This document discusses the molecular structures of DNA and RNA. It explains that DNA stores genetic information, while RNA has messenger, ribosomal and transfer functions. The basic units of nucleic acids are nucleotides composed of a nitrogenous base, a 5-carbon sugar and one to five phosphate groups. The four nucleotides in DNA contain the bases adenine, guanine, cytosine and thymine. RNA differs from DNA in using uracil instead of thymine. DNA exists in a double helix formation with base pairing between strands. The three common helical forms are A, B and Z-DNA.

1. Nucleic Acid Structures

2. Learning objectives
 Understand the molecular structure of DNA.
 Understand the molecular structure of RNA.
 Understand the relationships between purines,
pyrimidines, nucleosides, nucleotides, and nucleic
acids.
 Understand the structural differences between
DNA and RNA

3. Nucleic acid functions
 DNA –genetic information storehouse
 RNA
 message between the genome and the ribosome for the building
of proteins (messenger RNA, or mRNA),
 act as the catalytic moiety in a RNA-protein enzyme complex
(rRNA)
 serve as the translator between mRNA sequence and protein
sequence (tRNA)
 RNA and DNA- recent discovery catalyze some
reactions without protein involvement (ribozymes or
deoxyribozymes).

4. 9000 year old “Kennewick man” skull discovered along the banks of
the Columbia River in Washington state believed to have originated
from Southeast Asia, Japan or Polynesia
DNA from
archeological
samples
Compared to modern
DNA
Evidence of DNA
stability

7. SUGAR + BASE = NUCLEOSIDE

8. Nucleotide – monomeric unit in nucleic acid

11. Deoxythymidine 5’-monophosphate
Deoxyadenosine 5’-monophosphate
Deoxycytidine 5’-monophosphate
Deoxyguanosine 5’-monophosphate

12. N-glycosidic bond
3’,5’-phosphodiester bond

13. C1’-N1= Pyr
C1’-N9 = Pur

15. Structural features of the DNA
(secondary structure)
 Double helix; 2 topographic features – major groove and
minor groove
 Two strands antiparallel
 In aqueous environment phosphate-sugar backbone
outside of the helix; purine and pyrimidine rings inside
the structure
 Strand stabilized by :
H-bonds between complement bases
Van der Waals and
Hydrophobic interactions between stacked bases

18. Conformational varieties of the secondary
structure
A B Z

19. Conformational varieties of the
secondary structure
 B-DNA – crystallized from water; water
retained in the inner structure;
predominant form under physiological
conditions
10 base pairs/turn of helix
Right handed
Distance bet base pairs 0.34 nm
Diameter – 2.0 nm or 20 A

20.  A-DNA
Dehydrated form of B-DNA
Right handed helix
11 base pairs/helix
Diameter = 26 A

21.  Z-DNA
 Found in short stretches of native DNA
and synthetic DNA
 Left handed helix
 12 base pairs/helix
 Diameter = 18 A

22. 85-90 degrees as
the melting
temperature

23. Tm = midpoint of each curve

24. At normal temp Increased tem, DNA denatures
Sample becomes more turbid
More light is absorbed

25. Tertiary structure of the DNA
 Circular (relaxed) – in E.coli; simian virus 40;
bacteriophage; certain animal species
 Supercoiled DNA – extra twisting in the linear
duplex; allows DNA to be more compact in the cell;
regulatory role in replication
 Topoisomerase – change topology of the DNA
 Quadruplex DNA – four stranded; in protozoan
DNA; occur in G-rich regions; regulating and
stabilizing telomeres and regulation of gene expression