13. NUCLEIC ACIDS: INFORMATIONAL
POLYMER
• Nucleic acids are polymers of monomer
units called nucleotides
• Nucleic acids store and transmit hereditary
information: gene
• Two major forms of nucleic acid polymers:
DNA and RNA
15. DNA Structure
DNA is a nucleic acid.
The building blocks of DNA are nucleotides,
each composed of:
– a 5-carbon sugar called deoxyribose
– a phosphate group (PO4)
– a nitrogenous base
• adenine, thymine, cytosine, guanine
16.
17. 17
DNA Structure
The nucleotide structure consists of
– the nitrogenous base attached to the 1’ carbon of
deoxyribose
– the phosphate group attached to the 5’ carbon of
deoxyribose
– a free hydroxyl group (-OH) at the 3’ carbon of
deoxyribose
19. DNA Structure
Nucleotides are connected to each other to
form a long chain
phosphodiester bond: bond between adjacent
nucleotides
– formed between the phosphate group of one
nucleotide and the 3’ –OH of the next nucleotide
The chain of nucleotides has a 5’ to 3’
orientation.
21. 21
DNA Structure
Determining the 3-dimmensional structure of DNA
involved the work of a few scientists:
– Erwin Chargaff determined that
• amount of adenine = amount of thymine
• amount of cytosine = amount of guanine
This is known as Chargaff’s Rules
23. DNA Structure
Rosalind Franklin and Maurice Wilkins
– Franklin performed X-ray diffraction studies to
identify the 3-D structure
– discovered that DNA is helical
– discovered that the molecule has a diameter of
2nm and makes a complete turn of the helix every
3.4 nm
26. 26
DNA Structure
James Watson and Francis Crick, 1953
– deduced the structure of DNA using evidence
from Chargaff, Franklin, and others
– proposed a double helix structure
27.
28. 28
DNA Structure
The double helix consists of:
– 2 sugar-phosphate backbones
– nitrogenous bases toward the interior of the
molecule
– bases form hydrogen bonds with complementary
bases on the opposite sugar-phosphate backbone
30. 30
DNA Structure
The two strands of nucleotides are antiparallel
to each other
– one is oriented 5’ to 3’, the other 3’ to 5’
The two strands wrap around each other to
create the helical shape of the molecule.
31.
32. Double Helix
• 2 complementary strands of DNA
– sugar-phosphate backbone
– nitrogenous bases stacked in the center
– antiparallel
• 5’ end
• 3’ end
– twists to right
35. Base Pairing in DNA: The Watson-Crick
Model
According to the Watson–Crick model, a DNA molecule
consists of two polynucleotide strands coiled around
each other in a helical, screwlike fashion.
The sugar–phosphate backbone is on the outside of this
right-handed double helix, and the heterocyclic bases
are on the inside, so that a base on one strand points
directly toward a base on the second strand.
The double helix resembles a twisted ladder, with the
sugar–phosphate backbone making up the sides and the
hydrogen-bonded base pairs, the rungs.
42. DNA double Helix
Chargaff’s rules: the amount of adenine equals the
amount of thymine, and the amount of guanine
equals the amount of cytosine, and the total
amount of purines equals the total amount of
pyrimidine.
46. The three helical forms of DNA (and RNA)
Physiological
DNA
Very unusual
DNA
RNA
High-salt
DNA
47.
48. Type Shape Helix Base pair
per turn
Pitch
per bp
Width Occurrence
A Broadest Right
handed
11 0.256 nm 2.3 nm High salt
Medium
B Intermediat
e
Right
handed
10 0.338 nm 1.9 nm Normal
Form
Z Elongated Left
handed
12 0.571 nm 1.8 nm Some of
DNA
49. B-form DNA consists of a right-handed double helix with antiparallel strands
34 Å (10 bp)
per turn
major groove
minor groove
major
groove
minor
3.4 Å
per bp
These dimensions are for DNA fibers. In solution, there are ~10.5 base-pairs per turn.
5’ 3’
5’3’
52. Summary of the main structural features of B-form DNA
•Right-handed helix
•Two antiparallel strands held together by
Watson-Crick hydrogen bonds
•Pitch (repeat length) = 34 Å (3.4 nm)
•36o
rotation between residues
•Helix diameter of 20 Å (2.0 nm)
•Wide major groove, narrow minor groove
•Chargaff’s Rules: A = T; G = C
•Charged phosphates
•Bases in anti configuration
•The strands separate at high temperatures
•The solution structure is dynamic
53. DNA
• Functions
• 1. Storage of genetic information
• 2. Self-duplication & inheritance.
• 3. Expression of the genetic message.
• DNA’s major function is to code for proteins.
• Information is encoded in the order of the
nitrogenous bases.
