoxyribose Sugar Five-carbon cyclic ring structure Attached to
a phosphate group and nitrogeneous base The first four of the five
carbon atoms, together with an oxygen atom, form a five membered
ring The carbon atoms are numbered clockwise, starting with the
carbon atom to the immediate right of the oxygen atom This first
carbon atom is designated as 1 ' A deoxyribose sugar has a hydroxyl
group (OH) on the 3 ' carbon, and a hydrogen atom (H) on the 2 '
carbon
Slide 8
Deoxyribose sugar The nitrogeneous base is attached to the 1 '
carbon of the sugar by a glycosyl bond The phosphate group is
attached to the 5 ' carbon by an ester bond
Slide 9
Know your bonds: The nitrogeneous base is attached to sugar by
a glycosyl bond The phosphate group is attached to sugar by an
ester bond Nucleotides are held together by phosphodiester bonds
Base pairs are held together by hydrogen bonds
Slide 10
Building a model for DNA structure 1953- California: Linus
Pauling London: Rosalind Franklin & Maurice Wilkins Cambridge
University: James Watson & Francis Crick Franklins X-ray
crystallography (X-ray diffraction analysis) results helped Watson
and Crick to determine the molecular structure of DNA X-ray results
revealed that DNA has the shape of a helix DNA has a diameter of 2
nm A complete helical turn every 3.4 nm 1 nm=10 -9
Slide 11
Rosalind Franklin James Watson and Francis Crick (1953)
Slide 12
Slide 13
DNA Structure: The Double Helix DNA consists of two
anti-parallel strands of nucleotides Anti-parallel: parallel
running in opposite directions The bases of one strand are paired
with the bases of the other strand The 5 ' end of one strand of DNA
aligns with the 3 ' end of the other strand in a double helix.
Slide 14
Complementary Base Pairing The nitrogeneous base pairs are
arranged above each other, perpendicular to the axis of the DNA
melecule A purine is always bonded to a pyrimidine Adenine with
Thymine Guanine with Cytosine This type of pairing is termed
complementary base pairing
Slide 15
A purine is always bonded to a pyrimidine Adenine with
ThymineGuanine with Cytosine Diameter of DNA is 2 nm If 2 purines
are bonded together (A-A or A-G or G-G), the DNA molecule would be
wider If 2 pyrimidines are bonded together (T-T or T-C or C-C), the
DNA molecule would be less What about Thymine with Guanine? Or
Adenine with Cytosine?
Slide 16
DNA What about Thymine with Guanine? Or Adenine with Cytosine?
Even though, they would have the desired diameter, within the DNA,
the molecules would be unstable because of the lack of hydrogen
bonding. Only Adenine can hydrogen bond with Thymine and Guanine
can hydrogen bond with Cytosine. Double helix turns in a clockwise
direction DNA is a right-handed helix making one complete turn
every 10 molecules A full helical twist is 3.4 nm, therefore, the
distance between adjacent base pairs is 0.34 nm.
Slide 17
DNA The two strands of DNA run anti-parallelOne strand runs in
5 ' 3 ' directionThe other strand runs in 3 ' 5 ' direction The 3 '
end terminates with the hydroxyl group of the deoxyribose sugar The
5 ' end terminates with a phosphate group Therefore, every DNA
molecule has an intrinsic directionality