Chemistry of nucleic acid and classification .ppt

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1. Chemistry of Nucleic Acids
Nucleic acids (DNA and RNA) are made by joining
nucleotides in a repetitive way into long polymers
 Nucleotides have three components (a) phosphate (b) sugar
and (c) base
Nucleoside is a sugar-base compound

2. ___________________________________________________
DNA RNA
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Phosphate Phosphate
Deoxyribose (Sugar) Ribose (Sugar)
Guanine (Purine base) Guanine (Purine base)
Adenine (Purine base) Adenine (Purine base)
Cytosine (Pyrimidine base) Cytosine (Pyrimidine base)
Thymine (Pyrimidine base) Uracil (Pyrimidine base)
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Comparison of DNA and RNA

3. Figure 1. Components of nucleic acids (a) phosphate (b)
sugar (c) base

4. Figure 2. The structure of a nucleoside and two nucleotides:
nucleoside monophosphate and nucleoside triphosphate

5. Figure 3. Structure of four deoxyribose nucleotides

6. Chargaff’s ratio
Erwin Chargaff analyzed base composition of humans,
Mycobacterium tuberculosis and Sea Urchin and found that the
amount of adenine equaled that of thymine and cytosine equal
to guanine
 The Chargaff’s rule is that in DNA there is 1:1 correspondence
between purine and pyrimidine bases

7. Watson - Crick Model
•DNA a double helix with sugar backbone on the outside and the
bases on the inside
•There is one purine and one pyrimidine per turn
•The base pairing is such that a purine pairs with a pyrimidine
(A-T; C-G)
•There are two hydrogen bonds between adenine and thymine
and three between cytosine and guanine
• There is polarity in each strand of DNA molecule in that one
end of DNA will have 5' phosphate and the other has a 3'
hydroxyl group
•Nucleotides are connected across phosphates forming a
phosphodiester bonds

8. Figure 3. Double helical structure of DNA

9. DNA denaturation
Hydrogen bonding can be broken and DNA strands separated by
heating
Since cytosine-guanine pair has three hydrogen bonds to two of
adenine-thymine, the former will require more heat to denature

10. Requirements of Genetic material
For something to be classified as a genetic material it needs to
meet the following requirements:
- Control of protein synthesis - Later lectures will
show that DNA has the complexity to direct protein
synthesis
- Self-replication – DNA is capable of self
replication as explained in the next lecture
- Location in the nucleus – In prokaryote the
majority of the cell’s DNA is found in the
chromosome

11. Alternative forms of DNA
The form of DNA we have described above is a right handed
double helix and it is called B DNA and its turns clockwise
when viewed at its axis and bases are stacked perpendicular to
the main axis
If water content increases to about 75% bases are tilted in
regard to the axis and there are more bases per turn and this
DNA is called A DNA.
In 1979 Alexander Rich discovered Z DNA which is a left
handed helix. This DNA requires high salts to be stable but it
can be stabilized in physiologically normal conditions if
methyl groups are added to cytosine. This DNA may be
involved in regulating gene expression in eukaryotes